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Previous studies have reported that the trailing shoe application technique reduces phosphorus (P) in the runoff postslurry application when compared to the traditional splash-plate application technique. However, the effectiveness of the trailing-shoe technique as a means of reducing P losses has not been evaluated when slurry is applied during periods of high soil moisture levels and lower herbage covers. To address this issue, three treatments were examined in a 3 × 4 factorial design split-plot experiment, with treatments comprising three slurry treatments: control (no slurry), splashplate and trailing-shoe, and four slurry application dates: 7 December, 18 January, 1 March and 10 April. Dairy cow slurry was applied at a rate of 20 m3/ha, while simulated runoff was generated 2, 9 and 16 days later and analysed for a range of P fractions. Dissolved reactive P concentrations in runoff at day two was 41% lower when slurry was applied using the trailing-shoe technique, compared to the splash-plate technique (P < 0.05). In addition, P concentrations in runoff were higher (P < 0.05) from slurry applied in December and March compared to slurry applied in January or April, coinciding with periods of higher soil moisture contents. While the latter highlights that ‘calendar’-based non-spreading periods might not always achieve the desired consequences, the study demonstrated that further field-scale investigations into the trailing shoe as a mitigation measure to reduced P loss from agricultural soils is warranted.

The objective of the present study was to examine the effect of dietary forage proportion (FP) on metabolizable energy (ME) requirement for maintenance (MEm) and the efficiency of ME use for lactation (kl) in lactating dairy cows. Data used were derived from 32 calorimetric chamber experiments undertaken at our institute between 1992 and 2010, including data from 818 Holstein-Friesian cows (HF), 50 Norwegian Red cows, and 62 crossbred cows (Jersey × HF or Norwegian Red × HF). Animals were offered forage-only rations (n=66) or forage and concentrate rations (n=864) with FP ranging from 18 to 100% (dry matter basis). The effect of FP was evaluated by dividing the whole data set into 4 groups according to the FP ranges, categorized as FP

A four-treatment (UG-UG, UG-G, LG-G and HG-G) experiment (involving sixteen plots: 3·0 × 7·0 m) examined the effect of early spring grazing intensity on particulate phosphorus (PP) losses in surface run-off. Ten dairy cows fitted with manure collection ‘bags’ grazed during two short-term grazing events, Grazing-1 (23 February) and Grazing-2 (6 April). During Grazing-1, two treatments remained ungrazed (UG-), while treatments LG- and HG- were lightly grazed and heavily grazed respectively. At Grazing-2, three treatments were grazed to a similar intensity (-G), while one remained ungrazed (-UG). Run-off was generated at two and 16 days after Grazing-1 and Grazing-2 using rainfall simulators (40 mm h−1) and analysed for a range of P fractions. Grazing had no effect on either dissolved reactive phosphorus (DRP) concentrations or dissolved unreactive P concentrations (mean, 0·15 and 0·16 mg L−1 respectively) in run-off. However, PP concentrations increased (P

Low emission slurry spreading techniques such as the trailing shoe are known to increase nitrogen use efficiency and to increase the available window for slurry application, whilst minimising sward contamination. However, little is known about the ability of the trailing shoe to mitigate against incidental phosphorus (P) losses, especially following slurry application to mature swards. This 3 × 2 randomised block design experiment investigates the impact of sward cover on P losses in runoff following application of manure via simulated trailing shoe apparatus. Treatments comprised of three stages of grass growth (0, 10 and 20 day re-growths; equivalent to 1550, 2050 and 2900 kg DM/ha, respectively), and two slurry application rates: 25 m 3 /ha (Slurry) and 0 m 3 /ha (No slurry). Runoff was generated at day 2, 9 and 16 post slurry application using a drip type rainfall simulator. Throughout the experiment P concentrations in runoff were significantly greater from the Slurry treatments than from the No slurry treatments ( P P P > 0.05) on particulate P concentrations in runoff. No significant differences ( P > 0.05) between the three re-growth treatments for DRP, PP, or TP concentrations in runoff on day 9 and 16 post slurry application with the exception of the Slurry plots at day 16 when the 0 day re-growth treatment exhibited significant higher levels of DRP in runoff than the 10d ( P = 0.006) and 20d ( P = 0.003) RG treatments. This experiment demonstrated that allowing a grass sward to recover for at least 10 days post-harvest provides a potential mitigation strategy to reduce P losses in runoff following slurry application.

Low emission slurry spreading techniques are known to improve nitrogen use efficiency, but their impact on phosphorus (P) losses in surface runoff has received little attention. The current study was designed to examine the effect of slurry spreading technique on P losses in runoff. Twelve treatments were examined on 0.5- m by 1.0-m plots in a nominal 2 × 6 factorial design experiment. Treatments comprised grass swards at two different stages of growth, a stubble and a 4-wk regrowth, and six different slurry application treatments: control (no slurry), and slurry applied to simulate splash-plate, injection (across and down slope), and trailing shoe (across and down slope) spreading. Slurry was applied by hand (40 m ha). Rainfall simulations (40 mm h) were conducted at 2, 9, and 28 d post-slurry application. When slurry was applied to the stubble, dissolved reactive P (DRP) concentrations in runoff at Day 2 were 47 and 37% lower ( < 0.05) from the injection and trailing shoe treatments compared with the splash-plate treatment. Similarly, at Day 2, TP concentrations in runoff from the injection treatments were 27% lower ( < 0.05) than the splash-plate treatment. In contrast, application technique had no effect ( 0.05) on P concentrations in runoff following slurry application to the regrowth treatment. Phosphorus concentrations in runoff were unaffected by direction of slurry spreading (across or down) at both applications. Our results indicate that trailing shoe and injection techniques offer the potential to reduce DRP concentrations in runoff during the period immediately after slurry application.

The performance of spring calving Holstein-Friesian (HF) and Jersey×Holstein-Friesian (J×HF) dairy cows was examined during three successive years (mean of 35 HF cows and 31 J×HF cows per year). Throughout the experiment cows were managed on one of three grassland-based systems of milk production, namely low concentrate (LC), medium concentrate (MC) or high concentrate (HC). Post-calving, cows were housed and offered grass silage, supplemented with 6.0, 8.0 and 10.0 kg concentrate/cow/d in systems LC, MC and HC, respectively (mean period from calving to start of full time grazing, 69 days). During the grazing period target concentrate feed levels were 0, 2.5 and 5.0 kg/cow/d for systems LC, MC and HC, respectively (mean period from start of full time grazing to start full time re-housing, 206 days). Full lactation concentrate inputs were 530, 1092 and 1667 kg/cow, in systems LC, MC and HC, respectively. There were no significant genotype×system interactions for any of the milk production parameters examined. Food intake during the confinement and grazing periods was unaffected by genotype. Milk yield was highest with HF cows while milk fat and milk protein content were highest with the J×HF cows (P

The present study was undertaken to examine the effect of cow genetic merit on enteric methane (CH4) emission rate. The study used a data set from 32 respiration calorimeter studies undertaken at this Institute between 1992 and 2010, with all studies involving lactating Holstein-Friesian dairy cows. Cow genetic merit was defined as either profit index (PIN) or profitable lifetime index (PLI), with these two United Kingdom genetic indexes expressing the expected improvement in profit associated with an individual cow, compared with the population average. While PIN is based solely on milk production, PLI includes milk production and a number of other functional traits including health, fertility and longevity. The data set had a large range in PIN (n=736 records, −£30 to +£63) and PLI (n=548 records, −£131 to +£184), days in milk (18 to 354), energy corrected milk yield (16.0 to 45.6 kg/day) and CH4 emission (138 to 598 g/day). The effect of cow genetic merit (PIN or PLI) was evaluated using ANOVA and linear mixed modelling techniques after removing the effects of a number of animal and diet factors. The ANOVA was undertaken by dividing each data set of PIN and PLI into three sub-groups (PIN:£15, PLI:£67) with these being categorised as low, medium and high genetic merit. Within the PIN and PLI data sets there was no significant differences among the three sub-groups in terms of CH4 emission per kg feed intake or per kg energy corrected milk yield, or CH4 energy (CH4-E) output as a proportion of energy intake. Linear regression using the whole PIN and PLI data sets also demonstrated that there was no significant relationship between either PIN or PLI, and CH4 emission per kg of feed intake or CH4-E output as a proportion of energy intake. These results indicate that cow genetic merit (PIN or PLI) has little effect on enteric CH4 emissions as a proportion of feed intake. Instead enteric CH4 production may mainly relate to total feed intake and dietary nutrient composition.

A total of 139 treatment mean data of lactating dairy cows, offered mixed diets of concentrates and silages of grass ( n =33) and maize ( n =5) ad libitum, were obtained from 12 long term feeding experiments (8–10 weeks/period). In these experiments metabolisable energy ( ME ) concentrations in diets were either directly measured or estimated from in vivo digestibility data. Milk energy output ( E l ) was calculated from milk composition and live weight change ( LWC ) was estimated from linear regression. Energy balance ( E g ) and then E l(0) ( E l adjusted to zero E g ) were calculated using different energy contents per unit of LWC and energetic efficiencies as suggested in energy feeding systems [Livest. Prod. Sci. 5 (1978) 331; Ruminant Nutrition—Recommended Allowances and Feed Tables (1989); Nutr. Abstr. Rev. 60 (1990) 729; Energy and Protein Requirements of Ruminants (1993); Feeding Standards for Australian Livestock—Ruminants (1990); Nutrient Requirements of Dairy Cattle (2001)]. The objectives of the present study were to use these treatment mean data to derive the ME requirement for maintenance ( ME m ) and the efficiency of ME use for lactation ( k l ) for dairy cows and then validate these ME m and k l values using published calorimetric data. In the present study, ME intake, milk yield and LWC ranged respectively from 116 to 286 MJ/d, from 10.0 to 41.4 and from −0.405 to 0.825 kg/d. In the various feeding systems the energy content per unit of LWC and the energetic efficiencies are different, thus the mean calculated E l(0) values varied marginally (from 0.707 to 0.721 MJ/kg 0.75 ). Consequently, the ME m and k l values, derived from the linear regression of ME intake against E l(0) , ranged respectively from 0.59 to 0.62 (MJ/kg 0.75 ) and from 0.64 to 0.67. However, a parallel test on the linear relationships between ME intake and E l(0) revealed that the above difference in E l(0) between these systems had no significant effects on either the intercepts or the slopes, indicating that there were no significant differences in ME m or k l values derived from these systems. The mean ME m and k l values across the feeding systems therefore were 0.60 (MJ/kg 0.75 ) and 0.65 respectively and are within the range of ME m or k l values reported recently from the calorimetric data of lactating dairy cows. The ME m and k l values derived from the present production data set were then used to validate calorimetric data of lactating dairy cows (experimental mean, n =49) published since 1976. This demonstrated that the predicted ME requirement was comparable to the actual ME intake.

A set of data from 838 lactating dairy cows, drawn from 12 long-term feeding studies (at least 8 wk/period), was used to evaluate the energy feeding systems for dairy cows currently adopted in Australia, France, The Netherlands, the United Kingdom, and the United States. The animals were offered mixed diets of concentrates, forage [grass silages (n = 33) and corn silages (n = 5)] ad libitum. Data used in the present evaluation were either measured [dry matter (DM) intake, milk production and live weight], measured/estimated [dietary metabolizable energy (ME) concentration] or estimated [milk energy output and live weight change (LWC)]. The mean-square prediction error (MSPE) was used for the evaluation. Total ME intake, milk yields, and LWC varied from 91 to 338 MJ/d, 7.7 to 48.9, and -1.23 to 1.73 kg/d, respectively. Australian and French systems predicted total energy requirement and milk yield relatively well, while British, Dutch and American systems underpredicted total energy requirement by proportionately 0.06, 0.04, and 0.03, respectively; and overpredicted milk yield by 0.09, 0.06, and 0.04. The Agricultural and Food Research Council (AFRC) each produced a relatively larger error of the bias (predicted - actual data) over the total MSPE for ME requirement and milk yield and a relatively smaller error of random than other systems. However, an addition of proportionately 0.05 to the total predicted ME requirement of AFRC, as suggested in this system and currently used in the UK, indicated the prediction accuracy of ME requirement and milk yield is similar to Australian and French systems. Nevertheless, all the systems had a poor prediction of LWC. For each system, the total prediction error (total MSPE) was mainly derived from the line (slope; 0.49 to 0.64 of total MSPE), while less derived from the random (0.20 to 0.48 of total MSPE), indicating a large variation between the predicted and actual LWC existed among individual cows. The residual plots of the residual differences in LWC against predicted LWC revealed that the prediction error was greater with increasing LWC. It is concluded that Australian and French systems have a better prediction of total energy requirement and milk yield than other systems, and LWC is an inappropriate indicator of energy balance in lactating dairy cows.

Three experiments were carried out, each using eight Holstein-Friesian nonpregnant and late lactation cows, to determine the fasting heat production (FHP) of cows receiving forage diets at near ad libitum and to examine the effect of lactation on the utilisation of metabolisable energy (ME). In experiments 1 and 2, energy metabolism data were recorded in open-circuit calorimeter chambers during full lactation, half lactation (by drying off two of the four teats), and dry period (by drying off the remaining two teats), and afterwards the animals were fasted for FHP measurement. In experiment 3 similar data were recorded during full lactation and dry period (by drying off all four teats). The animals were offered dried grass nuts only in experiment 1, dried grass nuts and concentrate (gross energy (GE) ratio of 0.80:0.20) in experiment 2, and grass silage and concentrate (GE ratio of 0.80:0.20) in experiment 3. In each experiment, the feeding level was adjusted, on an individual animal basis, to provide an ME intake approximating to that required for both maintenance and milk production at the commencement of the experiments. This feeding level remained fixed throughout each experiment. The mean FHP data obtained in experiments 1 and 2 were similar, with an overall mean of 0.453 (s.d. = 0.0354) MJ/kg0.75. Total intakes of GE and ME each were similar between full lactation, (half lactation) and dry period within each of the three experiments. Heat production during the dry period was significantly higher than during full lactation in experiment 3 (p < 0.05), but in experiment 1 or 2 the differences were not significant, although the trend was similar. This effect on heat production resulted in a decrease in productive net energy (retained energy) during the dry period in comparison with that (milk energy and retained energy) during lactation. Consequently, in experiment 3 the efficiency of ME utilisation for tissue retention during the dry period was proportionately 0.18 lower than that for lactation (p < 0.05). Similar trends were also obtained in experiments 1 and 2, although none of the differences were significant. The results of the present study indicate that FHP (0.453 MJ/kg0.75) is proportionately 0.41 higher than fasting metabolism (0.321 MJ/kg0.75) predicted from Agricultural Research Council (1980). On average over the three experiments, the efficiency of ME utilisation for tissue retention during the dry period was proportionately 0.07 lower than that for milk secretion during lactation.

Twelve grass silages were offered to sheep as a sole diet at maintenance and to lactating dairy cows ad libitum as mixed silage and concentrates diets (n = 13 diets). Fecal and urinary energy outputs were measured for silages and mixed diets. Digestible energy (DE) and metabolizable energy (ME) concentrations for mixed diets with sheep at maintenance were estimated based on the silage dry matter (DM) proportion obtained in the cattle trials, the silage energy utilization values (methane energy-predicted) determined using sheep, and tabulated concentrate values. A comparison of dietary mean data (n = 13) indicated that concentrations of ME (P < 0.01) and DE (P < 0.001) in mixed diets were significantly lower for cows at production feeding level than for sheep at maintenance. The reductions were proportionately 0.015 and 0.020 with each unit increase in feeding level above maintenance, respectively. These ME and DE data were also used to evaluate the feeding level correction factors previously proposed by Van Es (1975) (ME, 0.018) and Yan et al. (2002) (ME, 0.016; DE, 0.025) using the mean square prediction error technique. The ME correction factor proposed by Yan et al. (2002) had a greater prediction accuracy than that proposed by Van Es (1975) for the prediction of ME concentration in mixed diets offered to dairy cattle at production feeding level.

Although interest in crossbreeding within dairy systems has increased, the role of Jersey crossbred cows within high concentrate input systems has received little attention. This experiment was designed to examine the performance of Holstein-Friesian (HF) and Jersey × Holstein-Friesian (J × HF) cows within a high concentrate input total confinement system (CON) and a medium concentrate input grazing system (GRZ). Eighty spring-calving dairy cows were used in a 2 (cow genotype) × 2 (milk production system) factorial design experiment. The experiment commenced when cows calved and encompassed a full lactation. With GRZ, cows were offered diets containing grass silage and concentrates [70:30 dry matter (DM) ratio] until turnout, grazed grass plus 1.0 kg of concentrate/day during a 199-d grazing period, and grass silage and concentrates (75:25 DM ratio) following rehousing and until drying-off. With CON, cows were confined throughout the lactation and offered diets containing grass silage and concentrates (DM ratio; 40:60, 50:50, 40:40, and 75:25 during d 1 to 100, 101 to 200, 201 to 250, and 251 until drying-off, respectively). Full-lactation concentrate DM intakes were 791 and 2,905 kg/cow for systems GRZ and CON, respectively. Although HF cows had a higher lactation milk yield than J × HF cows, the latter produced milk with a higher fat and protein content, so that solids-corrected milk yield (SCM) was unaffected by genotype. Somatic cell score was higher with the J × HF cows. Throughout lactation, HF cows were on average 37 kg heavier than J × HF cows, whereas the J × HF cows had a higher body condition score. Within each system, food intake did not differ between genotypes, whereas full-lactation yields of milk, fat plus protein, and SCM were higher with CON than with GRZ. A significant genotype × environment interaction was observed for milk yield, and a trend was found for an interaction with SCM. Crossbred cows on CON gained more body condition than HF cows, and overall pregnancy rate was unaffected by either genotype or management system. In summary, milk and SCM yields were higher with CON than with GRZ, whereas genotype had no effect on SCM. However, HF cows exhibited a greater milk yield response and a trend toward a greater SCM yield response with increasing concentrate levels compared with the crossbred cows.

A key concern in farming systems that seek to maximise the grass content of livestock diets is the management of grazing so as to avoid damage to soil structure during periods with high antecedent soil moisture conditions. The aim of this study was to investigate, under the current codes of good agricultural practice, the impact of two contrasting grazing systems on soil structure and nutrient export from grazed grasslands. The two grazing treatments were (1) WinterCalf treatment, involving winter-calving Holstein-Friesian dairy cows, which had a mean ‘start of turnout’ date of 28 March, and were offered 5.0kg concentrate/cow/day throughout a 214-day grazing season; and (2) SpringCalf treatment, involving spring-calving Jersey Holstein-Friesian dairy cows, which had a ‘start of turnout’ date of 17 February, and were offered between 1.0 and 2.0kg concentrate/cow/day throughout a 260-day grazing season. Maximum reliance on grazed grass was a key objective of the latter system. Mean stocking rates during the grazing season were 5.1 and 4.3 cows per hectare with WinterCalf and SpringCalf systems, respectively. The experimental site was divided into four replicated blocks (A, B, C and D), with each block containing two grazing plots (425m and 500m, for the WinterCalf and SpringCalf systems, respectively). In addition to the two grazed treatments, an ungrazed treatment (UG) was established within each of the WinterCalf and SpringCalf treatment plots by fencing off a 12m exclusion sub-plots, which livestock could graze but not tread on. Changes in soil structure were recorded before and after grazing on all plots and with rainfall simulation to investigate the impact of soil structural changes on nutrient export to water. The results of the study demonstrated that over a complete grazing season, under the current code of good grazing practice, nutrient export from the SpringCalf treatment was not significantly greater than that from the WinterCalf treatment. However, results highlighted an increase in nutrient export from the SpringCalf treatment compared to the ungrazed treatment. These increases were as a result of a significant increase in overland flow volume from the SpringCalf treatment compared to the UG treatment. The time taken to initiate overland flow was a key factor in this relationship, with an increase from an average value of 536 seconds for the SpringCalf treatment to 1709 seconds for the Ungrazed treatment. Allowing time for soil to recover from compaction could be an effective mitigation strategy for reducing nutrient export from soil, particularly in areas where soil P is significantly above the agronomic optimum for grass production.

High genetic merit dairy cows were managed on four contrasting grassland based systems of milk production over a three-year period (Ferris et al., 2000). Each of the four systems were designed to incorporate a range of different approaches to increasing nutrient intakes. Key performance data from this study are presented in Table 1. The aim of this paper is to examine the effects of the four milk production systems on dairy cow fertility.

The environmental and economic cost of concentrate protein ingredients is high, with the economic impact being especially important if these are certified free from genetically modified material. While reducing the crude protein (CP) content of the concentrate will reduce feed costs, animal performance is also likely to suffer (Mayne and Gordon, 1985). However it may be possible to maintain performance and yet reduce feed costs by reducing the protein content of the concentrate offered, with a simultaneous increase in concentrate feed levels. The current study was designed to examine the effects on animal performance of adopting this approach, and to quantify the ‘protein sparing’ effect of increasing the quantity of concentrate offered.

The response of grazing dairy cows to level of concentrate supplementation and to the concentration of protein in the supplement remains poorly defined. The response is of particular importance in view of the high costs associated with the concentrate feedstuffs, especially protein ingredients. The current study was designed to examine the response of grazing dairy cows to level of concentrate supplementation and to level of protein in the concentrate.

Traditional options to increase the energy intakes of lactating dairy cows include raising the level of concentrate feeding and/or improving the quality of grass silage. However, each of these options have limited potential to increase total ME intake (Ferris et al., 2000). The inclusion of an additional dietary component, such as whole crop cereal silage or fodder beet constitutes an alternative approach to increase energy intake (Phipps et al., 1995). An experiment was designed to examine the potential of fodder beet to increase the energy intake and performance of lactating dairy cows across a range of levels of concentrate feeding.

In a previous study, spring calving dairy cows exhibited considerable milk yield responses when given access to spring grass for short periods (Sayers and Mayne, 1998). The current study seeks to examine the performance of autumn calving (later lactation) dairy cows when given access to spring grass. In addition, this study seeks to identify if winter management system (forage or concentrate based systems) influences the response achieved.

Meeting the greater nutrient requirements of high genetic merit dairy cows in grassland based systems provides a very real challenge. This study examines the performance of high genetic merit animals, managed on four contrasting grassland based systems of milk production, including both the winter and summer periods.

In a previous short term study, Ferris et al. (1997) demonstrated that similar levels of nutrient intake and animal performance could be obtained by either increasing silage feed value and reducing concentrate feed level, or by reducing silage feed value and increasing concentrate feed level. The principles established in this study were incorporated into this trial to examine two systems of milk production over a full lactation, including both the winter and grazing periods.Forty high genetic merit dairy cows (PTA95 fat + protein = 38.2 kg), in their second or subsequent lactation, were used in a continuous design full lactation study. Animals had a mean calving date of 1 November and were allocated to one of two systems of milk production, HF or HC, within 36 hours of calving. During the winter, animals on system HF were offered a silage with high feed value characteristics, supplemented with 5.5 kg of concentrate (crude protein concentration of 280 g/kg DM) through an out-of-parlour feeding system.

To satisfy the increased nutrient requirements of high genetic merit dairy cows, diets of increased nutrient density and intake potential are required. This can be achieved through increasing the feed value of the forage component of the diet (grass silage or grazed grass) and by increasing concentrate feed levels. However both approaches tend to reduce the fibre concentration of the diet, and the physical effectiveness of the fibre in terms of its ability to stimulate ruminal processes. The incorporation of small quantities of straw into the diet has been suggested as a means of overcoming this problem and hence improving animal performance. This study was undertaken to examine the production response by dairy cows to straw inclusion in high quality grass silage and zero grazed grass diets.

High concentrate feed levels, with a consequent fall in the fibre content of the diet, are increasingly being offered to high genetic merit dairy cows. To improve fibre intake, small quantities of straw are often incorporated into the diet. This study was undertaken to examine the production response of dairy cows, offered a medium quality grass silage based diet together with relatively high levels of concentrate supplementation, to straw inclusion in the diet.Forty high genetic merit dairy cows, 20 multiparious and 20 first lactation animals (PTA95 fat + protein = 41.4 and 39.7 kg respectively) were used in this study. The study consisted of four treatments, arranged in a two period (4 week duration), partially balanced change-over design trial, with first lactation and multiparious animals being blocked separately. The experimental treatments consisted of grass silage mixed with one of four levels of straw, and offered ad libitum in the form of a mixed diet.

Earlier work has indicated low responses in milk yield to supplementation with concentrates during grazing (Leaver, 1985). The substitution effect of concentrates on herbage dry matter intake is high at generous allowances of grass, but is much lower when the allocation of grass is low (Meijs and Hoekstra, 1984). It is therefore expected that the responses in milk yield to supplementation with concentrates during grazing would be greater with a restricted allowance of grass. The aim of the experiment was to measure the milk yield responses of high yielding cows to increased energy and protein supplementation, when offered a restricted allowance of grass.

Supporting the higher milk yields of high genetic merit cows requires an increased supply of nutrients to the animal. In grass silage based production systems this is achieved by either offering additional concentrates or by improving the quality of the silage. However, the relationship between silage quality and the response to concentrate supplementation has not been defined for high merit animals. It might be expected that as level of concentrate supplementation increases, the quality of the silage offered will become less important. This study was undertaken to define the responses of high merit dairy cows to a range of concentrate levels given in addition to two contrasting silage types.

The genetic merit of the UK dairy herd is increasing rapidly and this trend is likely to continue for the forseeable future. With cows of higher milk yield potential and higher nutrient requirements, feeding systems developed in the past with medium genetic merit animals, may now need to be adapted to meet the needs of high genetic merit animals if nutrition x genotype interactions exist. This would be particularly true to increasing proportion of concentrates in grass silage based diets. This study was undertaken to compare the milk yield responses of high and medium merit dairy cows.

The objectives of the present study were to investigate the effects of cow group on energy expenditure and utilization efficiency. Data used were collated from 32 calorimetric chamber experiments undertaken from 1992 to 2010, with 823 observations from lactating Holstein-Friesian (HF) cows and 112 observations from other groups of lactating cows including Norwegian (n=50), Jersey × HF (n=46), and Norwegian × HF (n=16) cows. The metabolizable energy (ME) requirement for maintenance (MEm) for individual cows was calculated from heat production (HP) minus energy losses from inefficiencies of ME use for lactation, energy retention, and pregnancy. The efficiency of ME use for lactation (kl) was obtained from milk energy output adjusted to zero energy balance (El(0)) divided by ME available for production. The effects of cow groups were first evaluated using Norwegian cows against HF crossbred cows (F1 hybrid, Jersey × HF and Norwegian × HF). The results indicated no significant difference between the 2 groups in terms of energy digestibility, ratio of ME intake over gross energy intake, MEm (MJ per kg of metabolic body weight, MJ/kg(0.75)), or kl. Consequently, their data were combined (categorized as non-HF cows) and used to compare with those of HF cows. Again, we detected no significant difference in energy digestibility, ratio of ME intake over gross energy intake, MEm (MJ/kg(0.75)), or kl between non-HF and HF cows. The effects were further evaluated using linear regression to examine whether any significant differences existed between HF and non-HF cows in terms of relationships between ME intake and energetic parameters. With a common constant, no significant difference was observed between the 2 groups of cows in coefficients in each set of relationships between ME intake (MJ/kg(0.75)) and MEm (MJ/kg(0.75)), El(0) (MJ/kg(0.75)), HP (MJ/kg(0.75)), MEm:ME intake, El(0):ME intake, or HP:ME intake. However, MEm values (MJ/kg(0.75)) were positively related to ME intake (MJ/kg(0.75)), irrespective of cow group. We concluded, therefore, that cow groups evaluated in the present study had no significant effects on energy expenditure or energetic efficiency. However, the maintenance energy requirement (MJ/kg(0.75)) was not constant (as adopted in the majority of energy rationing systems across the world) but increased with increasing feed intake.