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Objective Our objective was to examine the influence of timing of liver abscess control and early rumen-protected AA supplementation on cattle growth performance (Exp. 1) and digestive function (Exp. 2), and related responses on liver abscess incidence and AA supply to the small intestine of feedlot steers. Materials and Methods In Exp. 1, a total of 200 calves were used in a 321-d experiment. Four strategies for liver abscess control using virginiamycin (VM; 16 mg of VM/kg of diet DM, Phibro Animal Health) were used: (1) 0% VM; (2) VM supplementation during the initial 168 d; (3) VM supplementation during the final 153 d; and (4) VM supplemented continuously for 321 d. During the initial 168-d feeding period, 2 levels of rumen-protected Met and Lys (Smartamine ML: SML; 0 vs. 0.6% DM basis, Adisseo) were supplemented across VM treatments in a 2 × 4 factorial arrangement. In Exp. 2, 4 Holstein steers (382 ± 23 kg) with cannulas in the rumen and proximal duodenum were used in a 4 × 4 Latin square design to evaluate treatment effects on digestion. Results and Discussion Cattle growth performance was not affected (P ≥ 0.18) by VM supplementation strategy. Moreover, VM supplementation did not affect (P ≥ 0.10) carcass characteristics. Liver abscess incidence was low (4%) and not affected (P ≥ 0.62) by supplementation strategy. Supplemental VM did not affect characteristics of ruminal and total-tract digestion (P ≥ 0.27). Supplementation of SML during the first 168 d decreased (P = 0.02) DMI during the last 153 d of the experiment, and tended to decrease (P = 0.07) overall (321-d) DMI. Smartamine ML supplementation did not affect (P ≥ 0.11) ADG, G:F, carcass characteristics, percentage of abscessed liver, or characteristics of ruminal and total-tract digestion. Implications and Applications Timing of VM supplementation did not influence cattle growth performance or incidence of liver abscess, which was low (4%) regardless of VM treatment. Protected Met and Lys supplementation increased Met or Lys supply to the small intestine. However, MP and AA supplies of the basal diet, alone, were apparently adequate to achieve expected growth performance.

Our objective was to evaluate the interaction of supplemental essential oils (EO) and monensin (MON) on performance and digestion. Two levels of EO (0 vs 0.011% Xtract®) and two levels of MON (0 vs 33 mg/kg) were evaluated in a 2 × 2 factorial arrangement. Treatment effects on 216-d growth performance were evaluated in 96 Holstein steers. Supplemental MON enhanced (P ≤ 0.05) ADG, DMI and gain efficiency. Without EO, MON improved (P

Objectives This study was conducted to examine the effect of urea fermentation potential (UFP) on digestive function, BUN, growth performance, and dietary energetics, in steers fed a finishing diet based on steam-flaked corn containing 15% dried distillers grains with solubles. Materials and Methods Four UFP levels were evaluated (−0.30, 0.10, 0.50, 0.90). The UFP levels were manipulated by dietary urea inclusion level of 1.20, 0.80, 0.40 and 0%, respectively (DM basis). Results and Discussion Dietary UFP did not affect (P > 0.10) ruminal digestion of feed N. Ruminal starch digestion decreased with increasing UFP (linear effect, P = 0.02). Ruminal digestion of starch and OM tended to be maximal (quadratic effect, P ≤ 0.07) at dietary UFP of 0.1%. Increasing UFP decreased flow to duodenum of nonammonia-N (linear effect, P = 0.04), and microbial N (linear effect, P = 0.05). Duodenal flow of feed N was not affected (P = 0.18) by dietary UFP. Increasing dietary UFP decreased total-tract digestion of OM (linear effect, P = 0.02), starch (linear effect, P = 0.03), and digestible energy (linear effect, P = 0.03). Increasing dietary UFP decreased ruminal pH (linear effect, P = 0.01) 1 h after feeding. Thereafter, the influence of dietary UFP on ruminal pH was not appreciable. Increasing dietary UFP decreased ruminal ammonia-N 1 h after feeding (linear effect, P Implications and Applications Even when finishing diet formulations otherwise exceed MP requirements, dietary UFP level remains a relevant factor to optimize dietary energy use. Dietary UFP levels greater than 0.50% could negatively affect digestion, gain efficiency, and dietary NE use.

Objectives The objective of this research was to evaluate the influence of rumen-protected methionine and lysine on growth performance of calf-fed Holstein steers during the initial 84-d feedlot phase and relate responses to measures of AA supply to the small intestine. Material and Methods Seventy-two Holstein steer calves (128 ± 9 kg) were blocked by weight and assigned to 12 pens (6 steers per pen). Steers were fed a diet based on steam-flaked corn formulated to provide 105% of theoretical MP requirements plus 0, 0.4, and 0.8% Smartamine ML (Adisseo, Alpharetta, GA; 55% l -lysine-HCl and 15% dl -methionine coated with a pH-sensitive polymer). Six Holstein steers (143 ± 13 kg) with cannulas in rumen and proximal duodenum were used in a crossover design to evaluate characteristics of digestion and AA supply to the intestine of the basal diet plus 0 versus 0.8% Smartamine ML. Results and Discussion Methionine and lysine supplementation did not affect DMI (P = 0.27) but enhanced ADG, gain efficiency (linear effect, P = 0.03), and estimated dietary NE (linear effect, P = 0.04). Amino acid supplementation increased (P Implications and Applications Supplementation with rumen-protected methionine and lysine may enhance gain efficiency and dietary energetics of growing Holstein calves during the early growing phase. The magnitude of the response is likely dependent on the otherwise general adequacy of nonspecific MP.

The objectives of this study were to evaluate the influence of level and source of supplemental Zn on growth performance and dietary energetics in calf-fed Holstein steers. A total of 168 steers (BW = 133 ± 7 kg) were used in a 336-d study. Treatments consisted of diets based on steam-flaked corn supplemented with (DM basis) (1) 20 mg of Zn/kg of DM as ZnSO4; (2) 10 mg of Zn/kg of DM as ZnSO4 plus 10 mg of Zn/kg of DM as Zn betaine; (3) 20 mg of Zn/kg of DM as Zn betaine; or (4) 40 mg of Zn/kg of DM as ZnSO4. Overall (d 1 to 336) there were no treatment effects (P > 0.10) on carcass-adjusted ADG, gain efficiency, and estimated dietary NE. At the 20 mg/kg level of Zn supplementation, the proportion of supplemental Zn provided as chelate did not influence overall ADG, gain efficiency, or dietary NE (P > 0.10). The proportion of Zn supplemented as chelate did not affect (P > 0.10) carcass weight, KPH, LM area, or marbling score. However, DP was greater (quadratic effect, P = 0.01) for steers receiving the 50:50 blend of ZnSO4 and Zn betaine. Carcass yield grade improved (linear component, P = 0.03) with proportion of supplemental Zn as chelate. We conclude that at a 20 mg/kg (DM basis) level of supplementation, Zn source does not affect growth performance of calf-fed Holstein steers. Increasing level of Zn supplementation (as ZnSO4) from 20 to 40 mg/kg will not further enhance overall performance responses.

Two experiments were conducted to examine the influence of protein and virginiamycin (VM) supplementation on feedlot growth performance, digestion, and metabolizable AA (MAA) supply of calf-fed Holstein steers. Growth performance and dietary energetics were evaluated in 120 Holstein steers (127 ± 9 kg). During the initial 112-d feeding period, a steam-flaked corn-based diet was balanced to meet either 100% (MAB) or 87% (UREA) of MAA requirements. Diets were supplemented with or without 22.5 mg/kg VM in a 2 × 2 factorial arrangement. Subsequently (d 112 to 308), all steers received the UREA diet with or without VM. During the initial 112-d, MAB increased ADG, G:F, and dietary NE (0.01). Thereafter, when all steers received the UREA diet, ADG, G:F, and dietary NE were not different (0.10) across initial supplementation treatments. Overall (d 1 to 308), MAB did not affect ADG (0.10) but enhanced G:F efficiency ( = 0.03) and dietary NE ( = 0.05). During the initial 112-d period and through the remainder of the experiment, VM increased G:F (0.01) and dietary NE (0.01). Four Holstein steers (146 ± 4 kg) with cannulas in the rumen and proximal duodenum were used in a 4 × 4 Latin square design to evaluate initial 112-d treatment effects on digestive function. There were no treatment effects (0.10) on ruminal digestion of OM, NDF, starch, microbial efficiency, or total tract digestion of OM and NDF. The MAB increased indispensable AA flow to the small intestine (0.01) and total tract digestion of N (0.01) and starch ( = 0.04). Observed AA supply to small intestine was in agreement with expected supply ( = 0.96). Virginiamycin decreased ( = 0.04) nonammonia N flow to the small intestine and did not affect (0.10) total tract N digestion. Extrapolating from AA supplies in the metabolism study, MAB satisfied indispensable AA requirements during the initial 112-d period, whereas the UREA diet met 73.5% and 79.2% of methionine and lysine requirements, respectively. During the subsequent periods (d 112 to 308) indispensable AA supplies exceeded theoretical requirements. We conclude that enhancements in energy utilization when diets are balanced to meet MAA requirements of calf-fed Holstein steers during the initial 112-d feedlot period remain appreciable throughout time on feed. Virginiamycin enhanced efficiency of energy utilization throughout the feedlot growing-finishing period.

Two experiments were conducted to examine the influence of kaolinite clay supplementation (0%, 1%, or 2% diet dry matter [DM] basis) on characteristics of digestion (Trial 1) and growth performance (Trial 2) in calf-fed Holstein steers fed a finishing diet. In Trial 1, 6 Holstein steers (539±15 kg) with ruminal and duodenal cannulas were used to evaluate treatment effects on characteristics of digestion. Kaolinite clay supplementation decreased total tract DM digestion (linear effect, p0.10) on average daily gain (ADG) and gain efficiency (ADG/dry matter intake). Kaolinite supplementation tended (linear effect, p = 0.08) to increase dietary net energy (NE) during the initial 112-d period. However, the overall (308-d) effect of supplementation dietary NE was not appreciable (p>0.20). However, due to the inertness of kaolinite, itself, the ratio of observed-to-expected dietary NE increased with kaolinite supplementation. This effect was more pronounced (linear effect, p 0.03) during the initial 224 d of the study. Overall (308 d), kaolinite supplementation tended to increase (linear effect, p = 0.07) dietary NE by 3% over expected. Kaolinite supplementation did not affect carcass weight, yield grade, longissimus area, kidney, pelvic and heart fat, and quality grade, but decreased (linear effect, p = 0.01) dressing percentage. It is concluded that kaolinite supplementation up to 2% of diet DM may enhance energetic efficiency of calf-fed Holstein steers in a manner independent of changes in characteristics of ruminal and total tract digestion.

Two trials were conducted to evaluate the influence of virginiamycin (26 mg/kg) and monensin (34 mg/kg) supplementation of a steam-flaked corn-based finishing diet containing 15% distiller dried grains plus solubles (DDGS) on growth performance and digestive function. In Trial 1, 135 steers (257 ± 21 kg) were used to evaluate treatment effects on 145-d growth performance. There were no treatment effects (P > 0.20) on dry matter intake (DMI). Antibiotic supplementation tended to increase average daily gain (ADG; 7%, P = 0.07); increased gain efficiency (11%, P < 0.01) and estimated dietary net energy (NE; 10%, P < 0.01). Carcass marbling score was greater (12%, P = 0.04) for virginiamycin vs. monensin. In Trial 2, three Holstein steers (300 ± 7 kg) were used in a 3 × 3 Latin square to evaluate treatment effects on digestion. Both monensin and virginiamycin depressed ruminal digestion of organic matter (OM; 6%, P < 0.01) and feed nitrogen (N, 15%, P = 0.03) and microbial protein synthesis (15%, P = 0.03). T...

Eight Holstein steers (216±48 kg body weight) fitted with ruminal and duodenal cannulas were used to evaluate effects of wheat straw processing (ground vs pelleted) at two straw inclusion rates (7% and 14%; dry matter basis) in dry rolled or steam-flaked corn-based finishing diets on characteristics of digestion. The experimental design was a split plot consisting of two simultaneous 4×4 Latin squares. Increasing straw level reduced ruminal (p0.10) by wheat straw level. Likewise, straw level did not influence ruminal acetate and propionate molar proportions or estimated methane production (p>0.10). Pelleting straw did not affect (p≥0.48) ruminal digestion of OM, NDF, and starch, or microbial efficiency. Ruminal feed N digestion was greater (7.4%; p = 0.02) for ground than for pelleted wheat straw diets. Although ruminal starch digestion was not affected by straw processing, post-ruminal (p0.14) by corn processing. However, microbial N flow to the small intestine and ruminal N efficiency (non-ammonia N flow to the small intestine/N intake) were greater (p

Two corn hybrid varieties [normal yellow dent corn (NC) and high oil corn (HOC)] and two levels of supplemental fat (0 vs. 30 g yellow grease/kg diet, YG) were evaluated in two 4 × 4 Latin square experiments involving eight Holstein steers with ruminal and duodenal cannulas. In Experiment 1, corn varieties were processed by dry rolling. In Experiment 2, they were processed by steam flaking. Non fat-supplemented NC and HOC diets contained 2.7 and 4.8 g of fatty acids (FA)/kg diet, respectively. Fat supplementation increased FA content to 4.9 and 7.1 g FA/kg diet for NC and HOC diets, respectively. With dry rolled corn varieties (Experiment 1), there were no treatment interactions on ruminal, postruminal and total tract digestion of OM, starch and N. There were interactions (P≤0.05) between corn variety and supplemental YG on ruminal aNDFom and postruminal FA digestion. With dry rolled NC, supplemental YG decreased ruminal aNDFom digestion, and postruminal FA digestion. Whereas, with dry rolled HOC, supplemental YG increased ruminal aNDFom digestion, but did not affect postruminal FA digestion. YG decreased (P 0.05) total tract digestion of OM, starch, aNDFom and N, but increased (2.5%, P≤0.05) dietary DE. Ruminal (P 0.05) on ruminal, postruminal and total tract digestion of OM, aNDFom, starch and N. There were interactions (P≤0.05) between corn variety and supplemental YG on intestinal FA digestion. With the NC diets, YG decreased FA digestion; whereas with the HOC diets, supplemental YG increased FA digestion. Supplemental YG did not affect (P>0.05) ruminal digestion, postruminal, or total tract digestion of OM, aNDFom, starch, N. Diet DE value was greater (3.1%, P≤0.05) for YG supplemented diets. There were no corn variety effects (P>0.05) on ruminal, postruminal and total tract digestion of OM, aNDFom, starch and N. Postruminal FA digestion was greater (6.7%, P

This study evaluated the effects of exogenous fibrolytic enzymes on intake, ruminal fermentation, and duodenal flow and digestion of nutrients in steers fed diets with whole Pima cottonseed (WPC) or cracked Pima cottonseed (CPC). Four Holstein steers (167±5 kg body weight) fitted with ruminal and duodenal cannulas with a 4×4 Latin square in a 2×2 factorial arrangement of treatments (WPC or CPC; 0 or 15 g enzyme) were used. Steers were fed wheat flaked-based diets with 9.95% of WPC or CPC as dry matter basis. Enzymes increased molar proportion of acetate in steer fed WPC, but in those fed CPC acetate decreased. Feeding enzymes increased duodenal flow of organic matter in steers fed WPC, but not in those fed CPC. Also, enzymes increased duodenal flow of N, non-ammonia N, and intestinal digestion of N in steers fed WPC and CPC.

Montano, M.F., Loya-Olguin, J.L., Plascencia, A., Salinas Chavira, J. and Zinn, R.A. 2010. Effect of variation in supplement inclusion rate and laidlomycin propionate on growth performance of feedlot cattle. J. Appl. Anim. Res., 37: 83–87. One hundred twelve yearling steers (364±3.4 kg) were used in a 123-d finishing trial to evaluate the interaction of laidlomycin propionate and variation in supplement inclusion rate on growth performance and carcass characteristics. Two levels of laidlomycin propionate (0 vs. 10 mg/kg) and two levels of variation in daily supplement (12 g urea, 18 g limestone, 2 g Mg02 and 5 g trace mineralized salt) inclusion rate (O vs 20% coefficient of variation) were evaluated in a 2x2 factorial arrangement. Variation was produced by altering the inclusion rate of the supplement portion of the diet to achieve a 20% CV in supplement concentration of the complete mixed, as follows: on d 1 and 5 of each week the diet contained 125% of the specified amount of supplement; on d ...

Plascencia, A., Alvarez, E.G., Montanto, M., Salinas-Chavira, J. and Zinn, R.A. 2009. Effects of dietary calcium levels on growth-performance and digestive function in cattle fed a high-fat finishing diet. J. Appl. Anim. Res., 36: 179–184. A growth performance and digestion trial was conducted to evaluate the influence of dietary calcium level on the feeding value of a high-fat (11%) finishing diet. Treatments consisted of three levels of dietary Ca: 0.50, 0.70 and 0.90% (DM basis). Ca source was limestone and yellow grease was included as supplemental fat. There were no treatment effects (P>0.20) on DMI. Increasing dietary Ca level enhanced ADG (P 0.10) on total tract digestion of OM, ...

Four Holstein steers with ruminal and duodenal cannulas were used in a 4 × 4 Latin square design to examine the effect of daily intake of 0, 2, 4 or 6 g/steer of standardized plant extract containing a mixture of quaternary benzophenanthridine alkaloids and protopine alkaloids (QBA+PA) on the characteristics of ruminal fermentation and characteristics of digestion. The basal diet consisted of a steam-flaked corn-based finishing diet that contained 62% corn and 12% sudangrass hay and the rest of diet was composed of mainly dried distillers grains, molasses, fat, and minerals. The source of QBA+PA used was Sangrovit-RS (Phytobiotics Futterzusatzstoffe GmbH, Eltville, Germany) and supplementation levels of 2, 4, and 6 g Sangrovit-RS∙steer∙d, which represented a net daily ingestion of approximately 6, 12, and 18 mg of QBA+PA compounds, respectively. Inclusion of QBA+PA linearly increased ( = 0.04) flow to the duodenum of nonammonia N and linearly decreased ( < 0.01) duodenal flows of ammonia N. Ruminal microbial efficiency (duodenal microbial N; g/kg OM fermented in the rumen) and protein efficiency (duodenal nonammonia N; g/g N intake) were increased ( < 0.05) as the level of QBA+PA increased. There were no effects of QBA+PA supplementation on ruminal, postruminal, and total tract digestion of OM, starch, and NDF, but postruminal and total tract digestion of N increased ( < 0.01) as the level of QBA+PA increased. Digestible energy of the diet tended to increase (linear affect, = 0.09) with QBA+PA supplementation. Ruminal pH and total VFA molar concentrations were not different between treatments. Ruminal NH-N concentration linearly decreased ( = 0.02) with QBA+PA supplementation. Ruminal molar proportion of acetate increased ( = 0.04) as the supplementation level of QBA+PA increased. It is concluded that QBA+PA supplementation enhances efficiency of N utilization in feedlot steers fed a steam-flaked corn-based finishing diet. This effect was due, in part, to enhanced ruminal microbial efficiency, decreased ruminal degradation of dietary nonammonia N, and enhanced postruminal N digestion.

Two trials were conducted to evaluate the interaction of the maceration process and surfactant (Tween 80) supplementation on feeding value of rice straw. Treatments were steam-flaked, corn-based diets containing 14% forage (DM basis), which was 1) Sudangrass hay; 2) ground rice straw; 3) ground rice straw plus 0.22% Tween 80; 4) macerated rice straw; and 5) macerated rice straw plus 0.22% Tween 80. In the maceration process, rice straw was passed through 2 sequentially placed pairs of corrugated rolls set at zero tolerance under a ram pressure of 62,050 millibars, similar to a conventional grain roller mill, except that the opposing rolls operated at different speeds (12 and 14 rpm, respectively). Sudangrass hay and rice straw (native and macerated) were ground through a 2.6-cm screen before incorporation into complete mixed diets. In trial 1, 125 Holstein steers (292 +/- 1.7 kg of BW) were used in a 188-d evaluation of the treatment effects on growth performance and carcass characteristics. In trial 2, 5 Holstein steers (224 +/- 3.5 kg of BW) with cannulas in the rumen and proximal duodenum were used in a 5 x 5 Latin square design to evaluate the treatment effects on digestion. There were no interactions between maceration and surfactant on growth or carcass characteristics. Tween 80 did not influence the feeding value of rice straw. Compared with grinding alone, maceration of rice straw increased the carcass-adjusted ADG (6%, P < 0.10), G:F (6%, P < 0.05), and dietary NE (5%, P < 0.05); DMI was similar across treatments. Assuming NE(m) and NE(g) of Sudangrass hay are 1.18 and 0.62 Mcal/kg, the NE(m) and NE(g) were 0.61 and 0.13 Mcal/kg for ground rice straw and 1.21 and 0.65 Mcal/kg for macerated rice straw. There were no treatment interactions on characteristics of digestion. Tween 80 did not influence ruminal or total tract digestion of OM, starch, NDF, or N. Compared with grinding alone, maceration of rice straw increased ruminal digestion of OM (7.7%, P < 0.10) and NDF (30.8%, P < 0.05), and total tract digestion of OM (2.3%, P < 0.10), NDF (21.1%, P < 0.01), and N (3.7%, P < 0.05). Total tract digestion of OM, NDF, starch, and N for the Sudangrass diet corresponded closely with that of the macerated rice straw diets. Maceration increases the feeding value of rice straw to a level similar to that of good-quality (flag stage of maturity) Sudangrass hay, which is attributable to increased OM and NDF digestion. Effects of surfactant supplementation on growth performance and digestion are not appreciable.

One hundred eight Holstein steer calves (114 ± 8 kg) were used to evaluate effects of phase feeding metabolizable amino acids (MAA) on growth performance and carcass characteristics. Three feeding strategies were evaluated: 1) control, single-phase feeding (steers were fed a single urea-based growing-finishing diet that meets the average MAA requirements for the overall feeding period); 2) 2-phase feeding (steers were fed a diet formulated to meet the average MAA requirements for the first 112 d on feed, and thereafter finished on the control urea-based diet); and 3) 3-phase feeding (2 diets were formulated to meet average MAA requirements during the first and second 56-d feedlot feeding periods, and thereafter cattle were finished on the control urea-based diet). Two-phase feeding and 3-phase feeding management strategies produced similar (P > 0.20) growth performance. Multiple-phase feeding increased ADG (18%, P 0.20) on growth performance; however, multiple-phase feeding increased overall (351-d) ADG (6.3%, P 0.20) of 3-phase vs. 2-phase feeding strategies on carcass characteristics. Multiple-phase feeding increased hot carcass weight (5.2%, P g was similar (96%) to that expected for the 2- and 3-phase programs during the first 112 d, but was only 83% of expected for the single-phase program. It was concluded that a 2-phase feeding program was needed to improve overall ADG, G:F, and energetic efficiency of calf-fed (placed in the feedlot as lightweight calf) Holstein steers compared with a conventional single-phase system. At a minimum, a 2-phase system would need to separately consider MAA requirements from feedlot placement to 280 kg BW (phase 1) and from 280 kg BW to harvest (phase 2). Methionine, Lys, and His appear to be the first limiting amino acids during the first feeding phase. During the second phase (280 kg to harvest), supply of all MAA acids will exceed requirements, even where urea is the sole source of supplemental N. Maximizing growth rate during the first feeding phase may enhance carcass dressing percent and LM area.

Forty-eight crossbred heifers (378.1±18 kg) were used in a 56-d feeding trial (four pens per treatment in a randomised complete block design) to evaluate the influence of ionophore supplementation on growth performance, dietary energetics and carcass characteristics in finishing cattle during a period of heat stress. Heifers were fed a diet based on steam-flaked corn (2.22 Mcal NEm/kg) with and without an ionophore. Treatments were: i) control, no ionophore; ii) 30 mg/kg monensin sodium (RUM30); iii) 20 mg/kg lasalocid sodium (BOV20), and iv) 30 mg/kg lasalocid sodium (BOV30). Both dry matter intake (DMI) and climatic variables were measured daily and the temperature humidity index (THI) was estimated. The maximum THI during the study averaged 93, while the minimum was 70 (THI average = 79.2±2.3). Compared to controls, monensin supplementation did not influence average daily gain, the estimated NE value of the diet, or observed-to-expected DMI, but tended (p = 0.07) to increase (4.8%) gain to feed. Compared to controls, the group fed BOV30 increased (p≤0.03) daily gain (11.8%), gain to feed (8.3%), net energy of the diet (5%), and observed-to-expected DMI (5.2%). Daily weight gain was greater (7.6%, p = 0.05) for heifers fed BOV30 than for heifers fed MON30. Otherwise, differences between the two treatments in DMI, gain to feed, and dietary NE were not statistically significant (p>0.11). Plotting weekly intakes versus THI, observed intake of controls was greater (p0.05) in the ionophores group (CV = 1.7%) than in the control group (CV = 4.5%). Inclusion of ionophores in the diet resulted in relatively minor changes in carcass characteristics. It is concluded that ionophore supplementation did not exacerbate the decline of DM intake in heat-stressed cattle fed a high-energy finishing diet; on the contrary, it stabilised feed intake and favoured feed efficiency. Ionophore supplementation reduced estimated maintenance coefficients around 10% in finishing cattle during a period of heat stress. This effect was greatest for heifers supplemented with 30 mg lasalocid/kg of diet.

We used 216 Holstein steers (151 kg) in a 262-d trial to evaluate the influence of dietary magnesium level (.19, .25, and .32%) and laidlomycin propionate (LP; 0 vs 11 ppm, air-dry basis) on growth performance and NE value of the diet. During the initial 112 d of the trial, LP increased (P.01) ADG (6.3%) and feed efficiency (4.2%). From d 112 until slaughter, LP increased (P.05) ADG (9.7%) and feed efficiency (4.5%). Across the 262-d feeding period, LP supplementation enhanced (P.01) ADG (8.9%) and feed efficiency (6.3%). There was an interaction (P.05) between dietary Mg and LP on NE value of the diet. The enhancement in NE value of the diets owing to LP with .19, .25, and .32% dietary Mg were .5, 3.0, and 5.9%, respectively. Six Holstein steers (302 kg) were used in a 6 x 6 Latin square experiment to evaluate treatment effects on characteristics of ruminal and total tract digestion. There were no treatment interactions (P.10) on site and extent of digestion of OM, starch, and N. Supplemental Mg increased (quadratic effect, P.10) ruminal OM digestion. Neither LP nor dietary Mg level affected (P.10) ruminal digestion of starch and feed N. Supplemental LP decreased (15%, P.05) ruminal microbial efficiency. Total tract digestion of OM and N increased (linear effect, P.01) with increasing dietary Mg level. There were interactions between LP and dietary Mg level on ruminal soluble-Mg concentration (linear effect, P.01) and Mg absorption (quadratic effect, P.05). Apparent total tract Mg digestion increased owing to LP (P.01) and dietary Mg level (linear effect, P.01). There were no treatment effects (P.10) on ruminal pH. Dietary Mg level did not influence (P.10) ruminal VFA concentrations or molar proportions. Supplemental LP increased (14%; P.10) total ruminal VFA concentration but did not affect (P.10) VFA molar proportions. We conclude that LP will increase daily weight gain and feed efficiency of calf-fed Holstein steers and that this response may be enhanced by increasing dietary Mg level.

Two trials were conducted to evaluate the influence of supplemental urea withdrawal on characteristics of digestion (Trial 1) and growth performance (Trial 2) of feedlot cattle during the last 40 days on feed. Treatments consisted of a steam-flaked corn-based finishing diet supplemented with urea to provide urea fermentation potential (UFP) of 0, 0.6, and 1.2%. In Trial 1, six Holstein steers (160 ± 10 kg) with cannulas in the rumen and proximal duodenum were used in a replicated 3 × 3 Latin square experiment. Decreasing supplemental urea decreased (linear effect, P ≤ 0.05) ruminal OM digestion. This effect was mediated by decreases (linear effect, P ≤ 0.05) in ruminal digestibility of NDF and N. Passage of non-ammonia and microbial N (MN) to the small intestine decreased (linear effect, P = 0.04) with decreasing dietary urea level. Total tract digestion of OM (linear effect, P = 0.06), NDF (linear effect, P = 0.07), N (linear effect, P = 0.04) and dietary DE (linear effect, P = 0.05) decreased with decreasing urea level. Treatment effects on total tract starch digestion, although numerically small, likewise tended (linear effect, P = 0.11) to decrease with decreasing urea level. Decreased fiber digestion accounted for 51% of the variation in OM digestion. Ruminal pH was not affected by treatments averaging 5.82. Decreasing urea level decreased (linear effect, P ≤ 0.05) ruminal N-NH and blood urea nitrogen. In Trial 2, 90 crossbred steers (468 kg ± 8), were used in a 40 d feeding trial (5 steers/pen, 6 pens/ treatment) to evaluate treatment effects on final-phase growth performance. Decreasing urea level did not affect DMI, but decreased (linear effect, P ≤ 0.03) ADG, gain efficiency, and dietary NE. It is concluded that in addition to effects on metabolizable amino acid flow to the small intestine, depriving cattle of otherwise ruminally degradable N (RDP) during the late finishing phase may negatively impact site and extent of digestion of OM, depressing ADG, gain efficiency, and dietary NE.