We used the [15N]glycine single-dose urea end-product technique to measure whole-body protein turnover in six Holstein steers (250 +/- 18 kg). Steers were implanted with Revalor-S and continuously infused abomasally with water (4 L/d) or amino acids (AA; in 4 L/d water) in a crossover experiment (two 14-d periods). The AA infusion contained the following (g/d): lysine (5.3), methionine (3.3), threonine (3.2), tryptophan (1.0), histidine (2.1), and arginine (5.5). Steers were fed a diet containing 85% rolled corn, 10% prairie hay, and 1.1% urea (DM basis) at 2.16% of body weight. Nitrogen retention tended (P = .15) to increase with AA infusion, from 27.9 to 32.9 g N/d. Amino acid infusion numerically increased whole-body protein turnover from 168.6 to 183.2 g N/d, protein synthesis from 152.6 to 169.3 g N/ d, and protein degradation from 124.7 to 136.4 g N/d. Enhanced protein accretion may have resulted from a larger increase in protein synthesis than in degradation. The tendency for increased N retention is interpreted to suggest that the implanted, lightweight Holstein steers fed a corn-urea diet in our study were able to respond to AA supplementation, suggesting that at least one of the infused AA was limiting in the basal diet. Protein turnover data suggest that cattle, like other animals, may increase protein synthesis and protein degradation in response to supplementation with limiting AA. The [15N]glycine single-dose urea end-product technique for measuring whole-body protein turnover in cattle may be useful.
Five ruminally and duodenally cannulated Holstein steers (305 kg) were used in a switchback experiment with three periods to evaluate two experimental treatments: a basal diet with or without 45 ppm of lasalocid. The basal diet contained approximately 43% rolled corn, 45% alfalfa hay, and 10% soybean meal (DM basis). Lasalocid did not affect feed intake or ruminal digestion of OM and NDF. Ruminal digestion of ADF tended to increase with supplemental lasalocid. Total tract digestion of OM, NDF, ADF, and N and intestinal flow of amino acids were not affected by lasalocid. Also, the ratio of microbial to nonmicrobial N fractions at the duodenum remained unchanged. Ruminal pH and concentrations of NH3, VFA, peptides, and amino acids were not affected by lasalocid. Ruminal protease activity decreased with supplemental lasalocid, but this decrease was not reflected in other variables, such as ruminal concentrations of peptides and amino acids. Ruminal deaminase activity remained unchanged. Thus, we concluded that dietary lasalocid did not alter ruminal protein degradation or postruminal flow of amino acids.
A 120-d finishing study utilizing 318 heifers (342 kg initial BW) was conducted to examine effects of ruminally protected choline (RPC) in diets containing graded concentrations of tallow. Heifers were blocked according to previous nutrition (full-fed or limit-fed) and allotted to 24 pens containing 11 to 15 heifers. Two pens, one within each block, were assigned to each of 12 factorially arranged treatments including dietary tallow (0, 2, or 4%) and supplemental RPC (0, 20, 40, or 60 g of product daily, estimated to supply 0, 5, 10, or 15 g/d choline postruminally). Heifers were implanted with Revalor-H and fed a finishing diet based on steam-flaked and dry-rolled corn (12.5% CP, 8% alfalfa on DM basis). Dry matter intake decreased (P < 0.10) by 5.4% when tallow was increased from 0 to 4% but was not affected by RPC. Heifers receiving 4% tallow had 7.3% lower gains than those receiving none (P < 0.10). Supplementation of RPC increased (P < 0.10) ADG, with 20 g/d resulting in an 8.6% increase. Similarly, gain efficiency improved (P < 0.10) by 7.6% with addition of 20 g/d RPC. Yield grade and kidney, pelvic, and heart fat both increased linearly (P < 0.10) with fat supplementation. The percentage of carcasses grading USDA Choice was not affected by intermediate levels of RPC but decreased with the highest level (60 g/d). Dressing percentage, hot carcass weight, marbling, and 12th-rib fat thickness were not affected significantly by either tallow or RPC. On d 90, jugular blood was collected from all heifers at 2 h postfeeding. Plasma urea and serum insulin concentrations were not affected by either tallow or RPC. Dietary tallow linearly increased (P < 0.10) NEFA, cholesterol, triglyceride, and total amino acid concentrations. Choline supplementation led to quadratic responses for total amino acids (P < 0.10), with concentrations being greatest for intermediate levels of RPC. Moderate levels of supplemental RPC improved growth performance of finishing cattle without negatively affecting carcass characteristics. Optimum performance was achieved with 20 g of product daily.
In Exp. 1, six steers (254 kg) were used in a 6 x 4 incomplete Latin square to determine the effects of solvent-extracted soybean meal alone or in combination with rumen-protected methionine and lysine on N balance in steers limit-fed a high-corn diet to gain 1.1 kg/d. The basal diet contained (DM basis) 80% rolled corn, 15% alfalfa, and .9% urea (13.9% CP), and 2 or 4% soybean meal replaced corn to give CP concentrations of 14.8 and 15.6%, respectively. Each diet was fed with and without 5 g/d of Smartamine-ML (.75 and 2.0 g of rumen-protected methionine and lysine, respectively). Nitrogen retention increased linearly (P = .09) with level of soybean meal. Rumen-protected methionine and lysine had no effect on N balance. In Exp. 2, seven steers (233 kg) were used in a 7 x 4 incomplete Latin square experiment to investigate optimal levels and sources of CP for steers limit-fed to gain 1 kg/d. Treatments included a negative-control diet (urea; 11.7% CP) and six diets containing either 13.5, 15.4, or 17.2% CP with either solvent-extracted or expeller-processed soybean meal. Diets provided 75, 87.5, 100, or 112.5% of estimated CP requirement for a gain of 1 kg/d. The basal diet contained 83% rolled corn, 15% alfalfa, and .2% urea. Nitrogen retention increased linearly (P = .006) with soybean meal addition, and no differences were observed between CP sources. The CP system underpredicted the protein requirements of limit-fed steers under our conditions.
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