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Hypomagnesaemia (grass tetany) is a metabolic disorder of ruminants due to a reduced dietary intake of magnesium (primary deficiency), incorrect digestibility or associated metabolic factors reducing Mg intake (secondary deficiency). Grass tetany is a production disease responsible for important economic losses in beef herds from Argentina. Several factors influence the development of grass tetany in cattle, including physiological status, weather, soil and forage. This research described a retrospective analysis over the past 20 years, revising the cases of beef cattle clinical hypomagnesaemia registered at the Veterinary Diagnostic Service in INTA Balcarce, Argentina. Hipomagnesemia é um distúrbio metabólico de ruminantes devido a uma redução na absorção de magnésio (deficiência primária), digestibilidade incorreta ou fatores metabólicos associados que reduzem a ingestão de Mg (deficiência secundária). Hipomagnesemia é uma doença de produção responsável por importantes perdas econômicas em rebanhos de corte da Argentina. Vários fatores influenciam o desenvolvimento da hipomagnesemia em bovinos, incluindo fatores fisiológicos, clima, solo e forragem. Este trabalho descreve uma análise retrospectiva dos últimos 20 anos, revisando os casos de hipomagnesemia clínica em bovinos de corte registrados no Serviço de Diagnóstico Veterinário do INTA Balcarce, Argentina. Fil: Cantón, Germán José. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce; Argentina. Fil: Fernández, Eduardo Luján. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce; Argentina. Fil: Poo, Juan Ignacio. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce; Argentina. Fil: Späth, Ernesto Juan Alfredo. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce; Argentina. Fil: Odriozola, Ernesto Raúl. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce; Argentina. Fil: Monterubbianesi, María Gloria. Universidad Nacional de Mar del Plata. Facultad de Ciencias Agrarias; Argentina. Fil: Moreno, Fabiana Carina. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce; Argentina.

Aim Magnesium (Mg) deficiency (known as grass tetany) is a serious metabolic disorder that affects grazing ruminants. We tested whether Mg-fertiliser can increase Mg concentration of Italian ryegrasses (Lolium multiflorum L.) including a cultivar (cv. Bb2067; ‘Magnet’), bred to accumulate larger concentrations of Mg. Methods Under controlled environment (CE) conditions, three cultivars (cv. Bb2067, cv. Bb2068, cv. RvP) were grown in low-nutrient compost at six fertiliser rates (0–1500 μM MgCl2.6H2O). Under field conditions, the three cultivars in the CE condition and cv. Alamo were grown at two sites, and four rates of MgSO4 fertiliser application rates (0–200 kg ha−1 MgO). Multiple grass cuts were taken over two-years. Results Grass Mg concentration increased with increasing Mg-fertiliser application rates in all cultivars and conditions. Under field conditions, cv. Bb2067 had 11–73% greater grass Mg concentration and smaller forage tetany index (FTI) than other cultivars across the Mg-fertiliser application rates, sites and cuts. Grass dry matter (DM) yield of cv. Bb2067 was significantly (p p ≥ 0.05). Conclusions Biofortification of grass with Mg through breeding and agronomy can improve the forage Mg concentration for grazing ruminants, even in high-growth spring grass conditions when hypomagnesaemia is most prevalent. Response to agronomic biofortification varied with cultivar, Mg-fertiliser rate, site and weather. The cost:benefit of these approaches and farmer acceptability, and the impact on cattle and sheep grazing on grasses biofortified with Mg requires further investigation.

An outbreak of hypomagnesemia is reported in Holstein dairy cattle grazing lush oat (Avena sativa) pasture in Uruguay. Nine of 270 (3.3%) cows died in May-July (autumn-winter) 2017. These nine cows were from 2 to 9-years-old (1st-6th lactation), with 22 to 194 days of lactation and 15.8 to 31.4L of daily milk production. Two cows with acute sialorrhea, muscle spasms, lateral recumbency, weakness, opisthotonos, and coma, were euthanized and necropsied. No significant macroscopic or histological lesions were found. One untreated clinically-affected cow and eight out of 14 clinically healthy cows of the same group under similar management and production conditions had low serum levels of Mg (lower than 0.7mmol/L). Secondarily, both clinically affected cows and six out of 14 healthy cows had low serum Ca levels. The K/(Ca+Mg) ratio of two oat forages, corn silage, and ration was 5.10, 7.73, 2.45, and 0.85, respectively. A K/(Ca+Mg) ratio lower than 2.2 represents a risk for hypomagnesemia. The difference between the contribution-requirement of minerals in the diet was established and a daily deficiency of Mg (-0.36g/day), Na (-25.2g/day) and Ca (-9.27g/day) was found, while K (184.42g/day) and P (12.81g/day) were in excess. The diet was reformulated to correct the deficiencies and the disease was controlled by the daily administration of 80g of magnesium oxide, 80g of calcium carbonate and 30g sodium chloride per cow. It is concluded that hypomagnesemia is a cause of mortality in dairy cattle in Uruguay, and that the condition can be prevented by appropriate diet formulation.

Metabolic diseases usually occur when the production’s peak is beyond the capacity of the animal’s reserves to sustain a particular metabolic demand (eg, energy, calcium, magnesium, etc) at physiologic concentrations. Diseases like ketosis, milk fever or grass staggers usually occur after calving, when the demands for milk production overwhelm the physiological regulatory mechanisms. Despite the availability of treatment strategies for these disorders in the field, the evidence supporting their use is scarce in some cases, and the debate regarding the optimal treatment strategies for recovery continues. This article reviews and compares the current available knowledge on various treatment strategies for ketosis, hypocalcaemia (milk fever), and hypomagnesaemia (grass staggers) in periparturient cattle. The practical applications of the research studies are emphasised to provide current evidence-based treatment protocols for practising veterinarians.

Dualpurpose wheat (Triticum aestivum L) can be used for cattle grazing while producing grain from the same crop. In a two-year study, wheat forage production, grain yield, and growth performance of beef cow&ndash, calf pairs grazed on wheat pasture for 2&ndash, 3 weeks in spring was compared with the conventional wheat grain production system and stockpiled tall fescue (Festuca arundinacea (L.) Schreb) pasturing. Grazing wheat resulted in grain yield (4.1 vs. 4.6 t ha&minus, 1) and test weight (65.9 vs. 66.7 kg hL&minus, 1) similar to the conventional grain production system. Wheat accumulated significantly lower forage dry matter yield (0.9 vs. 1.9 t ha&minus, 1) in spring with higher crude protein (190.2&ndash, 290.2 vs. 122.0&ndash, 151.0 g kg&minus, 1) and low fiber contents compared to the stockpiled tall fescue pasture. Wheat pasture presented risk for the development of grass tetany with regard to N, K, Na, and Mg contents. Calves grazed on wheat gained 1143&ndash, 1370 gd&minus, 1 body weight compared to the 826&ndash, 879 gd&minus, 1 in the stockpiled tall fescue pasturing. Cows had inconsistent and mixed body weight change response. With warmer temperatures and adequate precipitation controlled grazing of wheat in spring by beef cow&ndash, calves offered weight gain benefits exceeding the stockpiled tall fescue pasturing and grain production similar to the conventional wheat grain system.

Cattle hypomagnesemia has been typically a pasture disease and labeled “grass tetany”, but after its increase during indoor...

Due to a decrease in phosphorus (P) and potassium (K) mining, manure is incinerated to concentrate P and K in ash. To understand the alternative use of manure-derived ash as P and K sources, laboratory and greenhouse experiments were conducted to determine the relationship between extractability and P and K uptake in cattle manure ash (CMA) and that between CMA application and a grass tetany hazard. The results showed that more P was extracted with 2% citric acid (90% of the total P) than with 2% formic acid (72–84% of the total P). Ninety-one percent of the total K was soluble in water. A greenhouse pot experiment was conducted to test P and K availability to Guinea grass (Megathyrsus maximus). Cattle manure ash or calcium dihydrogen phosphate (CF) was incorporated into sandy soil at 10, 20, and 50 g P2O5 m−2. Two combinations of CMA and CF were tested at 20 g P2O5 m−2. Potassium rates followed K content in CMA applied at different rates of P equivalent to 19, 38, or 96 g K2O m−2. In four harvest...

Hypomagnesaemic tetany (HypoMgT) in ruminants is a physiological disorder caused by inadequate intake or impaired absorption of magnesium (Mg) in the gut. If it is not detected and treated in time, HypoMgT can cause the death of the affected animal. A semi-structured questionnaire survey was conducted from July 2016-2017 to assess farmers' awareness of HypoMgT in cattle and sheep in the UK. The questionnaire was distributed to farmers at farm business events and agricultural shows, and through a collaborative group of independent veterinary practices to their clients. Farmers were asked about (i) the incidence of presumed HypoMgT (PHT); (ii) their strategies to treat or prevent HypoMgT; (iii) mineral tests on animals, forage and soil, and (iv) farm enterprise type. A total of 285 responses were received from 82 cattle, 157 mixed cattle and sheep, and 46 sheep farmers, of whom 39% reported HypoMgT in their livestock, affecting 1-30 animals. Treatment and/or prevention against HypoMgT was reported by 96% respondents with PHT and 79% of those without. Mineral tests on animal, forage, and soil was conducted by 24%, 53%, and 66% of the respondents, respectively, regardless of PHT. There was a highly significant association between the use of interventions to tackle HypoMgT and the incidence of PHT (p < 0.01). The top three treatment/prevention strategies used were reported as being free access supplementation (149), in feed supplementation (59) and direct to animal treatments (drenches, boluses and injections) (45) although these did vary by farm type. Although some (9) reported using Mg-lime, no other pasture management interventions were reported (e.g., Mg-fertilisation or sward composition). Generally, the results indicate that UK farmers are aware of the risks of HypoMgT. A more integrated soil-forage-animal assessment may improve the effectiveness of tackling HypoMgT and help highlight the root causes of the problem.

Herbage minerals affect performance of grazing cattle. We investigated the response of herbage P, K, Ca and Mg contents and Ca/P and K/(Ca + Mg) ratios to long-term stocking rate, continuous vs. discontinued grazing practice, and sampling year. Cattle had been stocked at 2·4 and 4·8 animal unit months ha−1 since 1949. Exclosures were installed in April 1998. Herbage samples were collected near peak herbage mass in 2001, 2003, 2008 and 2012 and analysed for mineral content. Mineral contents were similar between the two stocking rates, but were lower (P

The aim of this study was to develop and evaluate mathematical models that predict mineral excretion, particularly calcium (Ca), magnesium (Mg) and selenium (Se), from lactating dairy cows. Mineral excretion can be affected by several dietary factors. A deficiency in Ca or Mg application to pasture, among other factors, can contribute to grass tetany or wheat pasture poisoning in cows, whereas an excess can cause runoff into water supplies. Manure application with high Se concentration can also result in runoff, causing the bioaccumulation of selenium in aquatic ecosystems, wetland habitats and estuaries, leading to toxic levels in fish. A database composed of studies relating to mineral utilisation in lactating dairy cows conducted after and including the year 2000 was compiled. A meta-analysis was conducted with the aim of creating multiple empirical equations to predict Ca, Mg and Se excretion from lactating dairy cows. Calcium intake, feed Ca content, milk yield, milk protein content and acid detergent fibre content in diet were positively and linearly related to Ca excretion. Dietary crude protein content and milk fat content were negatively related to Ca excretion. Magnesium intake, feed Mg content and milk yield were positively and linearly related to Mg excretion. Selenium content of diet and dry matter intake were linearly and positively related to Se excretion. Two sets of models were developed using or excluding the intake variable and both sets of models were evaluated with independent data originating from commercial herd or individual animals. In general, intake measurements improved prediction when evaluated with independent datasets (root mean square prediction error = 8% to 19% vs 14% to 26% of the average observed value). There were substantial mean biases, particularly those evaluated with data from a commercial farm, perhaps due to inaccurate feed intake measurements. Although there was generally good agreement between predicted and observed mineral excretion, model development and evaluation would benefit from an expanded database.

Grass tetany, sometimes called grass staggers or hypomagnesemia, can be a serious problem in Syria with cattle grazing small grain or ryegrass pastures. hypomagnesemia tetany has been known under a variety of names, including magnesium tetany, lactation tetany and grass staggers, but most of these terms have been discarded because the disease is not always associated with lactation or with grazing animals. The exact cause of hypomagnesemia tetany in ruminant animals is a dietary deficiency of magnesium that may be a contributory factor. Some research workers consider the condition to be caused by a cation-anion imbalance in the diet, and Some research workers believe that high intake of the element (K & Na) may interfere with the absorption and metabolism of magnesium in the animal, which may be an important factor in the etiology of hypomagnesemia tetany. Although the exact cause of hypomagnesemia is still uncertain, the primary factor would appear to be inadequate absorption of magnesium from the digestive tract. A high degree of success in preventing hypomagnesemia may be obtained by increasing the magnesium intake. This can be effected by feeding with magnesium rich mineral mixtures or, alternatively, by increasing the magnesium content of pasture by the application of magnesium fertilizers.

A biogeochemical investigation was carried out on Origanum majorana grown on limestone substrate in Greece. Possible health risks from consumption of dried herbs and infusions were assessed. Macronutrients and essential and toxic metals were determined in the leaves of O. majorana plants and in their soil substrates. Toxic metals were measured in the herbal infusions. Macroelements were found generally in low concentrations for normally developing plants, except for Ca. The ratios N/P and N/K were found lower than the optimum range for normal growth, while the values of K/(Ca + Mg) ratio prevent the development of grass tetany. Manganese and arsenic were enriched in distinct samples. O. majorana plants can be used as indicators for soil environmental assessment. They can also be applied in phytoremediation methods in metal-polluted soils. Hazard indices were far below 1. Carcinogenic risks were found to be within the acceptable range. No health risk is anticipated by the consumption of the specific plants investigated in the present study.

Previous research demonstrated that phosphorus (P) fertilization of tall fescue (Festuca arundinacea Schreb.) pastures on acidic soils increased leaf P, magnesium (Mg), and calcium (Ca) concentrations and reduced the risk of grass tetany. It has been suggested that liming could have a similar effect by increasing soil pH and remobilizing sorbed P. To investigate this option, calcitic or dolomitic limestone was applied to stockpiled tall fescue plots on two acidic soils, a fragipan and a claypan. As expected, increasing rates of limestone were correlated with higher soil pHs. Contrary to predictions, limestone applications had very little effect on soil or plant P. However, both types of limestone at both locations altered leaf macronutrient concentrations in a manner that improved the grass tetany equivalent ratio: increased Ca and/or Mg and decreased K concentrations, thus lowered the likelihood of grass tetany in grazing beef cattle.

Lime in pelletized form is potentially more convenient for farmers than ground limestone, as it can be applied using conventional fertilizer-spreading equipment. Pelletized lime is intended to maintain an optimum soil pH when applied annually at a rate of 350 kg lime/ha/yr. Interactions between lime and N fertilizer rate were examined by applying 0, 75, 150, 225 and 300 kg N/ha/yr (as calcium ammonium nitrate) in combination with 0, 175, 350 and 525 kg pelletized dolomitic lime/ha/yr over 3 yr to a permanent grassland sward used for silage production in County Down, Northern Ireland. Equal rates of ground lime, from the same source as the pelletized lime, were applied as a comparison. Effects on soil chemical properties, grass dry matter (DM) yield and herbage nutrient removal were examined. Lime maintained or slightly increased the soil pH, particularly in the top 2.5 cm of the profile, but there was no difference in the performance of pelletized lime compared to ground lime in any of the parameters measured. Lime had no significant effect on grass DM yield or grass quality; however, there was a significant (P

The objective was to determine changes in the nutritional quality of the three predominant botanical components (i.e., grasses, legumes, forbs) in the Preveza Prefecture grasslands in North-Western Greece. The study involved collection of herbage samples during the bloom stage from 12 experimental plots located in three altitudinal zones (i.e., lower, middle, and upper) in 2 consecutive years. Samples were manually separated into the three botanical components and analyzed for crude protein (CP), neutral detergent fibre (aNDFom), acid detergent fibre (ADFom), lignin(sa), in vitro dry matter (DM) digestibility (IVDMD), in vitro NDF digestibility (IVNDFD), metabolizable energy (ME) and minerals (i.e., Ca, P. K. Mg). Altitudinal zone strongly affected the nutritive value of grasses, legumes and forbs. Indeed, altitude above sea level was positively correlated with CP, IVDMD and IVNDFD and negatively correlated with fibre contents. We found that the Ca:P ratio exceeded the animal functional disorder threshold, which posed a hazard; a belief amplified by the high K/(Ca + Mg) ratio, which would increase the risk of grass tetany in ruminants that primarily ate grasses in the upper zone. (C) 2011 Elsevier B.V. All rights reserved. Animal Feed Science and Technology

During pregnancy, ewes graze pastures that may be marginal in calcium (Ca) and magnesium (Mg), and may also be low in sodium (Na) and high in potassium (K), with a high dietary cation–anion difference. Such pastures may increase susceptibility to hypocalcaemia and hypomagnesaemia, leading to lamb losses. Clinical hypocalcaemia and hypomagnesaemia do occur in Australian sheep; however, it is unknown whether subclinical forms of these disorders compromise ewe or lamb health and survival. The present study monitored the Ca and Mg status of ewes in late pregnancy, so as to evaluate the risk of subclinical mineral disorders in ewes grazing typical southern Australian pastures. Calcium and Mg concentrations in pasture, ewe plasma and urine were monitored in 15 flocks in southern Australia. Mineral concentrations in pasture did not indicate a widespread risk of Ca or Mg deficiency; however, urinary pH and Ca and Mg concentrations in the plasma and urine of the pregnant ewes were not entirely consistent with the expectations from pasture analysis. Urine pH was above 7 on all properties and 87.5% of properties had a mean Ca in urine below the adequate concentration of 1 µmol/mosmol. The mean plasma Ca concentration was below adequate (18 mg/L), more than 20% of ewes on six farms had below adequate concentrations of plasma Mg. Only one-third of the farms had 100% of ewes measured with adequate concentrations of plasma Ca (4/15) or Mg (5/15). The mineral concentrations in pre-lambing blood and urine samples suggested that a significant number of animals grazing southern Australian pastures during winter may be at risk of subclinical hypocalcaemia and hypomagnesaemia.

The mineral concentration of forage grasses plays a significant role in 2 metabolic disorders in dairy cattle production, namely, hypocalcemia (milk fever) and hypomagnesemia (grass tetany). Risks of occurrence of these 2 metabolic disorders can be evaluated by determining the dietary cation-anion difference (DCAD) and the grass tetany (GT) index of forages and specific rations. The objective of this study was to evaluate the feasibility of predicting timothy (Phleum pratense L.) mineral concentrations of Na, K, Ca, Mg, Cl, S, and P, the DCAD, and the GT index by near-infrared reflectance spectroscopy (NIRS). Timothy samples (n = 1,108) were scanned using NIRS and analyzed for the concentration of 7 mineral elements. Calculations of the DCAD were made using 3 different formulas, and the GT index was also calculated. Samples were divided into calibration (n = 240) and validation (n = 868) sets. The calibration, cross-validation, and prediction for mineral concentrations, the DCAD, and the GT index were performed using modified partial least squares regression. Concentrations of K, Ca, Mg, Cl, and P were successfully predicted with coefficients of determination of prediction (R(P)2) of 0.69 to 0.92 and coefficients of variation of prediction (CV(P)) ranging from 6.6 to 11.4%. The prediction of Na and S concentrations failed, with respective R(P)2 of 0.58 and 0.53 and CV(P) of 82.2 and 12.9%. The 3 calculated DCAD and the GT index were predicted successfully, with R(P)2 >0.90 and CV(P)

Scenarios of global climate change forecast an increase in air temperature of 3°C over the next 100 years in eastern Canada. Growth and nutritive value of cool-season grasses are known to be affected by air temperature. It is also believed that grasses grown at high latitude have a greater nutritive value. The objectives of this study were to assess the effect of four combinations of day/night temperature and photoperiod (15 h-17/5°C; 15 h-22/10°C; 15 h-28/15°C; and 17 h-17/5°C) on dry-matter (DM) yield, in vitro true DM digestibility (IVTD), in vitro digestibility of neutral-detergent fibre (NDF), and concentrations of NDF, acid-detergent fibre (ADF), lignin, minerals and non-structural carbohydrates (NSC) in timothy grown under controlled conditions. Furthermore, herbage was harvested in the morning and in the afternoon to assess the impact of the time of harvest. The dietary cation-anion difference [DCAD = (K⁺ + Na⁺) - (Cl⁻ + 0·6 S²⁻)] and the grass tetany index [GT index = K⁺/(Ca²⁺ + Mg²⁺)] were also calculated. Higher temperature regimes significantly decreased IVTD and digestibility of NDF but had a limited effect on concentrations of NDF, ADF and lignin. DM yield of herbage was less and the concentration of NSC was greater in timothy grown under a temperature regime of 28/15°C than the 17/5°C and 22/10°C regimes; this effect is mainly explained by a response to temperature stress. Values of DCAD and the GT index of herbage were also lower under the 28/15°C than the 17/5°C and 22/10°C regimes as a result of a decreased plant K concentration. Under the 17/5°C regime, an increase in 2 h of photoperiod resulted in increased DM yield, decreased concentrations of K, Ca, Mg, Cl and N, and an increased starch concentration; IVTD or digestibility of NDF were not affected, although lignin concentration was reduced. Harvesting timothy in the afternoon rather than in the morning resulted in higher NSC, mainly sucrose, concentrations, and decreased ADF and NDF concentrations. The forecasted increase in air temperature in eastern Canada over the next 100 years will result in lower yields and nutritive value of timothy.

Many wild weeds are used in Mexico as fodder. Due to their economic value, this investigation was undertaken to determine the chemical composition of 14 species. The mineral, crude protein, fiber, and total phenolic content, as well as the antioxidant activity, was determined. There was a significant variation in the mineral content among the 14 weeds, which all had calcium and potassium as the predominant elements. According to the results, some of the weeds might represent possible mineral deficiency sources, making the supplementation of certain minerals advisable. Although five of the species contained a low concentration of magnesium, the overall mineral ratios of the forage weeds were at safe levels, preventing grazing animals suffering from grass tetany, in contrast to the excessive calcium : phosphorus ratios that can affect their normal growth and bone development. The crude fiber and protein content in most of the weeds fell into the recommended range for the maintenance of cattle. The phenolic content of the studied weed extracts was higher than that reported for several Mediterranean forage species. These phenolic profiles correlate well with the radical scavenging activities of the extracts.

Timothy (Phleum pratense L.) is grown in several countries, mainly for livestock forage. The nutritive value and dry matter (DM) yield of timothy is well established but less information is available on mineral concentration and related indices, like the dietary cation-anion difference (DCAD) and the grass tetany (GT) index. These indices predict the risk of a forage diet to cause metabolic disorders, i.e., milk fever for DCAD and grass tetany for the GT index. Improving our knowledge of factors affecting timothy mineral concentration is important for livestock production. This review paper compares timothy with other common cool-season grasses for mineral concentrations, DCAD, and GT index. Also assessed are variations in timothy mineral concentration affected by harvest and fertilization management, soil mineral availability, and environmental conditions. These results demonstrate the high potential for improvement and use of timothy forage to decrease the incidence of metabolic disorders within the dairy industry. Key words: Milk fever, grass tetany, dietary cation-anion difference, grass tetany index, forage, cool-season grasses

Incidence of metabolic disorders increases when dairy cows (Bos taurus) are fed forages that have a high dietary cation-anion difference (DCAD) (>250 mmol c kg -1 dry matter, DM) or high grass tetany (GT) index (>2.2), both associated with high forage K concentration, often caused by applications of liquid swine manure (LSM). We determined how DCAD and GT index of timothy (Phleum pratense L.), grown on two soils with different K availability, were affected by mineral or LSM fertilization. Experimental treatments were: unfertilized control, mineral fertilizer, raw LSM, and liquid fractions of four treated LSM types (decanted, filtered, anaerobically digested, and flocculated), applied in spring and after the first of two harvests each year. Forage DCAD was lowest (-108 mmol c kg -1 DM) with the flocculated LSM due to its higher Cl content. Forage DCAD with other LSM types was similar to that with mineral fertilizer. Forage GT indices with LSM and mineral fertilizer were higher than that of the unfertilized control but still within an acceptable range for cows. The DCAD and GT index were greater on soils with high K availability. From spring growth to summer regrowth, these values decreased for soils with low K availability and increased for soils with high K availability. Compared with mineral fertilizer, LSM applied to timothy did not increase the risk of metabolic disorders for dairy cows; a Cl-enriched LSM can substantially decrease DCAD and lower the risk of milk fever.

HiMag tall fescue (Lolium arundinaceum (Schreb.) S.J. Darbyshire = Festuca arundinacea Schreb) was selected for high Mg concentration in the herbage to reduce grass tetany risk to ruminants; however, the mechanism of increased Mg uptake into shoots is unknown. The objective was to determine cation concentrations of roots, crowns, and leaves in plants of cv. HiMag and its parents, cv. Kentucky 31 and cv. Missouri 96, grown in nutrient solution for 42 days, and determine if cation ratios in roots, crowns, and leaves are different, indicating a differ- ence due to translocation. Treatments were "basal" (1.5 mM K and 0.5 mM Mg), "K" (3.2 mM K), "Mg" (1 mM Mg), and "K+Mg" (3.2 mM K and 1 mM Mg). For HiMag, Mg was lower in roots (Trial 2 only), not different in crowns, and greater in leaves than Kentucky 31 and Missouri 96. Doubling the K and Mg of the nutrient solution from basal levels resulted in a 44% reduction of root Mg in Kentucky 31 and Missouri 96, compared to a 17% reduction in root Mg for HiMag. The K inflow rate in HiMag for the basal treatment was lower than that in Kentucky 31 and Missouri 96. These results provide evidence for a process that limits K uptake and an active Mg translocation mechanism in tall fescue. HiMag was apparently selected for traits that promote transloca- tion of Mg from roots to shoots.

Hypomagnesemia is a metabolic disorder of ruminants not receiving adequate utilizable magnesium, either because of low Mg concentration in the forage or because of complicating factors affecting Mg availability. The disorder is more likely to affect cattle than sheep or goats and generally affects older, lactating animals. Magnesium deficiency may result in decreased milk or beef production (Grunes & Mayland, 1975; Grunes et al., 1970). An intensified hypomagnesemia, known as grass tetany, has caused a large number of cattle deaths in the temperate regions of the United States and other countries, resulting in losses of up to 3% in dairy or beef herds in a bad year (Baker & Gould, 1976; Grunes et al., 1970; Mansfield et al., 1975; Molloy, 1971). In some individual herds in some years losses have been even higher. Reid et al. (1978a) estimated annual losses in the United States at 1 to 2% of mature grazing animals. The severity of the problem is quite sporadic. Many factors affect Mg uptake by plants and its subsequent availability to animals. It is our objective to discuss the soil, climate, and plant factors that may ultimately affect Mg intake and availability to ruminants.

Annual ryegrass (Lolium multiflorum Lam.) provides livestock feed and captures nutrients from fields receiving manure application. The objective of this study was to determine relationships among maturity and yield, mineral uptake, and mineral concentration. Primary spring growth of ‘Marshall’ ryegrass was harvested every 7 d to 56 d maturity and was fertilized with swine effluent containing 254 and 161 kg nitrogen (N) and 42 and 26 kg phosphorus (P) ha−1 for two years. Yield increased linearly to a maximum of 13.6 mg ha−1 after 49 d in 2001 and 8.0 mg ha−1 after 56 d in 2002. Mineral uptake was highly correlated (r > 0.95) with yield and attained a maximum single harvest of 192 kg N ha−1 and 32 kg P ha−1 (mean of two years). Concentration of all minerals except calcium (Ca) declined as ryegrass matured. Low magnesium (Mg) concentration (< 2 g kg−1 dry matter) increases the risk of hypomagnesemic grass tetany.

Cycling of potassium in grassland systems has received relatively little attention in research and practice in recent years. Balanced nutrient systems require consideration of nutrients other than nitrogen (N). Potassium (K) is needed in large amounts and is closely related to N nutrition. In intensive dairy farming, surpluses of K arise from the input of concentrates and fertilizer and are returned to the grassland and may lead to increasing K content in the soil. Organic farming, on the other hand, is characterized by limitations in input of nutrient sources and quantities. Leaching of K from grassland is usually low, but high levels of available soil K, high K input from fertilizer or at urine patches lead to increasing losses. High K inputs have a negative influence on Mg and Ca uptake by plants and can cause accelerated leaching of these cations. High levels of K have been associated with inducing nutrition-related dairy cow health problems such as milk fever (hypocalcaemia) and grass tetany (hypoinagnesaemia). This review gives an overview of the cycling of potassium and related cations in grassland systems especially with regard to leaching losses and identifies limitations to knowledge.

High-quality forage for spring and fall grazing is an important need of ranchers in the Northern Great Plains and Intermountain-West regions of the United States of America and in the prairie provinces of Canada. Russian wildrye [Psathyrostachys juncea (Fischer) Nevski] has been used to meet this grazing need, especially in Canada. However, its use has probably been limited by its reputation for seedling establishment difficulties and scattered reports of grass tetany. The purpose of this research was to characterize the variation in nutritive quality of Russian wildrye accessions used in a multi-location grass tetany project, to access the effect of environment on quality components, and to determine the relationship between forage nutritive quality components and entities associated with grass tetany. Sixty-seven Russian wildrye accessions from the US National Plant Germplasm System were established in spaced-plant nurseries at Logan, UT, Mandan, ND, and Swift Current, SK, Canada. Plants were sampled at two stages of development over 2 yr. Location, year, and stage of development effects were significant (P < 0.01) for in vitro true dry matter digestibility (IVDMD), crude protein (CP), and neutral detergent fiber (NDF). At the early stages of development used in this study the range in nutritive quality traits among entries was rather narrow. Relatively high positive correlation coefficients between K and IVDMD, and K and CP suggest that breeding for higher nutritive quality may also produce a more tetany-prone forage, because high K concentration is usually associated with grass tetany. On the other hand, Mg concentrations were also highly correlated with IVDMD and CP, and higher Mg concentrations would be beneficial in preventing grass tetany. The K/(Mg + Ca) ratio, which has been suggested as an indicator of grass tetany, was only moderately related to IVDMD, CP, and NDF. Key words: Digestibility, crude protein, neutral detergent fiber, stage of plant development

There is ample experimental evidence that, Na, if supplied in separate fertiliser granules or crystals to N, i.e., in blended fertiliser form, can improve both the yield and the recovery of fertiliser N by grassland swards in situations of limited K supply, but not in situations of K abundance. There is some evidence, though, that in K-replete situations, Na, if supplied in the same fertiliser granule as N, i.e. in concentrated complex fertiliser (CCF) form, also improves dry matter production and N recovery by swards whilst lowering the risk of grass tetany in grazing animals. However, the mechanism for the latter effect of Na on N uptake has never been elucidated, nor has it been clarified whether Na stimulates NH 4 + and NO 3 − uptake by plants or simply NO 3 − uptake alone. The aim of the present study was to see if supplying Na in the same fertiliser pellets (NNa-CCF) as NH4NO3 (differentially labelled with15N), or in separate pellets (NNa-blend), had any effect on the recovery of15N-labelled NH 4 + and NO 3 − -N by perennial ryegrass plants growing in a glasshouse under K-replete conditions. The results of the experiment confirmed that using an NNa-CCF was more beneficial to shoot production than using an NNa-blend. However, the differential in shoot production occurred without any corresponding difference in total N (i.e. NH 4 + plus NO 3 − -N) recovery in shoot tissue. Instead, Na, in the CCF appears to have stimulated NO 3 − uptake at the expense of NH 4 + absorption, thereby altering the balance between NH 4 + and NO 3 − -nutrition in favour of NO 3 − -nutrition, and stimulating shoot production as a consequence. It was concluded that if grassland is already well supplied with K it would be more beneficial in terms of sward production to apply a Na and N-containing CCF than a blend of separate Na and N-containing granules or crystals.

HiMag, an accession of tall fescue (Festuca arundinacea Schreb.), was selected for high magnesium (Mg) concentration in leaves to reduce grass tetany risk to ruminants. However, the mechanism for enhanced Mg uptake in HiMag leaves has not been determined. The objective was to investigate if increased Mg uptake in HiMag could be explained by differences in elemental distribution among plant parts, root characteristics, or organic acid concentrations compared to its parental cultivars, “Kentucky 31” (KY31) and “Missouri 96” (MO96). The study was conducted on a surface-irrigated calcareous Portneuf silt loam (coarse-silty, mixed, mesic, Durinodic Xeric Haplocalcid). Vegetation and soil cores of 7.6-cm diameter were sampled to a 45-cm soil depth in 15-cm increments. Mass and ash were determined for leaves, crowns, and roots. Leaf area, root length, root area, root length density, elemental concentration, and uptake [potassium (K), calcium (Ca), Mg, sodium (Na), and phosphorus (P)], and malate and cit...

There is a continued need for breeding high-Mg grass cultivars with low grass tetany risk. Since interaction between magnesium (Mg) and potassium (K) exists, there is a possibility that genotypes characterized for higher Mg concentration may exhibit contrasting behavior for nutrient uptake with K application. A solution culture experiment was conducted to find a suitable K level for individual selection of high-Mg plants. Three K treatments of 1.0, 5.0, and 25 mM were superimposed. The uptake pattern of Mg, calcium (Ca), and K for approved high-Mg cultivars of Italian ryegrass (Magnet), orchard-grass (Mgwell), and tall fescue (HiMag) were compared with that of control cultivars. The seedlings were evaluated for shoot dry weight, amount of nutrient uptake, and average shoot nutrient concentrations. There was an increase in shoot dry weight grown with higher nutrient solution K levels. The effect of K on the trend of Mg uptake was paralleled among the high-Mg cultivars of the three species. The high-Mg cultivars showed higher Mg uptake and average shoot Mg concentrations than the control cultivars. Potassium uptake and average shoot K concentrations of high-Mg cultivars was always lower than the control cultivars. The greatest difference between the controls and high-Mg plants for average shoot Mg and K concentrations was observed in the plants grown under 5.0 mM level of K. This indicates the effectiveness of screening high-Mg grass seedlings using 5.0 mM level of K under solution culture condition. Methodology introduced in this experiment is aimed at expediting the term necessary for screening high-Mg plants.

Previous research has linked soil phosphorus level with forage magnesium concentration in tall fescue (Festuca arundinacea Schreb). Understanding this relationship might reduce the incidence of sub-acute hypomagnesaemia and/or grass tetany in cattle (Bos taurus). Our objective was to determine if P fertilization of tall fescue pastures affects the mineral element status of lactating, mature cows in spring and performance of their nursing calves. The treatments were: (i) tall fescue pasture fertilized to achieve 30 lb/acre of Bray I P; (ii) tall fescue pasture not fertilized with P but animals were given access to a mineral supplement that contained 12% Mg; and (iii) tall fescue pasture not fertilized with P and no supplement provided. With two exceptions, cows in all treatments showed similar serum and cerebrospinal fluid Mg concentrations. Average daily gain of nursing calves was 0.2 lb/day higher in the P-fertilized treatment compared to the other treatments. Thus, improving soil P level may alleviate symptoms of sub-acute hypomagnesaemia in cows and, more importantly, increase calf gain.

Bos indicus × B. taurus, especially Brahman × B. taurus, crossbred females express a high level of preweaning productivity during a relatively long life span; however, for reasons such as subpar reproductive performance of Brahman bulls, price discrimination for contemporary steer and heifer mates, and older age at puberty, these females can be expensive to produce. The longevity expressed by these crossbred cows can be partially attributed to the fact that these females are more tolerant of hot/humid environments and are less subject to death or being culled for maladies such as calving difficulty, ectoparasites (horn flies, mosquitoes, ticks), endoparasites (gastrointestinal helminths), eye disorders (infectious bovine keratoconjunctivitis, ocular squamous cell carcinoma), teeth deterioration, and grass tetany. In contrast, udder/teat abnormalities (“bottle” teats, pendulous udders) and vaginal/uterine prolapses tend to shorten the productive life span of some B. indicus × B. taurus genetic types.

Breeding for high magnesium (Mg) concentrations has been conducted for several forage species. Mgwell, Magnet, and HiMag are the first experimental strains, bred for increased Mg concentrations of orchardgrass, Italian ryegrass, and tall fescue, respectively. This experiment compared the performance and genetic variability of these high‐Mg cultivars grown in solution culture with other cultivars in each species. Three mineral absorption experiments were carried out with one month aged seedlings. Seedlings were evaluated for shoot dry weight, uptake and concentration of Mg, calcium (Ca), and potassium (K), and also the density of these minerals in the shoot. The cultivars of different species behaved differently among the experiments even though the over all environmental condition was kept similar. The high‐Mg cultivars showed higher Mg uptake per plant, but the differences were not so distinct. However, the trend in Mg concentration among the cultivars of different species was similar, and the d...

The main objective of this study was to compare potassium (K), calcium (Ca), and magnesium (Mg) utilization and grass tetany potential of orchardgrass (Dactylis glomerata L.) and tall fescue (Festuca arundinaceae Schreb.) fertilized with dairy manure or commercial fertilizer. The study was conducted from 1994 through 2000 in Willsboro, NY on a Kingsbury clay soil (very-fine, illitic, mesic Aeric Ochraqualf) of somewhat poor drainage. The design was a split-plot in a randomized complete block with two manure rates (16.8 Mg ha−1 and 33.6 Mg ha−1) and one fertilizer treatment (84 kg N ha−1 at spring greenup and 56 kg N ha−1 prior each regrowth harvest) as the main plots and grass species as subplots replicated six times. Potassium concentration and uptake increased after two years of manure application compared to commercial fertilizer treatment and residual effects of manure were large at least three years following manure application. Calcium concentration and uptake decreased after two years of m...

Grass tetany is a nutritional disease of ruminants caused by low dietary Mg. Previous research has shown that early spring P-fertilization increases the leaf Mg concentration of tall fescue (Festuca arundinacea Schreb.) hay. However, little is known about how P-fertilization alters the mineral concentration of tall fescue under grazing. Our objective was to compare, under grazing, the Mg, K, Ca, and P concentration of tall fescue when soil P was considered either adequate or low. The treatments were tall fescue grown on soil fertilized to achieve 30 lb/acre P (Pfertilized) or left unfertilized at 6 lb/acre P (Control). Three cow/calf pairs grazed each pasture from 15 February to 11 April, 2000, and 6 March to 1 May, 2001. Forage samples were collected at the start of grazing and at 14 day intervals thereafter. Under grazing, fertilization with P increased tall fescue forage Mg, K, and Ca concentration once spring growth started. However, the ratio of K/ (Ca+Mg) never approached the critical level thought to induce grass tetany. These results suggest that fertilizing tall fescue pastures to an adequate soil P level improves the amount of dietary Mg available to ruminants during early spring and decreases the grass tetany potential of the forage. Grass tetany is a nutritional disease that kills or affects approximately 350000 beef cows in the USA each year (6). Grass tetany typically occurs when cattle are moved from a winter diet of hay or stockpiled forage to lush, coolseason or winter annual grass pastures in early spring (11) (Fig. 1). Most research indicates that grass tetany is caused by a dietary Mg deficiency. When forage Mg concentration is below 0.20% dry matter (DM), the balance between Mg, Ca, and K in the animal is upset and grass tetany can occur (7). In addition to the low forage Mg concentration, the high water content in spring growth of cool-season or winter annual grass pastures makes it difficult for grazing cattle to consume adequate amounts of Mg (2). Other forage mineral concentrations have been implicated with the disease in addition to Mg. These include a forage Ca concentration less than 0.4% and/or a forage K level above 3.0% DM (20). Crop Management 16 August 2002 Fig. 1. (A) Tall fescue (Festuca arundinaceae Schreb.) beginning spring growth. (B) Mixtures of cereal rye (Secale cereale L.) and annual ryegrass (Lolium multiflorum Lam.) can contain too low Mg levels for grazing beef cows in lactation. Grass tetany is not always controlled by Mg supplementation (19). The disease is also difficult to diagnose in its early stages because symptoms are not always evident (10). Recent work to increase the amount of dietary Mg to grazing cattle has focused on boosting the early spring Mg concentration of grasses such as tall fescue (Festuca arundinacea Schreb.) (4,17,18). By increasing herbage Mg concentration, cattle might consume appropriate levels of Mg while grazing. Thus, producers would be less dependent upon unreliable supplements to provide cattle with enough dietary Mg. Recent research indicates that the Mg concentration of tall fescue leaves is dependant on soil P level (17,18). In these experiments, tall fescue was grown on plots that had between 8 and 97 lb/acre available P and received 0 or 25 lb/acre triple-super phosphate in early spring. When available soil P was 8 or 15 lb/acre and no P-fertilization occurred, leaf Mg concentration was 0.21% DM. Fertilization of these low P soils with 25 lb/acre P increased leaf Mg concentration to over 0.25% DM. In these experiments, 15 lb/acre Mg fertilization with MgCl2 only increased tall fescue leaf Mg concentration when 25 lb/acre P was also applied or when soil P was greater than 26 lb/acre (18). The impact of soil P on tall fescue forage Mg concentration could be an important consideration across the southeastern quarter of the United States. It is reported that half of the soils in this region test medium, low, or very low for soil P (5). In addition, more than 24 million acres of tall fescue are grown in this area (1) where grass tetany is a common disease. To determine if P-fertilization of tall fescue pastures increases the amount of dietary Mg available to cows, experiments are needed to determine if the Mg uptake of tall fescue is enhanced under grazing conditions. Our objective was to compare, under grazing conditions, the Mg, K, Ca, and P concentration of tall fescue forage when soil P was considered either adequate (30 lb/acre) or low (6 lb/acre). Grazing Experiment Testing Tall Fescue Mineral Concentration The grazing portion of the experiment was conducted from 15 February to 11 April, 2000, and from 6 March to 1 May, 2001. The location was the University of Missouri Southwest Research and Education Center near Mt. Vernon, MO (37°04’N, 93°53’W; elevation 1150 ft). The soil series was a Creldon silty clay loam (fine, mixed, mesic Mollic Fragiudalf). An area low in soil P was divided into three blocks, each with two 2.0-acre pastures of established ‘Kentucky 31’ tall fescue that received one of the following treatments: • Cows grazing tall fescue grown on a soil with an adequate level of P (30 lb/acre) (P-fertilized); • Cows grazing tall fescue grown on a soil low in P (6 lb/acre) (Control). A B Crop Management 16 August 2002 Pastures in the P-fertilized treatment were broadcast fertilized in early February and late August with calcium orthophosphate (triple-super phosphate; 46% P2O5 by mass) beginning in February, 1999 and ending in February, 2001. The total amount of P2O5 applied ranged between 286 and 313 lb/acre. These applications raised soil P to the level considered adequate for tall fescue forage production (13). All pastures were treated with 120 lb N/acre as ammonium nitrate and 300 lb K2O/acre as potassium chloride 14 days prior to the start of grazing each year. In addition, ammonium nitrate was broadcast in July, 1999 and 2000 at 60 lb N/acre to promote fall growth after pastures were cut for hay. Table 1 shows the soil pH, K, Ca, Mg, and P levels of the pastures during the experiment. Table 1. Soil pH, K, Ca, Mg, and P levels of P-fertilized and Control tall fescue pastures. Data were pooled across two years from samples taken before and after the experimental period each year. Pasture Management Grazing occurred during the typical grass tetany season, which begins approximately three weeks before new growth starts and ends six to eight weeks later (10). Three lactating cows with calves grazed each pasture to simulate a typical grazing system in the early spring. The cows grazed stockpiled tall fescue early in the study and actively-growing grass after spring growth began. Equal amounts of forage were allocated to animals grazing the pastures using the putand-take method so that forage availability was equal for both treatments. Forage Sampling and Analyses Forage samples were harvested with a sickle-bar mower. They were collected at the start of grazing and at 14 day intervals thereafter. Six 1.0-ft-x-15.0-ft strips were cut from each pasture to a residual height of 2 in. Forage from the six strips was combined and sub-samples were collected. Sub-samples were dried at 122° F for four days in a forced-air oven for dry matter determination. The subsamples were ground through a 0.04 in screen using a cyclone mill. Plant tissue was analyzed for K by flame emission; Mg and Ca by atomic absorption; and P by colorimetry (15). Statistical Analyses All data were pooled across years because error variance was homogeneous. The model used to analyze the data was a split-plot in time (9). Main plots were two pasture treatments and split plots were five harvest dates. All hypotheses were tested at the 0.10 alpha level. Means for all data were separated using least significant difference. Means are presented by treatment and date because treatment by date interactions occurred. Magnesium Concentration of Tall Fescue Forage Forage from both treatments showed low but equal Mg concentrations for the first 14 days of the experiment (Fig. 2). Research from West Virginia has shown that stockpiled tall fescue has a seasonal low in Mg concentration during mid-February (3). The lack of a treatment difference early in our study was likely a result of Mg leaching from the leaves over winter and little grass growth. However, by day 28, the Mg concentration of the Control was 0.13% DM, which was 15% less than at day zero. In contrast, the P-fertilized treatment did not Treatment pH K Ca Mg P

Grass tetany (hypomagnesemia) has caused severe economic losses in ruminant animals grazing cool-season grasses, including Russian wildrye [Psathyrostachys juncea (Fisch.) Nevski]. The malady has been associated with deficiencies in Mg, Ca, and carbohydrates, and high levels of K. The K/(Ca + Mg) ratio (KRAT), expressed as moles of charge, is often used to express the grass tetany potential of forage. Development and use of new cultivars with an improved balance of the associated minerals would be an economical approach to reduce the incidence of grass tetany. Objectives of this study were to characterize the genetic variability, genotype by environment interactions, and intercharacter relationships for P, K, Ca, Mg, KRAT, crude protein (CP), neutral detergent fiber (NDF), and in vitro dry matter digestibility (IVDMD), among 21 clonal lines of Russian wildrye and their polycross progenies. Evaluations were made for 2 yr at three diverse locations in the USA and Canada. The clonal lines were derived from cultivars and plant introductions. Although the clone x location interaction was usually significant, differences among the clonal lines were significant for K, Ca, Mg, and KRAT, and three forage quality estimates of CP, NDF, and true IVDMD. Although the magnitude of the genetic variability among the progenies was substantially less than that found among the clonal lines, we conclude that the grass tetany potential, CP, NDF, IVDMD, and P concentration of this breeding population can be altered through breeding. Opportunities for genetic improvement in forage quality were particularly favorable for CP. Genetic correlations among the clonal lines suggested that selection for higher levels of CP would be accompanied by increased K, Ca, Mg, and IVDMD and reduced KR AT and NDF. DUSSIAN WILDRYE is a caespitose, cool-season, perennial grass that is widely used in range improvement

Grass tetany or hypomagnesemic tetany in cattle (Bos taurus) is caused by an imbalance of K, Ca, and Mg in the diet. Indications of grass tetany range from reduced milk yield or weight gain to severe convulsions and death. The risk of grass tetany dramatically increases when the K/(Mg + Ca) ratio of forage exceeds 2.2, especially for dams during early lactation. Russian wildrye [Psathyrostachys juncea (Fisch.) Nevski], a valuable forage species, has ratios well above this level. Our objectives were to determine the mineral concentration and ratio values for 65 accessions of Russian wildrye to select germplasm sources with low tetany ratio and to determine the effects of year, location, and their interactions with accessions. Seedlings of each accession and two checks, Syn A and Mankota, were established in replicated space-plant nurseries at Logan, UT, Mandan, ND, and Swift Current, Saskatchewan, Canada. Years-within-location effects generally produced the largest variance component, while the accession variance was larger than location x accession and location X accession x year interaction variances for K, Ca, Mg, KJ(Mg + Ca) ratio, and Reduced Tetany Potential (RTP) index. Selection for these traits in Russian wildrye germplasm will require multiple years to characterize adequately accessions, breeding lines, or synthetics. The K/(Ca + Mg) ratio of the accessions tested ranged from 2.2 to 3.0 when averaged across sites and years for V4 growth stage. A similar range of ratio values and ranking of the accessions was observed at the E2 growth stage. The three tetraploid accessions evaluated were among the five accessions with the highest tetany ratios. Previously reported forage yield and seed yield means were significantly correlated with K, Mg, and N concentrations and K/(Ca + Mg) ratio. The RTP index was not correlated with forage yield, seed yield, or N concentration. Therefore, selection in Russian wildrye should be based on increased RTP index rather than K concentration or K/(Ca + Mg) ratio to avoid concomitant unintentional selection of reduced forage yield and seed yield

Grass tetany (hypomagnesemia) continues to be a problem for the livestock industry. An experimental cultivar of tall fescue (Festuca arundinacea Shreb.) selected for high Mg concentration, HiMag, was compared against four other cultivars for its ability to increase serum-Mg and reduce the risk of grass tetany. Forage from HiMag contained 22% more Mg, 18.5% more Ca, and 9% more P than other cultivars; K levels were not different. Regardless of cultivar, concentrations of Ca and Mg were lower while K and P were higher during the spring than the autumn. The tetany ratio [K/(Ca + Mg) on M c basis] was lower for HiMag than for other cultivars (1.34 vs. 1.65); all cultivars had a higher ratio during spring than autumn (1.91 vs. 1.31). In cattle (Bos taurus) grazing trials, blood serum from steers grazing HiMag contained 8% more Mg in autumn 1993 but was not different in spring 1994. Blood serum Mg was not different for cows (either dry or nursing calves) during autumn grazing, but approached significance (P = 0.09) during the spring 1995 calving season (2.32 vs. 2.07 mg/dL for HiMag vs.other cultivars). Except for differences between calves on HiMag compared with Kentucky-31 in autumn 1995 (58 vs. 43 Ib, respectively), animal weight change was not affected by cultivar. Normal forage Mg concentrations (>0.20%) and tetany ratios below 2.2 for all cultivars in every season except spring 1995 may explain the lack of consistent animal response. No clinical symptoms of grass tetany were observed at any time during these studies. However, the higher Mg concentration and lower tetany ratio suggest that HiMag could provide a means of reducing the incidence of grass tetany in livestock during periods when risk of the disorder is high.

A model of Mg metabolism in sheep is proposed. It is based on standard Michaelis-Menten enzyme kinetics to describe the transport of Mg across the rumen wall and passive diffusion to describe the absorption of Mg in the hindgut. Factors known to have an effect on Mg metabolism in farm animals, namely the concentrations of K and Mg in the diet, and the physico-chemical conditions within the rumen as determined by the type of diet, are incorporated into the model. Consideration of the rumen as the only site of Mg absorption provided an inadequate mechanistic description of Mg metabolism in sheep. To ensure compatibility between predicted Mg absorption and recent independent data sets for Mg balances, it was necessary to include in the model aspects of Mg absorption that operate in the hindgut. The results from this model suggest that there is a need for a series of experiments to determine the important aspects of Mg transport in the hindgut of sheep. Mechanisms of homeostasis are discussed.

Wheat seedlings (Triticum aestivum) grown on single salt solutions were analyzed for trans-aconitate and aconitate isomerase to determine the possible relationship between them and to determine if they might be related to the incidence of grass tetany. The trans-aconitate activity at the end of a two-day growth period and changes in trans-aconitate activity during this period varied widely and were related to the cation but not the anion of the single salt solution. Aconitate isomerase activity did not vary as much as trans-aconitate or change in trans-aconitate content, and was not closely correlated with the trans-aconitate content. Analysis of diverse species and tissues for aconitate isomerase and trans-aconitate showed that the presence of aconitate isomerase was necessary for appreciable trans-aconitate accumulation. We conclude that the activity of aconitate isomerase does not control trans-aconitate content but that its presence is necessary for any significant accumulation of trans-aconitate.

`HiMag' tall fescue (Festuca arundinacea Schreb.) was selected for high Mg concentration to reduce grass tetany risk to ruminants, but neither animal preference nor consumption of HiMag were known. The objectives were to evaluate methods of quantifying preference and to determine intake and preference by cattle (Bos taurus L.) of HiMag relative to seven other tall fescues. All entries were free of a fungal endophyte [Neotyphodium coenophialum (Morgan-Jones & Gams) Glen, Bacon & Hanna] that reduces cattle performance. The experimental design was a randomized complete block with three replications of eight cultivars nested within each of three test pastures. Six heifers grazed the vegetative to boot-stage pastures for 48 h in May, June, August, and September of 1993 and 1994. The pastures, located at 1200 m elevation, were furrow irrigated. Pre- and post-grazed forage were clipped and weighed to determine yield and utilization (48-h utilization < 50%). Preference scoring of 0 to 10 (0 to 100% of forage eaten) was done by four trained observers at 24, 30, and 48 h. The heifers quickly learned to distinguish between cultivars, and their order of preference was Kenhy > KY 31 > HiMag = Barcel = Cl = Stargrazer > M096 = Mozark. The cultivar x trial(year) interaction for preference indicated that cultivars responded differently to weather conditions, which in turn affected animal preference. Preference scoring had high repeatability and ranked cultivars similarly to the clip-and-weigh method of measuring utilization. Preference scoring was accomplished with 27% of the experimental error and only 6% of the time required for clip-and-weigh. Only 44% of the variation in preference score (PS) was explained by the model: PS = 8.8 - 1.1(Mg DM yield ha-1). Estimated dry matter (DM) intake of HiMag was 6.4 kg (animal unit day)-1. Consumption and preference of HiMag by cattle are satisfactory relative to other tall fescue cultivars.