Effects of ammonia load on methionine utilization by growing steers

Seven ruminally cannulated Holstein steers (194 +/- 16 kg) housed in metabolism crates were used in a 6 x 6 Latin square, with one additional steer, to study effects of ruminal ammonia load on methionine (Met) use. All steers received a diet based on soybean hulls (2.6 kg DM/d), ruminal infusions of 200 g/d of acetate, 200 g/d of propionate, and 50 g/d of butyrate, as well as abomasal infusion of 300 g/d of glucose to provide energy without increasing microbial protein supply, and abomasal infusions of a mixture (248 g/d) of all essential AA except Met. Treatments were arranged as a 3 x 2 factorial and included urea (0, 40, or 80 g/d) infused ruminally to supply metabolic ammonia loads and Met (2 or 5 g/d) infused abomasally. Supplementation with the greater amount of Met decreased (P < 0.05) urinary N excretion from 68.8 to 64.8 g/d and increased (P < 0.05) retained N from 22.0 to 27.5 g/d. Urea infusions linearly increased (P < 0.05) urinary N excretions, plasma urea concentrations, and urinary urea excretions, but retained N was not affected. The efficiency of deposition of supplemental Met, calculated by assuming that Met deposition is 2.0% of protein deposition (6.25 x retained N), ranged between 18 and 27% when steers received 0 or 80 g/d of urea, respectively. There were no (P > or = 0.40) effects of treatments on serum insulin or IGF-I concentrations. In our model, increasing ammonia load did not affect whole-body protein deposition in growing steers when Met was limiting.     

Excess amino acid supply improves methionine and leucine utilization by growing steers

In 2 experiments, 6 ruminally cannulated Holstein steers (205 +/- 23 and 161 +/- 14 kg initial BW in Exp. 1 and 2, respectively) housed in metabolism crates were used in 6 x 6 Latin squares to study the effects of excess AA supply on Met (Exp. 1) and Leu (Exp. 2) use. All steers received a diet based on soybean hulls (DMI = 2.66 and 2.45 kg/d in Exp. 1 and 2, respectively); ruminal infusions of 200 g of acetate/d, 200 g of propionate/d, and 50 g of butyrate/d, as well as abomasal infusion of 300 g of glucose/d to provide energy without increasing the microbial protein supply; and abomasal infusions of a mixture of all essential AA except Met (Exp. 1) or Leu (Exp. 2). Periods were 6 d, with 2-d adaptations and 4 d to collect N balance data. All treatments were abomasally infused. In Exp. 1, treatments were arranged as a 2 x 3 factorial, with 2 amounts of l-Met (0 or 4 g/d) and 3 AA supplements (no additional AA, control; 100 g/d of nonessential AA + 100 g/d of essential AA, NEAA + EAA; and 200 g/d of essential AA, EAA). Supplemental Met increased (P < 0.01) retained N and decreased (P < 0.01) urinary N and urinary urea N. Retained N increased (P < 0.01) with NEAA + EAA only when 4 g/d of Met was provided, but it increased (P < 0.01) with EAA with or without supplemental Met. Both AA treatments increased (P < 0.01) plasma urea and serum insulin. Plasma glucose decreased (P = 0.03) with supplemental Met. In Exp. 2, treatments were arranged as a 2 x 3 factorial with 2 amounts of L-Leu (0 or 4 g/d) and 3 AA supplements (control, NEAA + EAA, and EAA). Supplemental Leu increased (P < 0.01) retained N and decreased (P < 0.01) urinary N and urinary urea N. Both AA treatments increased (P < 0.01) retained N, and they also increased (P < 0.01) urinary N, urinary urea N, and plasma urea. Serum insulin increased (P = 0.06) with supplemental Leu and tended (P = 0.10) to increase with both AA treatments. Supplementation with excess AA improved Met and Leu use for protein deposition by growing cattle.     

Histidine utilization by growing steers is not negatively affected by increased supply of either ammonia or amino acids

Two experiments were conducted with ruminally cannulated Holstein steers to determine effects of N supply on histidine (His) utilization. All steers received 2.5 kg DM/d of a diet based on soybean hulls; abomasal infusion of 250 g/d amino acids, which supplied adequate amounts of all essential amino acids except His; abomasal infusion of 300 g/d glucose; and ruminal infusion of 180 g/d acetate, 180 g/d propionate, and 45 g/d butyrate. Both experiments were 6 x 6 Latin squares with treatments arranged as 3 x 2 factorials. No significant (P < 0.05) interactions between main effects were noted for N balance criteria in either Exp. 1 or 2. For Exp. 1, steers (146 +/- 7 kg) received 0, 1.5, or 3 g/d of L-His infused abomasally in combination with 0 or 80 g/d urea infused ruminally to supply a metabolic ammonia load. Urea infusions increased (P < 0.05) ruminal ammonia concentration from 8.6 to 19.7 mM and plasma urea from 2.7 to 5.1 mM. No change in N retention occurred in response to urea (35.1 and 37.1 g/d for 0 and 80 g/d urea, respectively, P = 0.16). Retained N increased linearly (P < 0.01) with His (31.5, 37.8, and 39.0 g/d for 0, 1.5, and 3 g/d L-His, respectively). Efficiency of deposition of supplemental His between 0 and 1.5 g/d averaged 65%. In Exp. 2, steers (150 +/- 6 kg) were infused abomasally with 0 or 1 g/d of L-His in combination with no additional amino acids (Control), 100 g/d of essential + 100 g/d of nonessential amino acids (NEAA+EAA), or 200 g/d of essential amino acids (EAA). Retained N increased (P = 0.02) from 34.2 to 38.3 g/d in response to His supplementation. Supplementation with NEAA+EAA increased (P < 0.05) N retention (33.9, 39.3, and 35.6 g/d for Control, NEAA+EAA, and EAA, respectively), likely in response to increased energy supply. Plasma urea concentrations of steers receiving NEAA+EAA (3.8 mM) and EAA (3.8 mM) were greater (P < 0.05) than those of Control steers (2.7 mM). The average efficiency of His utilization was 63%, a value similar to the value of 65% observed in Exp. 1, as well as the 71% value predicted by the Cornell net carbohydrate and protein system model. Under our experimental conditions, increases in N supply above requirements, as either ammonia or amino acids, did not demonstrate a metabolic cost in terms of His utilization for whole-body protein deposition by growing steers.     

Effects of energy supply on leucine utilization by growing steers at two body weights

The effects of energy supplementation on Leu utilization in growing steers were evaluated in 2 experiments by using 6 ruminally cannulated Holstein steers. In Exp. 1, steers (initial BW = 150 +/- 7 kg) were limit-fed (2.3 kg of DM/d) a diet based on soybean hulls and received a basal ruminal infusion of 100 g of acetate/d, 75 g of propionate/d, and 75 g of butyrate/d, as well as abomasal infusions of 200 g of glucose/d and a mixture (215 g/d) containing all essential AA except Leu. Treatments were arranged as a 3 x 2 factorial, with 3 amounts of Leu infused abomasally (0, 4, and 8 g/d) and supplementation of diets with 2 amounts of energy (0 and 1.9 Mcal/d of GE). Supplemental energy was supplied by ruminal infusion of 100 g of acetate/ d, 75 g of propionate/d, and 75 g of butyrate/d, as well as abomasal infusion of 200 g of glucose/d to provide energy to the animal without affecting the microbial protein supply. When no supplemental energy was provided, Leu supplementation increased N balance, with no difference between 4 and 8 g/d of Leu (24.5, 27.0, and 27.3 g/d for 0, 4, and 8 g/d of Leu), but when additional energy was supplied, N retention increased linearly in response to Leu (25.6, 28.5, and 31.6 g/d for 0, 4, and 8 g/d of Leu; Leu x energy interaction, P = 0.06). The changes in N balance were the result of changes in urinary N excretion. The greater Leu retentions in response to energy supplementation when Leu was the most limiting nutrient indicate that energy supplementation improved the true efficiency of Leu utilization. In addition, supplemental energy increased the gross efficiency of Leu utilization when the Leu supply was not limiting by increasing the maximal rates of protein deposition. Experiment 2 was similar to Exp. 1, but steers had an initial BW of 275 +/- 12 kg and were limit-fed at 3.6 kg of DM/d. Retention of N was not affected (P = 0.22) by Leu supplementation, indicating that Leu did not limit protein deposition. Energy supply increased N retention (P < 0.01) independently of Leu supplementation (33.0 vs. 27.8 g/d). Overall, energy supplementation improved Leu utilization by modestly increasing N retention when Leu was limiting and by increasing the ability of steers to respond to the greatest amount of supplemental Leu. We conclude from these results that the assumption of a constant efficiency of AA utilization is unlikely to be appropriate for growing steers.     

Branched-chain amino acids for growing cattle limit-fed soybean hull-based diets.

Five ruminally cannulated Holstein steers (176 kg) were used in a 5 x 5 Latin square to evaluate the effects of branched-chain AA supplementation on N retention and plasma AA concentrations of steers. Steers were limit-fed (3.0 kg/d of DM) twice daily diets low in ruminally undegradable protein (72% soybean hulls, 19% alfalfa, 5% molasses, and 4% vitamins and minerals). Acetate (400 g/d) was continuously infused into the rumen. Treatments were continuous abomasal infusions of 1) 115 g/d of a mixture of 10 essential AA designed to exceed the steers' requirements (10AA), 2) 10AA with Leu removed, 3) 10AA with Ile removed, 4) 10AA with Val removed, and 5) 10AA with all three branched-chain AA removed. Experimental periods were 7 d, with 3 d for adaptation to treatments and 4 d for total fecal and urinary collections for N balance. Blood samples were collected 5 h after feeding on d 7. Retained N decreased in response to removal of Leu (P < 0.06), Val (P < 0.05), or all three branched-chain AA (P < 0.05). Plasma Leu concentrations decreased (P < 0.05) in response to removal of Leu and all three branched-chain AA. Plasma Ile concentrations decreased (P < 0.05) in response to removal of Ile and all three branched-chain AA but increased (P < 0.05) in response to removal of Leu. Plasma Val concentrations decreased (P < 0.05) in response to removal of Val and all three branched-chain AA but increased (P < 0.05) in response to removal of Leu. Responses in N balance and plasma AA concentrations of growing cattle limit-fed soybean hull-based diets demonstrate limitations in the basal supply of Leu and Val but not Ile provided that supplies of all other essential AA are met.     

Effects of energy level on methionine utilization by growing steers

We evaluated the effect of energy supplementation on Met use in growing steers. Six ruminally cannulated Holstein steers (228 +/- 8 kg of BW) were used in a 6 x 6 Latin square and fed 2.8 kg of DM/d of a diet based on soybean hulls. Treatments were abomasal infusion of 2 amounts of Met (0 or 3 g/d) and supplementation with 3 amounts of energy (0, 1.3, or 2.6 Mcal of GE/d) in a 2 x 3 factorial arrangement. The 1.3 Mcal/d treatment was supplied through ruminal infusion of 90 g/d of acetate, 90 g/d of propionate, and 30 g/d of butyrate, and abomasal infusion of 30 g/d of glucose and 30 g/d of fat. The 2.6 Mcal/d treatment supplied twice these amounts. All steers received basal infusions of 400 g/d of acetate into the rumen and a mixture (125 g/d) containing all essential AA except Met into the abomasum. No interactions between Met and energy levels were observed. Nitrogen balance was increased (P < 0.05) by Met supplementation from 23.6 to 27.8 g/d, indicating that protein deposition was limited by Met. Nitrogen retention increased linearly (P < 0.05) from 23.6 to 27.7 g/d with increased energy supply. Increased energy supply also linearly reduced (P < 0.05) urinary N excretion from 44.6 to 39.7 g/d and reduced plasma urea concentrations from 2.8 to 2.1 mM. Total tract apparent OM and NDF digestibilities were reduced linearly (P < 0.05) by energy supplementation, from 78.2 and 78.7% to 74.3 and 74.5%, respectively. Whole-body protein synthesis and degradation were not affected significantly by energy supplementation. Energy supplementation linearly increased (P < 0.05) serum IGF-I from 694 to 818 ng/mL and quadratically increased (P < 0.05) serum insulin (0.38, 0.47, and 0.42 ng/mL for 0, 1.3, and 2.6 Mcal/d, respectively). In growing steers, N retention was improved by energy supplementation, even when Met limited protein deposition, suggesting that energy supplementation affects the efficiency of AA use.     

Nitrogen balance in lambs fed a high-concentrate diet and infused with differing proportions of casein in the rumen and abomasum

Twenty-five wether lambs (34 +/- 0.9 kg) fitted with ruminal and abomasal infusion catheters were used in a completely randomized design to determine the effects of differing proportions of ruminal and abomasal casein infusion on N balance in lambs fed a high-concentrate diet (85% corn grain, 1.6% N; DM basis) for ad libitum intake. Wethers were infused with 0 (control) or 10.4 g/d of N from casein with ruminal:abomasal infusion ratios of 100:0, 67:33, 33:67, or 0:100% over a 14-d period. Feed, orts, feces, and urine were collected over the last 5 d. Total N intake and excretion were greater (P < 0.01) in lambs infused with casein than in controls; however, N retention did not differ in lambs infused with casein compared with controls, suggesting that N requirements were met without casein supplementation. Total N intake and total N excretion did not differ among casein infusion treatments. Urinary N excretion decreased linearly (P = 0.07) with decreasing ruminal infusion of casein. Site of casein infusion quadratically (P = 0.06) influenced N retained (g/d), with the greatest retention observed in the 33:67 ruminal:abomasal infusion treatment. Dry matter intake from feed decreased from 1,183 to 945 g/d (P = 0.02) in lambs infused with casein compared with controls, but apparently digested DM did not differ among treatments. These data indicate that decreasing the ruminal degradability of supplemental protein above that required to maximize N retention results in decreased urinary excretion of N without greatly affecting apparent diet digestion.     

The effect of abomasal methionine supplementation on nitrogen retention of growing steers postruminally infused with casein or nonsulfur-containing amino acids

An experiment was conducted to develop a system useful for measuring methionine requirements of growing steers. Seven ruminally cannulated steers (312 kg, gaining .91 kg/d) were fed a diet based on ammoniated corn cobs, corn starch, molasses and urea. Quantities of N and sulfur-containing amino acids disappearing from the small intestine were 96.0 and 14.8 g/d, respectively. Postruminal infusions of Na-caseinate (CAS) resulted in linear (P less than .01) increases in N retention with values increasing from 30.1 g/d with no postruminal CAS infusion to 39.3, 50.8 and 59.2 g/d (averaged across methionine supplementation) when 100, 200 and 300 g/d CAS were infused. Postruminal infusions of a mixture of crystalline L-amino acids (simulating the nonsulfur-containing essential amino acid pattern of casein; SIM) at levels of 100, 200 and 300 g/d also led to linear increases (P less than .01) in N retention with steers retaining 30.9, 38.9 and 50.5 g N/d (averaged across methionine supplementation), respectively. Postruminal infusion of 12 g/d L-methionine across CAS and SIM infusions improved (P less than .01) N retention by 7.6 g/d but infusion of SIM, which is devoid of sulfur amino acids, also increased N retention. Responses to methionine supplementation was greatest when 200 or 300 g/d SIM were abomasally infused. The data are interpreted to demonstrate that, for steers fed a diet containing little true protein, postruminal supplementation with nonsulfur-containing amino acids tended to increase the ability of growing steers to respond to methionine supplementation.     

Effects of ruminal casein and glucose on forage digestion and urea kinetics in beef cattle

Effects of supplemental glucose and degradable intake protein on nutrient digestion and urea kinetics in steers (Bos taurus) given ad libitum access to prairie hay (4.7% CP) were quantified. Six ruminally and duodenally cannulated steers (initial BW 391 kg) were used in a 4 × 4 Latin square with 2 extra steers. Treatments were arranged as a 2 × 2 factorial and included 0 or 1.2 kg of glucose and 240 or 480 g of casein dosed ruminally once daily. Each period included 9 d for adaptation, 4 d for total fecal and urine collections, and 1 d for ruminal and duodenal sampling. Jugular infusion of (15)N(15)N-urea with measurement of enrichment in urine was used to measure urea kinetics. Glucose reduced forage intake by 18% (P < 0.01), but casein did not affect forage intake (P = 0.69). Glucose depressed (P < 0.01) total tract NDF digestion. Glucose supplementation decreased ruminal pH 2 h after dosing, but the effect was negligible by 6 h (treatment × time; P = 0.01). Providing additional casein increased the ruminal concentration of NH(3), but the increase was less when glucose was supplemented (casein × glucose; P < 0.01). Plasma urea-N was increased (P < 0.01) by additional casein but was reduced (P < 0.01) by glucose. Microbial N flow to the duodenum and retained N increased (P ≤ 0.01) as casein increased, but neither was affected by glucose supplementation. Urea-N entry rate increased (P = 0.03) 50% with increasing casein. Urinary urea-N excretion increased (P < 0.01) as casein increased. The proportion of urea production that was recycled to the gut decreased (P < 0.01) as casein increased. Glucose supplementation decreased (P < 0.01) urinary urea excretion but did not change (P ≥ 0.70) urea production or recycling. The amount of urea-N transferred to the gut and captured by ruminal microbes was less for steers receiving 480 g/d casein with no glucose than for the other 3 treatments (casein × glucose interaction, P = 0.05), which can be attributed to an excess of ruminally available N provided directly to the microbes from the supplement. Overall, the provision of supplemental glucose decreased forage intake and digestibility. Increasing supplemental casein from 240 to 480 g/d increased urea production but decreased the proportion of urea-N recycled to the gut.     

Effect of ruminal vs postruminal administration of degradable protein on utilization of low-quality forage by beef steers

An experiment was designed to determine the effects of ruminal and postruminal infusions of ruminally degradable protein (casein) on intake and digestion of low-quality hay by beef steers. Twelve ruminally fistulated Angus x Hereford steers (initial BW = 563 kg) were blocked by weight and assigned to one of three treatments: control (C; hay only) or hay plus ruminal (R) or postruminal (P) infusion of 400 g/d of sodium caseinate. The trial consisted of five periods: 1) 10-d adaptation to the hay diet; 2) 7-d measurement of hay intake (without infusions); 3) 10-d adaptation to protein infusion treatments (intake measurements continued); 4) 7-d measurement of hay intake and digestibility (infusions continued); and 5) 3-d ruminal sampling period (infusions continued). Steers were given ad libitum access to tallgrass-prairie hay (3.4% CP, 76.6% NDF) throughout the study. Casein was administered once daily before feeding, either directly into the rumen or via anchored infusion lines into the abomasum. Hay intake was increased by supplementation (P < 0.01). Ruminal infusion elicited a greater (P = 0.04) increase in hay intake than postruminal infusion. Intake tended (P = 0.11) to be lower in period 4 than in period 2 for control steers but was greater in period 4 than in period 2 (P < or = 0.03) for both R and P steers. The increase in intake between periods 2 and 4 was greater for R than for P steers (P = 0.03). Supplementation improved diet OM digestion (P = 0.04) but not NDF digestion (P = 0.18); however, greater relative error for NDF digestion may have limited the ability to elucidate significant treatment effects. There were no differences in either OM digestion (P = 0.42) or NDF digestion (P = 0.35) between R and P steers. Plasma urea N at 0 and 3 h after feeding on the last day of the experiment was lower (P = 0.05) for C than for R and P steers, but no difference (P = 0.48) was evident between R and P steers. Ruminal ammonia N levels also were increased by supplementation (P < 0.01), with a much larger increase for R than for P steers (P < 0.01). Total VFA concentrations were not affected (P = 0.21) by treatment, but R steers exhibited lower proportions of acetate and higher proportions of isobutyrate, valerate, and isovalerate than P steers (P < 0.01). In conclusion, ruminal and postruminal infusion of a degradable protein source improved forage utilization, although the response in forage OM intake and total digestible OM intake was greater for ruminal infusion than for postruminal infusion.     

Sulfur-containing amino acid requirement of rapidly growing steers

Eight ruminally cannulated steers (294 kg, ADG = 1.3 kg/d) were used in a N retention study (8 x 8 latin-square design) to evaluate sulfur-containing (S) amino acid (AA) requirements for growth. Treatments were abomasal infusions of seven levels of L-methionine (0, 3, 6, 9, 12, 15 and 18 g/d) and one level of DL-methionine (6 g/d). All steers were fed a semipurified diet based on ammoniated corn cobs (DMI = 6.56 kg/d) and were abomasally infused with 400 g/d dextrose and 296.4 g/d of crystalline AA that simulated the non-S-AA pattern of casein. Infusion of 3 g/d supplemental L-methionine maximized N retention in steers. Intestinal flows of absorbable S-AA were determined to be 1.89 g/kg DMI. Breakpoint analysis of retained N as a function of total absorbable S-AA yielded a total S-AA requirement of 14.7 g/d. Nitrogen retention for DL-methionine (36.4 g/d) was not different (P greater than .05) from that for 6 g/d L-methionine (38.8 g/d), but because this value was not in the linear response range, the efficacy of DL-methionine in meeting S-AA needs could not be evaluated. Plasma methionine concentrations increased linearly (P less than .05) in response to L-methionine infusion and were greater (P less than .05) for steers infused with 6 g/d DL-methionine (45.3 microM) than for steers receiving 6 g/d L-methionine (30.5 microM). Plasma cystine increased when up to 9 g/d L-methionine was infused.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nitrogen metabolism of beef steers fed endophyte-free tall fescue hay: effects of ruminally protected methionine supplementation

Level of nitrogen (N) intake and ruminally protected methionine supplementation were evaluated in eight Angus growing steers (initial BW 253+/-21 kg, final BW 296+/-21 kg) in a replicated 4+/-4 Latin square design. The steers were fed two endophyte-free tall fescue (Festuca arundinacea) hays that contained 2.2 (LO) or 2.8% (HI) of DM as N and were either supplemented or not with ruminally protected methionine (10 g metabolizable methionine/d). Diets were fed to provide adequate energy for 0.5 kg ADG and sufficient protein for maintenance (LO), or protein to support 0.5 kg ADG (HI). Following at least 14 d of adjustment, N balance was measured for 6 d. Isotopic urea was infused (15N15N-urea, 0.164 mmol urea N/h) via a jugular catheter for 56 h and urine was collected from 48 to 56 h to measure urea kinetics. Jugular blood was collected during the balance trial, and serum was analyzed for serum urea N (SUN). By design, daily N intake was greater (P < 0.05) for HI (112 g) than for LO (89 g). Compared with LO, steers when fed HI had greater (P < 0.05) daily DMI (4,217 vs 4,151 g), fecal N (34.4 vs 31.2 g), N digested (77.1 vs 57.7 g), urine N (48.3 vs 37.5 g), urine urea N excretion (34.6 vs 24.8 g), and N retained (29.8 vs 21.1 g). When fed HI steers also had higher (P < 0.05) urine urea N concentration (276 vs 219 mM), SUN (8.7 vs 6.7 mM), N digestibility (69.1 vs 64.9%), percentage of urinary N present as urea (71.5 vs 66.7%, P < 0.053), and rate of urea N production (59.6 vs 49.2 g/d) but lower (P < 0.05) percentage of urea N produced that was returned to the ornithine cycle (15.03 vs 19.2 1%) than when fed LO. Methionine supplementation decreased daily urine N (41.2 vs 44.6 g, P = 0.10) and increased both the amount of N retained daily (27.9 vs 23.7 g, P < 0.089) and the percentage of N digested that was retained (40.4 vs 34.6%, P < 0.094). In summary, supplemental methionine met a specific dietary limitation by increasing the amount of digested N that was retained by the steers.     

Nitrogen balance in lambs fed low-quality brome hay and infused with differing proportions of casein in the rumen and abomasum

Twenty wether lambs (46 +/- 2 kg) fitted with ruminal and abomasal infusion catheters were used in a completely randomized design to determine the effects of differing proportions of ruminal and abomasal casein infusion on N balance in lambs fed low-quality brome hay (0.8% N, DM basis) for ad libitum intake. Wethers were infused with 0 (control) or 10.7 g/d of N from casein with ratios of ruminal:abomasal infusion of 100:0 (100R:0A), 67:33 (67R:33A), 33:67 (33R:67A), or 0:100% (0R:100A), respectively, over a 12-d period. Total N supply (hay N intake + N from casein infusion) was greater (P = 0.001) in lambs receiving casein infusion than in controls. Urinary N excretion (g/d) was greater (P = 0.001) in lambs receiving casein infusion than in controls. Urinary N excretion decreased as casein infusion was shifted from 100R:0A to 33R:67A and then slightly increased in lambs receiving 0R:100A (quadratic, P = 0.02). Total N excretion was greater (P = 0.001) in lambs receiving casein infusion than in controls and decreased linearly (P = 0.005) as casein infusion was shifted to the abomasum. Retained N (g/d, % of N intake, and % of digested N) was greater (P = 0.001) in lambs receiving casein than in controls. Retained N increased as infusion was shifted from 100R:0A to 33R:67A and then slightly decreased in lambs receiving 0R: 100A (quadratic, P < 0.07). Based on regression analysis, the predicted optimum proportion of casein infusion to maximize N retention was 68% into the abomasum. The regression suggests that supplementation with undegradable intake protein had an additional benefit over supplementation with ruminally degradable intake protein (100R:0A) and that changing the percentage of ruminally undegradable intake protein in supplemental protein from 33 to 100% resulted in minimal differences in N retention. Apparent N, DM, OM, and energy digestibility (% of intake) was greater (P < 0.03) in lambs infused with casein than controls but did not differ among casein infusion groups. These data suggest that feeding protein supplements containing a portion (greater than 0%) of the crude protein as ruminally undegradable intake protein, as compared to 100% ruminally degradable intake protein, to lambs consuming low-quality forage increases N retention and the efficiency of N utilization without influencing total-tract nutrient digestion.     

The net portal and hepatic flux of metabolites and oxygen consumption in growing beef steers given postruminal casein

Changes in net portal and hepatic nutrient flux and oxygen consumption in response to 3-d abomasal casein infusions were studied in seven multicatheterized beef steers. Steers were fed 4.3 kg DM/d of a high-concentrate diet in 12 equal meals. Blood flow (para-aminohippurate dilution) and net flux (venoarterial concentration difference x blood flow) across portal-drained viscera (PDV) and hepatic tissues were measured on d 3 of the abomasal infusions. In two experiments, the response to 300 (300C) and 150 (150C) g casein/d were compared, respectively, to a control water infusion. The 300C increased (P less than .05) arterial blood concentrations of alpha-amino N (AAN), urea N and ammonia; 150C increased (P less than .05) arterial urea N. Urinary urea N excretion was increased (P less than .01) by 300C and 150C. Although 300C increased net PDV release of AAN (P less than .07) and alanine (P less than .10), there was no net change in total splanchnic (TSP) flux due to an increased net hepatic uptake of AAN (P less than .01) and alanine (P less than .05). Net PDV glucose flux was decreased (P less than .05) by 300C, but net hepatic glucose flux was not affected by either level of casein. The 150C increased TSP oxygen consumption (P less than .05) and hepatic oxygen extraction (P less than .10). Approximately 26 and 30% of the casein N infused abomasally appeared in the portal blood as AAN for 150C and 300C, respectively. The sum of net PDV ammonia and AAN fluxes accounted for 47 and 88% of the N infused for 150C and 300C, respectively. These data emphasize the importance of intestinal and liver tissues in regulating the flux of nitrogenous compounds absorbed from the diet.     

Nitrogen metabolism and somatotropin secretion in beef heifers receiving abomasal arginine infusions

Twelve Angus x Hereford heifer calves (233 kg) were fitted with abomasal infusion cannulas and used to study N and endocrine responses to abomasally infused arginine (Arg). Heifers were allotted randomly to three treatment groups and received continuous abomasal infusions (2 liters/d) of water (CON) or Arg solutions providing .33 g Arg.HCl/kg BW (LOW) or .50 g Arg.HCl/kg BW (HIGH) each day. A 12-d dietary adjustment period preceded a 7-d infusion and collection period. Each calf received 4,544 g DM/d of a basal diet in equal portions at 0600, 1200, 1800 and 2400. Calves were housed in individual metabolism crates and fitted with urinary bladder catheters for total excreta collection. On d 1 and 5, blood samples were collected at 15-min intervals for 8 h between 1200 and 2000. Single samples were obtained at 1400 on remaining days. The infusion of Arg increased the quantity of N retained by heifers (P less than .01) and the percentage of total N retained (P less than .10); however, no differences were observed between LOW and HIGH heifers. Increased (P less than .01) urinary N excretion by Arg heifers was associated with greater (P less than .05) quantities of urinary urea N and ammonia N. Blood urea N and serum Arg concentrations were highest (P less than .05) in Arg heifers, whereas total serum AA concentrations were lower (P less than .05) in Arg heifers than in CON heifers. Serum glucose and insulin concentrations were not affected (P greater than .10) by treatment. Characterization of somatotropin (STH) profiles revealed that amplitude and frequency of STH pulses were not affected (P greater than .10) by treatment, whereas mean (P less than .10) and basal (P less than .05) STH concentrations were elevated in HIGH compared to LOW heifers on d 1 and 5. The similar N retention responses of LOW and HIGH heifers and similar STH profiles of CON and LOW heifers suggest that the stimulatory effect of the HIGH dose on STH secretion occurred only after tissue N requirements had been satisfied.     

Effects of ethanol and heat treatments of soybean meal and infusion of sodium chloride into the rumen on ruminal degradation and escape of soluble and total soybean meal protein in steers

Soybean meal (SBM) treated with 70% ethanol at 80 C (ET), nontreated SBM (NT) or a ureacasein-corn mix (UC) was fed to steers fitted with ruminal and duodenal cannulae to study ruminal N metabolism. Sodium chloride (NaCl) was ruminally infused at 0 or 500 g/d. Nitrogen supplements provided approximately 70% of total dietary N. Experimental design was a 6 X 6 Latin square with a 3 X 2 factorial arrangement of treatments. Total duodenal N flows and non-ammonia, non-bacterial-N (NANB-N) flows were higher (P less than .05) when steers were fed SBM treatments compared with UC, and higher (P less than .05) when steers were fed ET compared with NT. Percentage of SBM-N escaping ruminal degradation was greater (P less than .05) when steers were fed ET compared with NT, and greater (P less than .05) when NaCl was infused into the rumen. Duodenal flows of total, indispensible and dispensible amino acids were increased (P less than .05) when steers were fed SBM treatments compared with UC, and greater (P less than .05) when steers were fed ET compared with NT. No differences in soluble N flows at the omasum were observed due to treatment. Bacterial protein comprised the majority of the N leaving the rumen. Both ruminal NaCl infusion and ethanol and heat treatment of SBM increased ruminal SBM-N escape.     

Net absorption and utilization of nitrogenous compounds across ruminal, intestinal, and hepatic tissues of growing beef steers fed dry-rolled or steam-flaked sorghum grain

Our objectives were to determine effects of grain processing on splanchnic (gut tissues and liver) N metabolism and whole-body N balance by growing steers and to ascertain the relative contributions of ruminal and intestinal tissues to net absorption and utilization of N-containing nutrients. Seven beef steers (348 kg initial BW), surgically implanted with appropriate catheters, were fed diets containing 77% steam-flaked (SF) or dry-rolled (DR) sorghum grain. Blood flows and net output or uptake of ammonia N, urea N, and alpha-amino N (estimate of amino acids) were measured across portal-drained viscera (PDV or gut tissues) and intestinal, ruminal, hepatic, and splanchnic tissues (PDV + hepatic). The experimental design was a crossover between DR and SF diets, with six samplings of blood at 2-h intervals on 2 d for each steer. Nitrogen intake (139 +/- 3 g/d), output in urine (43 +/- 2 g/d), and retention (40 +/- 3 g/d) were similar for both processing treatments. When steers were fed SF sorghum compared to DR sorghum, N retention as a percentage of N intake was numerically greater (P < 0.12), output of fecal N was numerically lower (P < 0.13), and urinary urea N was lower (P < 0.04). For SF vs DR, net uptake of alpha-amino N by liver was higher (P < 0.04; 20 vs 9 g/d) and was numerically lower (P < 0.16) for ruminal tissues (15 vs 33 g/d). Feeding steers SF compared to DR tended to increase net transfer (cycling) of blood urea N to PDV (57 vs 41 g/d; P < 0.07), increased cycling to intestinal tissues (15 vs 6 g/d; P < 0.05), and numerically increased transfer to ruminal tissues (42 vs 32 g/d; P < 0.12) but did not alter other net output or uptake of N across splanchnic tissues. Total urea N transfer (blood + saliva) was similar for both treatments. Net uptake of alpha-amino N by ruminal tissues was about 30% of the net amount of alpha-amino N absorbed across the intestinal tissues. In summary, most of the blood urea N cycled from the liver to gut tissues was transferred to ruminal tissues for potential microbial protein synthesis, and the net ruminal utilization of alpha-amino N was about 30% of that absorbed from intestinal tissues. Feeding growing steers SF compared to DR sorghum diets numerically increased whole-body N retention (percentage of N intake) by about 15% and tended to increase transfer of blood urea N to the gut by about 40%, which could increase the supply of high-quality microbial protein for absorption.     

Intestinal starch disappearance increased in steers abomasally infused with starch and protein

Steers (379 +/- 10 kg) with ruminal, duodenal, and ileal cannulas were used in a 5 x 5 Latin square digestion trial to quantify and evaluate the relationship between intestinal protein supply and intestinal starch disappearance. Treatments were infusions of 0, 50, 100, 150, or 200 g/d of casein along with 1,042 g/d of raw cornstarch. Abomasal infusions were accomplished by passing tubing and a pliable retaining washer through the reticular-omasal orifice into the abomasum. Steers were fed a 93% corn silage, 7% supplement diet that contained 12% crude protein at 1.65% body weight in 12 equal portions/d. Periods lasted 17 d (12 d for adaptation, 2 d of collections, and 3 d of rest). The quantity and percentage of organic matter and protein disappearance from the small intestine increased linearly (P < 0.03) with infused casein. Greater quantities of starch disappeared with increased casein infusion (P < 0.01). The infusion of 200 g/d of casein increased small intestinal starch disappearance by 226 g/d over the control. Casein infusion did not affect the quantity or percent of organic matter, starch, or protein disappearance in the large intestine. Treatments did not change ruminal ammonia N, ruminal pH, or plasma glucose concentrations. Starch disappearance from the small intestine was increased with greater protein flow to the duodenum of steers.     

Methionine as a methyl group donor in growing cattle

Holstein steers were used in two 5 x 5 Latin square experiments to evaluate the sparing of methionine by alternative sources of methyl groups (betaine and choline). Steers were housed in metabolism crates and limit-fed a soybean hull-based diet high in rumen degradable protein. To increase energy supply, ruminal infusions of volatile fatty acids and abomasal infusions of glucose were provided. An amino acid mixture, limiting in methionine, was infused abomasally to ensure that nonsulfur amino acids did not limit protein synthesis. Treatments for Exp. 1 were abomasal infusion of 1) water, 2) 2 g/d L-methionine, 3) 1.7 g/d L-cysteine, 4) 1.6 g/d betaine, and 5) 1.7 g/d L-cysteine + 1.6 g/d betaine. Treatments for Exp. 2 were abomasal infusion of 1) water, 2) 2 g/d L-methionine, 3) 8 g/d betaine, 4) 16 g/d betaine, and 5) 8 g/d choline. In both experiments, nitrogen retention increased in response to methionine (P < 0.05), demonstrating a deficiency of sulfur amino acids. Responses to cysteine, betaine, and choline were all small and not significant. The lack of response to cysteine indicates that the response to methionine was not due to transsulfuration to cysteine or that cysteine supply did not alter the flux of methionine through transsulfuration. The lack of response to betaine suggests that the steers' needs for methyl groups were met by the dietary conditions or that betaine was relatively inefficient in increasing the remethylation of homocysteine to methionine and, thereby, reducing the synthesis of cysteine from homocysteine. Under our experimental conditions, responses to methionine were likely due to a correction of a deficiency of methionine per se rather than of methyl group donors.     

Influence of protein type and level on nitrogen and forage use in cows consuming low-quality forage

Minimal quantities of ruminally degradable protein from supplements may improve supplement use efficiency of ruminants grazing dormant forages. In Exp. 1, N retention, ruminal NH(3), serum urea N, and NDF digestibility were evaluated for 12 ruminally cannulated cows (Bos spp.) in an incomplete Latin Square design with 3 periods of 42 d each. Cows were fed weeping lovegrass [Eragrostis curvula (Schrad.) Nees] hay (4.1% CP, 75% NDF, OM basis) at 1.3 % BW/d and offered 1 of 3 sources of CP [urea, cottonseed (Gossypium spp.) meal (CSM); or 50% blood meal and 50% feather meal combination (BFM)] fed to supply 0, 40, 80, or 160 g/d of CP. Beginning on d 22 of supplementation, ruminal contents and serum samples were collected at -2, 0, 3, 6, 9, 12, 18, 24, 30, 36, and 48 h relative to the morning offering of hay. On Day 24, feces and urine were collected for 72 h. In Exp. 2, 4 ruminally cannulated steers were used in a replicated 4 by 4 Latin Square to evaluate use of supplements differing in quantity and ruminal CP degradability. Steers were fed 6.8 kg/d chopped sudangrass [Sorghum bicolor (L.) Moench nothosubsp. drummondii (Steud.) de Wet ex Davidse] hay (3.7% CP, 74% NDF on OM basis) and supplemented with 56 g/d of a salt mineral mix (CON); CON + 28 g/d blood meal + 28 g/d feather meal (BFM); CON + 98 g/d CSM (LCS); or CON + 392 g/d CSM (HCS). Treatments provided 0, 40, 40, or 160 g/d of CP for CON, BFM, LCS, and HCS respectively. In Exp. 1, N use and total tract NDF digestibility were not affected by protein sources or amounts (P ≥ 0.18). Ruminal NH(3) concentrations exhibited a quadratic response over time for UREA (P < 0.05) and was greater with increasing inclusion of urea (P < 0.05); whereas BFM or CSM did not differ (P > 0.05) by amount or across time. In Exp. 2, supplementation had a tendency (P = 0.09) to increase DM disappearance. Supplementation also increased (P < 0.01) serum glucose concentrations; however, no difference (P ≥ 0.28) was found between supplements. Serum urea N and ruminal NH(3) concentrations were increased (P ≤ 0.01) in steers fed HCS. Feeding low quantities of a high-RUP supplement maintained rumen function without negatively affecting DM or NDF digestibility of a low-quality forage diet.