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.     

Effects of energy source on methionine utilization by growing steers

We evaluated the effects of different supplemental energy sources on Met use in growing steers. Ruminally cannulated Holstein steers were used in two 6 x 6 Latin squares, and data were pooled for analyses. In Exp. 1, steers (148 kg) were fed 2.3 kg of DM/d of a diet based on soybean hulls. Treatments (2 x 3 factorial) were abomasal infusion of 0 or 3 g of l-Met/d, and supplementation with no energy or with glucose (360 g/d) or fat (150 g/d) continuously infused into the abomasum. In Exp. 2, steers (190 kg) received 2.6 kg of dietary DM/d and were provided (2 x 3 factorial) with 0 or 3 g of l-Met/d, and with no supplemental energy or with acetate (385 g/d) or propionate (270 g/ d) continuously infused into the rumen. In both experiments, the energy sources supplied 1.3 Mcal of GE/d, and all steers received basal infusions of 400 g of acetate/d into the rumen and a mixture (125 g/d) of all essential AA except Met into the abomasum. Nitrogen balance (18.8 vs. 23.5 g/d; P < 0.01) and whole-body protein synthesis (2.1 vs. 2.3 kg/d; P < 0.07) were increased by Met supplementation, indicating that protein deposition was limited by Met. Supplemental energy reduced (P < 0.01) urinary N excretion and increased (P < 0.01) N retention without differences among energy sources. Increases in N retention in response to Met were numerically greater when energy was supplemented. Efficiency of supplemental Met use was 11% when no energy was supplemented but averaged 21% when 1.3 Mcal of GE/d was provided. Whole-body protein synthesis and degradation were not affected by energy supplementation. Serum insulin concentrations were increased by glucose and propionate supplementation. Serum IGF-I concentrations were increased by supplementation with Met or glucogenic sources of energy. In growing steers, N retention was increased by energy supplementation even though protein deposition was limited by Met, suggesting that energy supplementation improves the efficiency of AA use. These responses were independent of the source of energy.     

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.     

Effect of rumen-degradable intake protein supplementation on urea kinetics and microbial use of recycled urea in steers consuming low-quality forage

We evaluated the effect of increasing amounts of rumen-degradable intake protein (DIP) on urea kinetics in steers consuming prairie hay. Ruminally and duodenally fistulated steers (278 kg of BW) were used in a 4 x 4 Latin square and provided ad libitum access to low-quality prairie hay (4.9% CP). The DIP was provided as casein dosed ruminally once daily in amounts of 0, 59, 118, and 177 mg of N/kg of BW daily. Periods were 13 d long, with 7 d for adaptation and 6 d for collection. Steers were in metabolism crates for total collection of urine and feces. Jugular infusion of (15)N(15)N-urea, followed by determination of urinary enrichment of (15)N(15)N-urea and (14)N(15)N-urea was used to determine urea kinetics. Forage and N intake increased (linear, P < 0.001) with increasing DIP. Retention of N was negative (-2.7 g/d) for steers receiving no DIP and increased linearly (P < 0.001; 11.7, 23.0, and 35.2 g/d for 59, 118, and 177 mg of N/kg of BW daily) with DIP. Urea synthesis was 19.9, 24.8, 42.9, and 50.9 g of urea-N/d for 0, 59, 118, and 177 mg of N/kg of BW daily (linear, P = 0.004). Entry of urea into the gut was 98.9, 98.8, 98.6, and 95.9% of production for 0, 59, 118, and 177 mg of N/kg of BW daily, respectively (quadratic, P = 0.003). The amount of urea-N entering the gastrointestinal tract was greatest for 177 mg of N/kg of BW daily (48.6 g of urea-N/d) and decreased (linear, P = 0.005) to 42.4, 24.5, and 19.8 g of urea-N/d for 118, 59, and 0 mg of N/kg of BW daily. Microbial incorporation of recycled urea-N increased linearly (P = 0.02) from 12.3 g of N/d for 0 mg of N/kg of BW daily to 28.9 g of N/d for 177 mg of N/kg of BW daily. Provision of DIP produced the desired and previously observed increase in forage intake while also increasing N retention. The large percentage of urea synthesis that was recycled to the gut (95.9% even when steers received the greatest amount of DIP) points to the remarkable ability of cattle to conserve N when fed a low-protein diet.     

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.     

Ruminal ammonia load affects leucine utilization by growing steers

Six ruminally cannulated Holstein steers (initial BW = 189 +/- 11 kg) housed in metabolism crates were used in a 6 x 6 Latin square to study effects of ruminal ammonia load on Leu utilization. All steers received a diet based on soybean hulls (2.7 kg of DM/d), ruminal infusions of 200 g of acetate/d, 200 g of propionate/d, and 50 g of butyrate/d, as well as an abomasal infusion of 300 g of glucose/d to provide energy without increasing microbial protein supply and an abomasal infusion of a mixture (238 g/d) of all essential AA except Leu. Treatments were arranged as a 3 x 2 factorial and included Leu (0, 4, or 8 g/d) infused abomasally and urea (0 or 80 g/d) infused ruminally. Abomasal Leu infusion linearly decreased (P < 0.05) both urinary and fecal N excretions and linearly increased (P < 0.05) retained N, but the decreases in urinary N excretion in response to Leu tended (P = 0.07) to be greater, and the increases in retained N in response to Leu were numerically greater in the presence of the urea infusion. Although urea infusions increased (P < 0.05) plasma urea concentrations, urinary N excretions, and urinary urea excretions, retained N also was increased (P < 0.05). The efficiency of deposition of supplemental Leu ranged from 24 to 43% when steers received 0 or 80 g of urea/d, respectively. Under our experimental conditions, increasing ammonia load improved whole-body protein deposition in growing steers when Leu supply was limiting.     

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.     

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.     

Effect of frequency and amount of rumen-degradable intake protein supplementation on urea kinetics and microbial use of recycled urea in steers consuming low-quality forage

We evaluated the effect of frequency and amount of rumen-degradable intake protein (DIP) on urea kinetics in steers consuming prairie hay. Five ruminally and duodenally fistulated steers (366 kg of BW) were used in a 5 x 5 Latin square and provided ad libitum access to low-quality prairie hay (4.7% CP). Casein was provided daily in amounts of 61 and 183 mg of N/kg of BW (61/d and 183/d) and every third day in amounts of 61, 183, and 549 mg of N/kg of BW per supplementation event (61/3d, 183/3d, and 549/3d). Periods were 18-d long with 9 d for adaptation and 9 d for collection. Steers were in metabolism crates for total collection of urine and feces. Jugular infusion of (15)N(15)N-urea followed by determination of urinary enrichment of (15)N(15)N-urea and (14)N(15)N-urea was used to determine urea kinetics. Treatment means were separated to evaluate the effects of increasing DIP supplementation and the effects of frequency at the low (61/d vs. 183/3d) and at the high (183/d vs. 549/3d) amounts of DIP provision. Forage OM and total digestible OM intakes were linearly (P < or = 0.05) increased by increasing DIP provision but were not affected by frequency of supplementation at either the low or high amounts. Production and gut entry of urea linearly (P < or = 0.006) increased with DIP provision and tended to be greater (P < or = 0.07) for 549/3d than 183/d but were not different between 61/d and 183/3d. Microbial N flow to the duodenum was linearly (P < 0.001) increased by increasing DIP provision. Additionally, 183/d resulted in greater (P = 0.05) microbial N flow than 549/3d. Incorporation of recycled urea-N into microbial N linearly (P = 0.04) increased with increasing DIP. Microbial incorporation of recycled urea-N was greater for 549/3d than 183/d, with 42 and 23% of microbial N coming from recycled urea-N, respectively. In contrast, there was no difference due to frequency in the incorporation of recycled urea-N by ruminal microbes at the low level of supplementation (i.e., 61/d vs. 183/3d). This study demonstrates that urea recycling plays a substantial role in the N supply to the rumen and to the animal, particularly in steers supplemented infrequently with high levels of protein.     

Supplemental methionine and urea for gestating beef cows consuming low quality forage diets

A study was conducted to evaluate Met requirements of late-gestation beef cows consuming low quality forages on the premise that inadequate supply of metabolizable AA may limit protein accretion during pregnancy. Five ruminally cannulated, multiparous late-gestation beef cows (490 +/- 27 kg), of predominantly Angus (> or =75%) with Hereford and Simmental breeding, were used in a 5 x 5 Latin square experiment to evaluate the effects of postruminal dl-Met supplementation on N retention, serum metabolites, and plasma AA concentrations during the third trimester of pregnancy. The basal diet was fed individually, and weights of refusals were recorded for N intake determination. Treatments consisted of no urea, urea (0.053 +/- 0.002 g/kg of BW daily), urea + 5 g of Met/d, urea + 10 g of Met/d, and urea + 15 g of Met/d. Cows were adapted to the experimental diet 30 d before the beginning of the study, with periods lasting for 14 d; 4 d to allow for clearance of the previous treatment effects, 4 d for adaptation to the treatments, and 6 d for total fecal and urine collection. Blood samples were collected every 4 h on d 13 of each period for analysis of serum metabolites and plasma AA. Inclusion of urea increased DM and OM intakes (urea vs. no urea; P = 0.05), but no further improvement in intake was observed with inclusion of Met. Serum urea concentrations increased with inclusion of urea (P = 0.03) and responded quadratically (P = 0.06) when Met was added, with the lowest concentration observed in the urea + 5 g of Met/d treatment. More N was retained with the inclusion of urea (P = 0.04), and N retention increased linearly (P = 0.07) with inclusion of Met. Plasma Met concentration increased linearly (P < 0.01) with inclusion of Met. These data suggest that Met was a limiting AA and that supplementation of a combination of urea and 5 g/d of rumen-protected Met to low quality, forage diets will improve N retention and promote protein accretion during late pregnancy.     

Metabolize methionine and lysine requirements of growing cattle

Two growth studies were conducted to determine the Met and Lys requirements of growing cattle. In each 84-d trial, steer calves were fed individually diets containing 44% sorghum silage, 44% corn cobs, and 12% supplement (DM basis) at an equal percentage of BW. In Trial 1, 95 crossbred steers (251 kg) were supplemented with urea or meat and bone meal (MBM). Incremental amounts of rumen-protected Met were added to MBM to provide 0, .45, .9, 1.35, 3, and 6 g/d metabolizable Met. In Trial 2, 60 steers (210 kg) were supplemented with urea or corn gluten meal (CGM). Incremental amounts of rumen-protected Lys were added to CGM to provide 0, 1, 2, 3, 4, 5, 6, 8, and 10 g/d metabolizable Lys. Supplementation with MBM and CGM increased the supply of metabolizable protein to the animal. Steers fed MBM plus 0 Met gained 49 g/d more than steers fed urea, whereas steers fed CGM plus 0 Lys gained 150 g/d more than steers fed urea. Supplementation of rumen-protected Met and Lys improved ADG in steers fed MBM and CGM, respectively (P < .10). Nonlinear analysis, comparing gain vs supplemental Met and Lys intake, predicted supplemental Met and Lys requirements of 2.9 and .9 g/d, respectively. This amount of additional Met promoted .13 kg/ d gain greater than MBM alone, and this amount of additional Lys promoted .10 kg/d gain greater than the CGM alone. Metabolizable Met and Lys requirements were predicted from Level 1 of NRC (1996) calculated metabolizable protein supply, amino acid analysis of abomasal contents, and the maximum response to supplemental AA. Steers gaining .39 kg/d required 11.6 g/ d Met or 3. 1% of the metabolizable protein requirement, whereas steers gaining .56 kg/d required 22.5 g/d Lys or 5.7% of the metabolizable protein requirement.     

Daily and alternate-day supplementation of urea or biuret to ruminants consuming low-quality forage: II. Effects on site of digestion and microbial efficiency in steers

Five steers (491 +/- 21 kg BW) were used in an incomplete 5 x 4 Latin square with four 24-d periods to determine the influence of supplemental non-protein N (NPN) source and supplementation frequency (SF) on nutrient intake and site of digestion in steers consuming low-quality grass straw (4% CP). Treatments (TRT) included an unsupplemented control and a urea- or biuret-containing supplement placed directly into the rumen daily (D) or every other day (2D) at 0700. The NPN treatments were formulated to provide 90% of the estimated degradable intake protein requirement. Daily TRT were supplemented CP at 0.04% of BW/d, whereas the 2D TRT were supplemented at 0.08% of BW every other day. Therefore, all supplemented TRT received the same quantity of supplemental CP over a 2-d period. Forage OM intake was not affected (P > 0.05) by NPN supplementation, NPN source, or SF; however, total OM and N intake were increased (P < 0.01) with CP supplementation. Duodenal flow of N was greater (P = 0.04) with CP supplementation compared with the control. In addition, duodenal bacterial N flow was increased with CP supplementation (P = 0.04) and for biuret compared with urea (P < 0.01). Bacterial efficiency (g bacterial N/kg OM truly digested in the rumen) was greater (P = 0.05) for biuret than for urea. Apparent total-tract N digestibility was increased with NPN supplementation (P < 0.01) but not affected by NPN source or SF. These results suggest that urea or biuret can be used effectively as a supplemental N source by steers consuming low-quality forage.     

Evaluation of models of acute and subacute acidosis on dry matter intake, ruminal fermentation, blood chemistry, and endocrine profiles of beef steers

Crossbred steers (n = 20; 316 +/- 4 kg BW), each fitted with a ruminal cannula, were used to evaluate the effects of acute acidosis (AA) and subacute acidosis (SA) on DMI, ruminal fermentation, blood chemistry, and endocrine profiles. Animals were blocked by BW and assigned to treatments including 1) intraruminal (via cannula) steam-flaked corn (3% of BW; AA); 2) intraruminal dry-rolled wheat:dry-rolled corn (50:50; 1.5% of BW; SA); 3) offering forage-adapted steers ad libitum access to a 50% concentrate diet (AA control; AC); and 4) offering 50% concentrate diet-adapted steers ad libitum access to a 50% concentrate diet (SA control; SC). Samples of ruminal fluid and whole blood were collected on the day of the challenge (d 0) and 3, 7, 10, and 14 d after the challenge. Daily DMI responded quadratically (P < 0.01) through d 7 for AA and SA steers and increased linearly (P < 0.01) for AC steers. Dry matter intake by AA steers reached a nadir (< 3 kg/d) on d 3 and gradually increased to a level similar to other treatments (7 kg/d) by d 10, whereas DMI by SA steers increased through d 3. Blood pH, bicarbonate, base excess, and total CO2 were decreased (P < 0.03) for AA steers and increased (P < 0.03) for SC steers through d 7. Ruminal pH decreased quadratically (P < 0.01) in AA and AC steers and increased (P = 0.01) in SA steers through d 7. Ruminal total lactate concentration and osmolality responded quadratically (P < 0.01) for AA and AC steers. Ruminal total lactate peaked on d 3 for AA steers and on d 0 for AC and decreased to basal concentrations by d 7. Plasma NEFA concentration increased (P < 0.04) on d 3 and 7 for AA steers. Serum Na decreased (P < 0.05) on d 0 for AA and SA steers and on d 7 and 14 for AA steers. Serum P decreased (P = 0.01) for AA steers through d 7 and decreased quadratically (P = 0.01) for AC steers through d 7. Serum albumin and cholesterol decreased (P < 0.02) for AA and AC steers through d 7. Area under the GH curve decreased (P = 0.02) for AA and AC steers through d 7. Considerable variation was evident in the ability of an animal to cope with a carbohydrate challenge. Results of data modeling generally suggest that serum amylase activity, cholesterol and potassium concentrations, and plasma NEFA concentrations were useful in distinguishing between steers classified as experiencing subacute acidosis or not affected by a carbohydrate challenge.     

Effect of glycine and vitamin supplementation on sulphur amino acid utilization by growing cattle

Effects of glycine (Gly) and B-vitamins on sulphur amino acid (AA) utilization were studied in growing steers maintained under conditions where methionine (Met) was first limiting. Conditions were generated by limit feeding a diet low in ruminally non-degraded protein and abomasally infusing an AA mixture limiting in Met. Retained N tended (p = 0.07) to improve when steers received 10 mg folate, 10 mg vitamin B6, and 0.10 mg vitamin B12 daily. Hepatic vitamin B12 (p = 0.08) and folate (p = 0.05) concentrations increased with vitamin supplementation. In another trial, factorial treatments were 2 or 5 g/day L-Met and 0 or 50 g/day Gly infused abomasally. Retained N increased (p < 0.05) in response to Met, and responses were numerically larger in the presence of supplemental Gly. In a different trial, factorial treatments were 0 or 2.4 g/day L-cysteine (Cys) and 0 or 40 g/day Gly. Retained N was not affected by Cys in the absence of Gly, but was increased by Cys when Gly was supplemented (interaction, p = 0.01). B-vitamin status may affect sparing of Met by Cys. Supplemental Gly improved responses to supplemental Met and Cys.     

Ruminal and host adaptations to changes in frequency of protein supplementation

The effect of altering supplementation frequency on host N balance and key N transactions in the ruminal ecosystem were monitored. Four ruminally fistulated beef steers (BW = 513 kg; SEM = 6.5) were used in a 2 x 2 crossover design with two periods and two supplementation frequency treatments. Supplementation frequencies were 2 and 7 d/wk. Steers were fed tallgrass prairie hay (73.1% NDF, 5.3% CP) ad libitum. Supplement (42% CP; DM basis) was fed at 0.36% BW/d to steers supplemented 7 d/wk. Steers supplemented 2 d/wk received the same amount of supplement per week, but it was equally split among the two supplementation events. Steers supplemented 7 d/wk had higher forage (P < 0.02) and total digestible OM intake (P < 0.06), total N intake, fecal N excretion, and N retention. Although both supplementation frequencies were characterized by positive N balance, the decrease in N retention in the steers supplemented 2 d/wk was due to higher (P < 0.01) urinary N loss. Ruminal fluid was sampled at 0, 2, 4, 6, 12, 24, 48, and 72 h after supplementation beginning on a day when both treatments were supplemented. Frequency x hour interactions (P < 0.02) were observed for ruminal N metabolism criteria. Counts of peptide- and AA-fermenting bacteria peaked at 2 h and returned to nadir by 12 h for steers supplemented 7 d/wk. Steers supplemented 2 d/wk peaked at 6 h with a greater population and returned to nadir at 48 h. Ruminal ammonia concentrations followed a similar trend. Specific activity of ammonia production was lower (P < or = 0.05) immediately after supplementation for steers supplemented 2 d/wk, but by 12 h was the same as for 7 d/wk steers. Ruminal peptides and free AA peaked at 2 h for steers supplemented 2 d/wk and were generally higher (P < or = 0.05) during the first 6 h compared with steers supplemented 7 d/wk. Total VFA concentration was not different (P = 0.35) due to supplementation frequency. Frequency x hour interactions (P < 0.01) were observed for all molar proportions of VFA. The molar proportion of acetate and acetate:propionate ratio were lower (P < 0.01) and the molar proportions of propionate and butyrate were higher for steers supplemented 2 d/wk from 4 h to 24 h. In conclusion, forage use and N balance improved with supplementation 7 d/wk, but supplementation 2 d/wk was associated with some desirable shifts in select ruminal events that may contribute to moderating potential negative impacts of supplementing infrequently.     

Effect of supplementation frequency and supplemental urea level on dormant tallgrass-prairie hay intake and digestion by beef steers and prepartum performance of beef cows grazing dormant tallgrass-prairie

Effect of supplementation frequency and supplemental urea level on forage use (Exp. 1) and performance (Exp. 2 and 3) of beef cattle consuming low-quality tallgrass-prairie were evaluated. For Exp. 1 and 2, a 2 x 2 factorial treatment structure was used, such that two supplements (30% CP) containing 0 or 30% of supplemental degradable intake protein (DIP) from urea were fed daily or on alternate days. In Exp. 1 and 2, supplement was fed at 0.41% BW daily or at 0.83% BW (DM basis) on alternate days. For Exp. 3, a 2 x 4 factorial treatment structure was used, such that four supplements (40% CP) containing 0, 15, 30, or 45% of supplemental DIP from urea were fed daily or 3 d/wk. Supplements were group-fed at 0.32% BW daily or at 0.73% BW (DM basis) 3 d/wk. In Exp. 1, 16 Angus x Hereford steers (initial BW = 252 kg) were blocked by BW and assigned to treatment. Urea level x supplementation frequency interactions were not evident for forage intake, digestion, or rate of passage. Forage OM intake (OMI) and total digestible OMI (TDOMI) were not significantly affected by treatment. Total-tract digestion of OM (P = 0.03) and NDF (P = 0.06) were greater for steers supplemented daily. In Exp. 2, 48 Angus x Hereford cows (initial BW = 490 kg) grazing winter tallgrass prairie were used. Significant frequency x urea interactions were not evident for BW and body condition (BC) change; similarly, the main effects were not substantive for these variables. In Exp. 3, 160 Angus x Hereford cows (initial BW = 525 kg) grazing dormant, tallgrass prairie were used. Supplement refusal occurred for cows fed the highest urea levels, particularly for cows fed the supplement with 45% of the DIP from urea 3 d/wk, and supplement refusal increased closer to calving. A frequency x urea interaction (P = 0.02) was observed for prepartum BW changes. As supplemental urea level increased, prepartum BW loss increased quadratically (P = 0.02); however, a greater magnitude of loss occurred when feeding supplements containing > or = 30% of DIP from urea 3 d/ wk. Cumulative BC change followed a similar trend. In conclusion, moderate protein (< or = 30% CP) supplements with < or = 30% of supplemental DIP from urea can be fed on alternate days without a substantive performance penalty. However, infrequent feeding of higher protein (> 30% CP) supplements with significant urea levels (> 15% of DIP from urea) may result in decreased performance compared with lower urea levels.     

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 slow-release urea on ruminal digesta characteristics and growth performance in beef steers

Two experiments were conducted to evaluate the effects of slow-release urea (SRU) versus feed-grade urea on ruminal metabolite characteristics in steers and DMI, gain, and G:F in growing beef steers. Experiment 1 used 12 ruminally cannulated steers (529 +/- 16 kg of BW) to monitor the behavior of SRU in the ruminal environment. Compared with feed-grade urea, SRU decreased ruminal ammonia concentration (P = 0.02) and tended to increase ruminal urease activity (P = 0.06) without affecting ruminal VFA molar proportions or total concentrations (P > 0.20). After 35 d of feeding, the in situ degradation rate of SRU was not different between animals fed urea or SRU (P = 0.48). Experiment 2 used 180 Angus-cross steers (330 +/- 2.3 kg) fed corn silage-based diets supplemented with urea or SRU for 56 d to evaluate the effects on feed intake, gain, and G:F. The design was a randomized complete block with a 2 x 4 + 1 factorial arrangement of treatments. Treatments included no supplemental urea (control) or urea or SRU at 0.4, 0.8, 1.2, or 1.6% of diet DM. Over the entire 56 d experiment, there were interactions of urea source x concentration for gain (P = 0.04) and G:F (P = 0.01) because SRU reduced ADG and G:F at the 0.4 and 1.6% supplementation concentrations but was equivalent to urea at the 0.8 and 1.2% supplementation concentrations; these effects were due to urea source x concentration interactions for gain (P = 0.06) and G:F (P = 0.05) during d 29 to 56 of the experiment. The SRU reduced DMI during d 29 to 56 (P = 0.01) but not during d 0 to 28, so that over the entire experiment there was no difference in DMI for urea source (P = 0.19). These collective results demonstrate that SRU releases N slowly in the rumen with no apparent adaptation within 35 d. Supplementation of SRU may limit N availability at low (0.4%) concentrations but is equivalent to urea at 0.8 and 1.2% concentrations.     

Limiting amino acids for growing Holstein steers limit-fed soybean hull-based diets

Studies were conducted to determine limiting amino acids (AA) for cattle limit-fed soybean hull-based diets. Ruminally cannulated Holstein steers were maintained in metabolism crates, fed the same basal diet (73% soyhulls, 19% alfalfa, DM basis), and given the same intraruminal infusions (400 g/d acetate; to supply energy without increasing microbial protein supply). Treatments were infused abomasally. In Exp. 1, steers (200 kg) were provided 1) water, 2) 10 g/d of methionine (MET), or 3) a mixture of 10 essential AA (10AA). Nitrogen retention (13.7 g/d) was greatest (P < .05) for steers receiving 10AA. Steers receiving MET (7.9 g/d) had greater (P < .05) N retention than control steers (5.4 g/d). In Exp. 2, steers (200 kg) were provided 10AA or 10AA with L-Lys deleted from the mixture. Steers receiving 10AA tended (P < .09) to have greater N retention (19.0 g/d) than those receiving no lysine (16.3 g/d). In Exp. 3, steers (194 kg) were provided 10AA or 10AA with L-Thr deleted from the mixture. Nitrogen retention was not affected by removal of threonine. In Exp. 4, steers (152 kg) were provided 10AA or 10AA with L-His, L-Trp, L-Arg, L-Phe, or branched-chain AA (L-Leu, L-Ile, and L-Val) removed. Nitrogen retention was reduced (P < .05) by removal of either L-His or the branched-chain AA. For steers limit-fed soybean hull-based diets, methionine was first-limiting; histidine, at least one of the branched-chain AA, and possibly lysine were also limiting.     

Effects of ruminal and postruminal infusion of starch hydrolysate or glucose on the microbial ecology of the gastrointestinal tract in growing steers

Forty crossbred steers were used to determine the effects of carbohydrate supply site on the indigenous bacteria of the gastrointestinal tract. Steers were fitted with ruminal and abomasal infusion catheters and assigned randomly to one of eight groups in a complete randomized block design. The experimental period was 36 d. Treatments included: 1) a pelleted basal diet fed at 0.163 Mcal ME x (kg BW(0.75)) x 1 x d(-1) (LE); 2) the basal diet fed at 0.215 Mcal ME x (kg BW(0.75)) (-1) x d(-1) (HE); 3) the basal diet fed at 0.163 Mcal ME x (kg BW(0.75))(-1) x d(-1) with ruminal infusion of starch hydrolysate (SH) (RSH); 4) the basal diet fed at 0.163 Mcal ME x (kg BW(0.75))(-1) x d(-1) with abomasal infusion of SH (ASH); and 5) the basal diet fed at 0.163 Mcal ME x (kg BW(0.75))(-1) x d(-1) with abomasal infusion of glucose (AG). The total volume ofinfusate (5 kg x site(-1) x d(-1)) was equalized across treatments and infusion sites by infusion of water. Glucose and SH were infused at rates of 14.35 and 12.64 g x (kg BW(0.75)) x d(-1), respectively. Ruminal, cecal, and fecal samples were obtained on d 36. Ruminal pH was low (5.79) in LE steers and unaffected (P > 0.10) by increased energy intake or carbohydrate infusion. Cecal and fecal pH were 6.93 and 7.00, respectively, for LE steers. Increasing energy intake (P < 0.10) and the rate of carbohydrate infusion (P < 0.01) significantly decreased cecal and fecal pH compared with LE. Ruminal counts of anaerobic bacteria in LE steers were 8.99 log10 cells/g and abomasal carbohydrate infusion had no affect (P > 0.10) on these numbers. However, ASH and AG steers had approximately 1.5 log10 cells/g more (P < 0.01) cecal and fecal anaerobic populations. Ruminal, cecal, and fecal aerobic bacterial counts were 40, 22, and 23%, respectively, lower than anaerobic counts. Generally, aerobic counts responded similarly to the anaerobic counts. Less than 1% of the anaerobic bacteria enumerated in the rumen, cecum, and feces were coliforms, and 97% of the coliforms were Escherichia coli. Carbohydrate infusions resulted in only numerical increases in fecal coliform and E. coli concentrations (P > 0.10). Fecal E. coli were highly acid sensitive in all steers, with less than 1% surviving a 1-h exposure to low pH (2.0). This suggests that cecal or fecal pH is not a good indicator of acid resistance, and it supports the concept that there are other factors that may induce acid resistance.