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.     

Subclinical ammonia toxicity in steers: effects on hepatic and portal-drained visceral flux of metabolites and regulatory hormones

Four calves (avg wt 161 kg) were surgically fitted with indwelling catheters in the femoral artery and femoral, portal, hepatic and mesenteric veins to study the effects of subclinical ammonia toxicity on portal-drained viscera (PDV) and hepatic (HEP) net flux of key metabolites and pancreatic hormones. Hyperammonemia was induced via administration of ammonium chloride (NH4Cl; 12 mumol.kg BW-1.min-1) via the femoral vein catheter for 240 min; infusions were preceded (PRE) and followed (POST) by 60- and 180-min control periods, respectively. Blood samples were obtained from the arterial catheters, and portal and hepatic vein catheters. Net flux rates were calculated by multiplying venoarterial differences by blood flow. Arterial plasma ammonia N peaked (P less than .01) at 327 micrograms/dl; hepatic ammonia extraction increased (P less than .01) from 10 to 23% during NH4Cl infusion. Arterial plasma glucose concentrations increased (P less than .05) during NH4Cl infusion (90.5 vs 82.6 mg/dl) concomitant with trends toward a reduction in net HEP glucose output. Portal-drained visceral release of insulin did not increase (P greater than .10) during NH4Cl infusion despite the steady rise in circulating glucose concentration; however, cessation of NH4Cl infusion resulted in a 109% increase (P less than .05) in PDV insulin release at +60 min POST. Plasma L-lactate, nonesterified fatty acids, urea N and glucagon concentrations and net fluxes were variable throughout the experiment. Results tend to indicate that hyperammonemia reduced hepatic glucose output and glucose-mediated pancreatic insulin release.     

Urea flux in beef steers: effects of forage species and nitrogen fertilization

The effects of two forage species and N levels on urea kinetics and whole-body N metabolism were evaluated in eight Angus steers (initial BW 217+/-15 kg). In a replicated, 4 x 4 Latin square design, steers were fed gamagrass (Tripsacum dactyloides L.) or switchgrass (Panicum virgatum L.), each of which had 56.2 (LO) or 168.5 (HI) kg of N fertilization per hectare. Diets provided adequate energy for 0.5 kg ADG. Nitrogen balance and urea kinetics were measured from d 22 to 27 of each period. Urine samples collected during intravenous infusion of bis 15N urea were used to calculate production and recycling of urea N from relative abundance of urea isotopomers. Jugular blood serum was analyzed for serum urea N (SUN). Gamagrass differed from switchgrass (P < 0.05) in daily DMI (4,273 vs 4,185 g), N intake (72 vs 67 g), DM digestibility (61.0 vs 63.6%), fecal N (30.6 vs 28.3 g/d), urine urea N (10.5 vs 8.0 g/d), and percentage of urinary N present as urea N (53.5 vs 40.0%). After adjustment for differences in N intake, fecal N still tended to be greater (P < 0.09) for gamagrass than for switchgrass. The LO differed from the HI (P < 0.01) in daily N intake (63 vs 76 g), DM digestibility (61.3 vs 63.3%), urine N (13.6 vs 25.9 g/d), and N retained as a percentage of N digested (57.3 vs 43.5%). Compared to switchgrass, gamagrass had greater SUN, N digestibility, and N digested as N level increased (forage x N level interactions, P < 0.05). As N level increased, N retention increased from 19.5 to 23.5 g/d in gamagrass and decreased from 20.5 to 18.1 g/d in switchgrass (interaction, P < 0.07). The HI group was greater than the LO intake group (P < 0.03) in endogenous production of urea N (44.4 vs 34.0 g/d), gut entry rate of urea N (31.6 vs 28.2 g/d), and the amount of urea N that re-entered the ornithine cycle (9.4 vs 7.9 g/d). However, the percentage of urea N entering the gastrointestinal tract that was recycled was constant among treatments (29.1%), indicating that almost 70% of the urea N that entered the gastrointestinal tract was potentially available for anabolic purposes of the steers as a component of microbial products that were absorbed or excreted in the feces. In summary, N levels affected N metabolism of steers more when they were fed gamagrass than when they were fed switchgrass. Although the absolute amounts of N moving through the system changed with variations in intake, the proportions remained similar, with a greater efficiency of N use at low N intakes.     

Influence of chlortetracycline and dietary protein level on visceral organ mass of growing beef steers

Thirty-two beef steers (285 +/- 3 kg BW) were used to determine the effects of chlortetracycline and dietary protein level on visceral tissue mass, chemical composition, intestinal morphology, and proliferation rate indices. Steers were allotted randomly by weight to a factorial arrangement of dietary treatments consisting of either 10 or 13% CP diets top-dressed with a corn meal carrier (500 g/d) containing either 0 or 350 mg of chlortetracycline. After 84 d, steers were slaughtered and visceral organs removed and separated. Rinsed wet tissue mass was recorded; total RNA, total DNA, tissue DM, and tissue N content were determined; and tissue sections were prepared for immunohistochemical analysis. Thin tissue sections were evaluated to determine crypt depth and villus height as well as proliferation rate by immunohistochemical detection of the nuclear antigen Ki67. Rumen and abomasum weights and small intestinal length were greater (P < 0.04) in steers fed the 13% CP diet than in those fed the 10% CP diet on both an absolute weight basis and a percentage of empty BW. Chemical composition of the small intestinal and ruminal segments were largely unaffected by increased dietary protein. Increasing the dietary CP also increased the villus height in duodenal (P = 0.02) and the crypt depth of jejunal (P = 0.03) sections. Dietary administration of chlortetracycline decreased (P < 0.01) small intestinal weight both on absolute and empty BW bases. Nitrogen and RNA concentrations of the small intestinal segments were unaffected (P > 0.1) by dietary administration of subtherapeutic levels of chlortetracycline; however, because of increases (P < 0.05), or tendencies for an increase (P < 0.1), in the tissue content of DNA, the ratio of N to DNA was decreased (P < 0.05) or tended to be decreased (P < 0.1) in the small intestinal segments of the chlortetracycline-treated animals. The observed decrease in small intestinal epithelial mass does not appear to be due to alterations in cell proliferation rate but rather cell size. Consistent with this finding, cell proliferation, as determined by Ki67 antigen staining, was not affected by dietary treatment. Chlortetracycline administration decreased small intestinal mass that may be a result of decreased cell size.     

Effects of growth hormone-releasing factor and feed intake on energy metabolism in growing beef steers: net nutrient metabolism by portal-drained viscera and liver.

Effects of growth hormone-releasing factor (GRF) and intake on net nutrient metabolism by portal-drained viscera (PDV) and liver were measured in six growing Hereford x Angus steers fed a 75% concentrate diet at two intakes in a split-plot design with 4-wk saline or GRF injection periods within 8-wk intake periods. Daily rations were fed as 12 equal meals delivered every 2 h. Steers were injected s.c. for 21 d with either saline or 10 micrograms/kg of (1-29)NH2 human GRF at 12-h intervals. Six hourly measurements of net nutrient flux (venous-arterial concentration different [VA] x blood flow) across PDV and liver were obtained 8 to 10 d after injections began. Energy and N balances were measured using respiration calorimetry during the last week of injections. Greater intake increased blood flow (P less than .01) and net visceral release or removal of most nutrients (P less than .10). Exceptions included a decrease (P less than .10) in net PDV glucose release with greater intake in saline-treated steers and a decrease (P less than .01) in net liver removal of lactate with greater intake. Treatment of steers with GRF decreased net liver removal of alpha-amino N (AAN; P less than .05) and ammonia N (NH3N; P less than .10) and release of urea N (UN; P less than .05), increased liver release of glutamate (P less than .05), and decreased net PDV release of NH3 N (P less than .10). Decreased liver extraction ratio for AAN in GRF-treated steers (P less than .01) implies a direct effect of GRF treatment on liver metabolism separate from changes in liver AAN supply. Proportions of body N retention not accounted for by net total splanchnic AAN release increased with GRF treatment. This suggests a change in peripheral utilization of dietary AAN supply or an increase in total splanchnic N retention.     

Effects of isoenergetic infusions of propionate and glucose on portal-drained visceral nutrient flux and concentrations of hormones in lambs maintained by total intragastric infusion

Three lambs were used in a repeated Latin square design to determine the influence of isoenergetic infusions of propionate or glucose on portal-drained visceral flux (PDV) of nutrients and concentrations of insulin, glucagon, growth hormone and prolactin. Lambs were fitted with appropriate catheters for blood sampling and maintained on total intragastric infusion of nutrients. Basal VFA, casein, mineral and vitamin infusions (isocaloric and isonitrogenous) were supplemented with an additional 22 +/- .5 kcal/h from propionate, glucose or a combination of propionate plus glucose. Ruminal fluid proportion and arterial blood concentration and PDV flux of propionate increased (P less than .10) by 17 mol/100 mol, .02 mM and 40 mmol/h, respectively, with infusion of an additional 61 mmol/h of propionate. Regression equations predicted that, on a net basis, 67% of ruminally infused propionate and 43% of abomasally infused glucose appeared in portal blood. Arterial L-lactate, beta-hydroxybutyrate and acetate concentrations, and beta-hydroxybutyrate flux were increased (P less than .10) by .34 mM, .20 mM, .50 mM and 4.2 mmol/h, respectively, with infusion of 33 mmol/h of added glucose. Net utilization of glucose by the PDV was approximately 4.4 mmol/h when no glucose was infused. Increased infusion of propionate resulted in a 22.2-micrograms/h increase in PDV flux of insulin (P less than .08) but had no effect on arterial insulin, glucagon and prolactin concentrations (P greater than .10). Arterial growth hormone increased by 3.8 ng/ml with increasing glucose infusion (P less than .08).(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of ruminal and postruminal carbohydrate infusion on visceral organ mass and adipose tissue accretion in growing beef steers

Forty crossbred beef steers (243 +/- 2 kg of BW) with ruminal and abomasal infusion catheters were used to test 2 hypotheses: 1) visceral mass is responsive to energy input and site of carbohydrate (CHO) infusion and 2) rate and site of adipose accretion are dependent on site of CHO infusion and complexity. Treatments included a pelleted, forage-based, basal diet fed at 161 (LI) or 214 (HI) kcal of ME/(kg of BW(0.75) x d), LI plus ruminal (R-SH) or abomasal (A-SH) infusion of a partial starch hydrolysate (SH), and LI plus abomasal infusion of glucose (A-G). The basal diet was fed in 12 equal portions daily at 2-h intervals, with starch and glucose infused over a 22-h period at rates of 12.6 and 14.4 g/(kg of BW(0.75) x d). After 35 d of infusion, steers were slaughtered; and visceral organ and adipose mass, subcutaneous adipose thickness over the 5th and 12th rib, and LM intramuscular fat concentration were determined. Total intake energy (IE) increased (P = 0.0001) with ME intake. Dietary IE was similar between LI and CHO treatments, but total IE increased (P < 0.001) with CHO infusion. Greater dietary ME intake and CHO infusion increased or tended (P < or = 0.09) to increase final BW and HCW. As a percentage of empty BW, total stomach complex, rumen, omasum, liver, pancreas, and kidney weights were greater (P < or = 0.05) for HI vs. LI. Stomach complex, rumen, pancreas, and kidney weights as a percentage of empty BW were greater (P < or = 0.05) for R-SH vs. A-SH. Compared with ASH, A-G increased (P < or = 0.02) total and mucosal weights from the 10-cm sections of the ileum. Increases in rumen mass were associated with no change or an increase in rumen total and mucosal DNA concentrations. Greater dietary ME tended (P = 0.06) to increase subcutaneous fat thickness at the 5th rib but did not affect alimentary adipose accretion on an empty BW basis. Omental and total alimentary adipose weights were increased (P < or = 0.04) by A-G compared with A-SH. Although SH infusion did not alter adiposity, there was a consistent numerical pattern in total alimentary and subcutaneous fat depots with CHO infusion (A-G > ASH > R-SH). Our findings demonstrate that increasing ruminal CHO supply results in a disproportionate increase in rumen mass, whereas increasing small intestinal CHO supply does not alter gastrointestinal organ mass. Small intestinal energy in the form of glucose resulted in greater adipose accretion, particularly the omental depot.     

Net nutrient flux in visceral tissues of lambs fed diets differing in supplemental nitrogen source.

Eight multicatheterized wethers (35.9 +/- .8 kg BW) were used in a replicated 4 x 4 Latin square design to measure N retention and net uptake and release of plasma metabolites across the portal-drained viscera (PDV), hepatic (HEP), and total splanchnic (TS) tissues in response to changes in supplemental N source. Treatments selected to provide different amounts of undegradable intake protein (UIP) were urea, soybean meal (SBM), poultry by-product meal (PBM), and bloodmeal:corn gluten meal (BMCGM; 50:50 CP basis). Diets (urea, SBM, PBM, and BMCGM) contained 12.9, 13.8, 13.6, and 13.2% CP, respectively. Periods were 10 d, with total feces and urine collected on d 7 to 10 and blood sampled on d 10. Wethers were fed at 2% of BW in 12 daily portions. Nitrogen retention was 2.2, 3.3, 4.1, and 4.4 g/d for urea, SBM, PBM, and BMCGM, respectively. Urea had less (P < .01) N retention than SBM, PBM, and BMCGM; SBM had less N retention (P < .01) than PBM and BMCGM. Arterial, portal, and hepatic plasma flows were greater (P < .09) for SBM than for PBM and BMCGM (21 vs 16, 17; 84 vs 72, 72; 105 vs 87, 88 L/h). Portal plasma flow was greater (P < .10) for urea than for SBM, PBM, and BMCGM (85 vs 84, 72, 72 L/h). Portal-drained viscera and TS alpha-amino N (AAN) fluxes were less (P < .05) for PBM than for BMCGM (20.5 vs 26.6 and 7.2 vs 15.1 mmol/h), but TS AAN flux was less (P < .05) for urea than for SBM, PBM, and BMCGM (6.9 vs 16.9, 7.2, 15.1 mmol/h). Portal-drained viscera flux and HEP removal of NH3 N were greater (P < .001) for SBM than for PBM and BMCGM (27.7 vs 19.4, 20.6; -28.1 vs -20.0, -21.4 mmol/h). Gut use was less (P = .07) and HEP and TS fluxes of urea N were greater (P < .01) for SBM than for PBM and BMCGM (-4.92 vs -8.32, -7.93; 25.87 vs 16.54, 20.00; 20.95 vs 8.22, 12.07 mmol/h). These data suggest that PBM and BMCGM improved efficiency of N use compared with urea and SBM by reducing urinary N loss.     

Effects of phase-feeding of crude protein on performance, carcass characteristics, serum urea nitrogen concentrations, and manure nitrogen of finishing beef steers

As cattle mature, the dietary protein requirement, as a percentage of the diet, decreases. Thus, decreasing the dietary CP concentration during the latter part of the finishing period might decrease feed costs and N losses to the environment. Three hundred eighteen medium-framed crossbred steers (315 +/- 5 kg) fed 90% (DM basis) concentrate, steam-flaked, corn-based diets were used to evaluate the effect of phase-feeding of CP on performance and carcass characteristics, serum urea N concentrations, and manure characteristics. Steers were blocked by BW and assigned randomly to 36 feedlot pens (8 to 10 steers per pen). After a 21-d step-up period, the following dietary treatments (DM basis) were assigned randomly to pens within a weight block: 1) 11.5% CP diet fed throughout; 2) 13% CP diet fed throughout; 3) switched from an 11.5 to a 10% CP diet when approximately 56 d remained in the feeding period; 4) switched from a 13 to an 11.5% CP diet when 56 d remained; 5) switched from a 13 to a 10% CP diet when 56 d remained; and 6) switched from a 13 to an 11.5% CP diet when 28 d remained. Blocks of cattle were slaughtered when approximately 60% of the cattle within the weight block were visually estimated to grade USDA Choice (average days on feed = 182). Nitrogen volatilization losses were estimated by the change in the N:P ratio of the diet and pen surface manure. Cattle switched from 13 to 10% CP diets with 56 d remaining on feed or from 13 to 11.5% CP with only 28 d remaining on feed had lower (P < 0.05) ADG, DMI, and G:F than steers fed a 13% CP diet throughout. Steers on the phase-feeding regimens had lower (P = 0.05) ADG and DMI during the last 56 d on feed than steers fed 13.0% CP diet throughout. Carcass characteristics were not affected by dietary regimen. Performance by cattle fed a constant 11.5% CP diet did not differ from those fed a 13% CP diet. Serum urea N concentrations increased (P < 0.05) with increasing dietary CP concentrations. Phase-feeding decreased estimated N excretion by 1.5 to 3.8 kg/steer and nitrogen volatilization losses by 3 to 5 kg/steer. The results suggest that modest changes in dietary CP concentration in the latter portion of the feeding period may have relatively small effects on overall beef cattle performance, but that decreasing dietary CP to 10% of DM would adversely affect performance of cattle fed high-concentrate, steam-flaked, corn-based diets.     

Influence of protein degradability in range supplements on abomasal nitrogen flow, nitrogen balance and nutrient digestibility

Two metabolism trials were conducted with yearling steers fed mature native forage to measure the effect of supplemental protein degradability on selected metabolic variables. Supplements contained 40% crude protein equivalence. In Trial 1, four abomasal-cannulated steers weighing 290 to 379 kg were fed supplements containing the following N sources: (1) 15% corn, 85% urea (U); (2) 100% soybean meal (SBM); (3) 10% corn, 40% soybean meal, 50% urea (SBM-U) and (4) 14% corn, 36% blood meal, 50% urea (BM-U). Equal portions of the daily diet (2.2% of body weight) were fed every 2 h. Treatment differences were not significant for organic matter digestibility, abomasal organic matter flow, nonammonia N flow, feed N flow, bacterial N flow and efficiency of microbial protein synthesis. There was a positive (P less than .05) relationship between quantity of slowly degraded protein fed and nonammonia N flow (r = .97) or feed N flow (r = .98). Escape N was determined to be 21.5, 16.5 and 54.2% in SBM, SBM-U and BM-U supplements, respectively. In the second trial, no supplement, SBM, SBM-U and BM-U were fed in a N balance trial. Dry matter, crude protein and neutral detergent fiber digestibilities were higher (P less than .05) for steers fed supplemented diets. Acid detergent fiber digestibility was higher (P less than .05) for steers supplemented with SBM than steers fed the unsupplemented diets. Nitrogen retention was greater (P less than .05) for cattle fed SBM and BM-U than for cattle fed SBM-U or no supplement.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of crude protein concentration and degradability on performance, carcass characteristics, and serum urea nitrogen concentrations in finishing beef steers

Two experiments were conducted at two locations to determine the effects of dietary CP concentration and source on performance, carcass characteristics, and serum urea nitrogen (SUN) concentrations of finishing beef steers. British x Continental steers were blocked by BW (357 +/- 28 and 305 +/- 25 kg initial BW; n = 360 and 225; four and five pens per treatment in Exp. 1 and 2, respectively). Steam-flaked corn-based diets were arranged in a 3 x 3 factorial with three CP concentrations (11.5, 13, or 14.5% of DM) and three sources of supplemental CP (N basis): 100% urea; 50:50 blend of urea and cottonseed meal; or 100% cottonseed meal. Steers in both experiments were initially implanted with Ralgro and reimplanted with Revalor-S on d 56. Performance and carcass data were pooled across locations. Crude protein concentration x source interactions were not observed (P = 0.22 to 0.93) for performance and carcass data. Crude protein concentration affected ADG (P = 0.02) and carcass-adjusted (to a common dressing percent within location) ADG quadratically (P = 0.06). Increasing the concentration of supplemental urea linearly increased carcass-adjusted ADG and G:F (P < 0.05) and carcass-adjusted G:F (P < 0.001). Dry matter intake was not affected (P = 0.93) by either CP concentration or source. Hot carcass weight (HCW; P = 0.02), LM area (P = 0.05), and dressing percent (P = 0.03) increased linearly with increasing urea concentration, whereas increasing CP concentration quadratically affected HCW (P = 0.02), with a maximum at 13% CP. Differences in backfat thickness and yield grade were negligible across treatments. Neither marbling score nor percentage of carcasses grading USDA Choice was affected by CP concentration or source. At all times measured, SUN concentrations increased (P < 0.05) with increasing CP concentration, but effects of CP source were small and variable across time. Results indicate that increasing CP concentrations from 11.5 to 13% slightly increased ADG and carcass-adjusted ADG, whereas increasing the proportion of supplemental urea increased carcass-adjusted ADG, G:F, and carcass-adjusted G:F and increased HCW, LM area, and dressing percent. A CP concentration above 13% seemed detrimental to ADG and HCW. Serum urea N increased over time, with CP concentration having a greater effect than CP source.     

乙酸钠对西门塔尔牛日粮能量平衡和氮平衡的影响

选用4头装有永久性瘤胃瘘管,年龄3.5岁,体重约500kg,体况良好的西门塔尔阉牛。采用4×4拉丁方设计,以混合精料和玉米秸秆为基础日粮,研究乙酸钠(0、200、400和600g/d)对西门塔尔牛日粮能量平衡和氮平衡的影响。结果表明:400g/d组和600g/d组消化能、代谢能、沉积能显著高于对照组和200g/d组(P<0.05),其中200g/d组沉积能显著高于对照组(P<0.05);400g/d组和600g/d尿能、消化能/总能、代谢能/总能、沉积能/消化能显著高于对照组(P<0.05);各处理组总能、气体能、产热量差异不显著(P>0.05);400g/d组和600g/d组粪氮含量显著降低(P<0.05);尿氮含量差异不显著(P>0.05);400g/d组和600g/d组可消化氮和沉积氮含量、氮利用率显著高于对照组和200g/d组(P<0.05),400g/d组显著高于600g/d组(P<0.05)。由此推断日粮中乙酸钠的适宜添加量为400g/d。     

柠檬酸对西门塔尔牛日粮能量平衡和氮平衡的影响

选用4头年龄4岁、体况良好、平均体重500kg,装有永久性瘤胃瘘管的中国西门塔尔牛阉牛,采用4×4拉丁方设计．以混合精料和玉米秸秆为基础日粮,研究柠檬酸（0、100、200和300g／d）对日粮养分表观消化率的影响。结果表明：日粮添加柠檬酸200州组代谢能、沉积能、代谢能／总能、沉积能／消化能和沉积氮较对照组显著提高（P〈0．05）,沉积氮／消化氮较对照组极显著提高（P〈0．01）。根据试验结果推断日粮中柠檬酸的适宜添加水平为200g/d。     

Effects of feeding endophyte-infected fescue hay on portal and hepatic nutrient flux in steers

Six Holstein steers (313 +/- 10 kg BW) surgically fitted with hepatic portal, mesenteric venous, mesenteric arterial, and hepatic venous catheters were used in a replicated crossover design experiment to evaluate the feeding of Acremonium coenophialum-infected fescue hay on portal-drained visceral and hepatic nutrient metabolism. Only four steers had functional hepatic catheters. Infected (INF) and endophyte-free (EF) fescue hays were harvested on the same day in May, at the soft dough stage of maturity, from a similar location in southeast Kansas. The hay was chopped through a 2.5-cm screen and fed in 12 portions daily. Intake was limited to 5.2 kg of DM/d to equalize consumption. Each experimental period lasted 21 d. Dietary CP concentration was greater for INF than for EF (9.9 vs 8.6%); however, apparent digestibilities of DM (52.6%) and N (37%) were not different. Ruminal total VFA concentrations and molar proportions were not different with the exception of butyrate, which was increased (P less than .10) for steers when they were fed INF. Feeding of INF increased (P less than .05) arterial beta-hydroxybutyrate concentration and decreased (P less than .10) arterial butyrate concentration. Steers fed EF showed a greater (P less than .05) portal-arterial concentration difference for acetate and an increased (P less than .05) net portal flux of acetate (500 vs 620 mmol/h). No differences in net flux were noted for any of the other VFA, glucose, lactate, urea N, insulin, glucagon, or prolactin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of increasing ruminal butyrate on portal and hepatic nutrient flux in steers.

Six Holstein steers (mean +/- SE BW = 344 +/- 10 kg) fitted with hepatic, portal, and mesenteric vein and mesenteric artery catheters and a ruminal cannula were used in a 6 x 6 Latin square design to evaluate the effects of increasing ruminal butyrate on net portal-drained visceral and hepatic nutrient flux. Steers were fed a 40% brome hay, 60% concentrate diet in 12 portions daily at 1.25 x NEm. Water (control) or butyrate at 50, 100, 150, 200, or 250 mmol/h was supplied continuously via the ruminal cannula. Simultaneous arterial, portal, and hepatic blood samples were taken at hourly intervals from 15 to 20 h of ruminal infusion. Portal and hepatic blood flow was determined by continuous infusion of P-aminohippurate, and net nutrient flux was calculated as the difference between venous and arterial concentrations times blood flow. Ruminal and arterial concentrations and total splanchnic flux of butyrate increased (P less than .01) with increased butyrate infusion. Arterial concentrations of acetate (P less than .10), alpha-amino-N (P less than .05), and glucose (P less than .01) decreased with increased butyrate, whereas arterial beta-hydroxybutyrate (P less than .01) and acetoacetate (P less than .05) increased. Increased butyrate produced an increased portal-drained visceral flux of acetoacetate and an increased net hepatic flux of beta-hydroxybutyrate. Urea N and glucose net portal and hepatic fluxes were not affected by ruminal butyrate. Alpha-amino-N uptake by the liver decreased with increased butyrate (P less than .10). Simple linear regression (r2 = .985) indicated that 25.8% of ruminally infused butyrate appeared in portal blood as butyrate. Only 14% could be accounted for as net portal-drained visceral flux of acetoacetate plus beta-hydroxybutyrate.     

Effects of dietary energy level and supplemental protein source on performance of growing steers and nutrient digestibility and nitrogen balance in lambs

Four experiments were conducted to evaluate three crude protein (CP) sources (urea, U; soybean meal, SBM; corn gluten meal, CGM) in diets based on corn silage (high energy) or grass hay (low energy). In Exp. 1 and 2, growing steers were fed all combinations of energy and protein source at 10.5 or 12% CP. Steers fed high energy diets or 12% CP had improved (P less than .05) daily gains and feed:gain over 84 d. Protein source had no effect (P greater than .05) on performance except that steers fed U consumed more (P less than .05) feed than those fed CGM. Steers were fed experimental diets to a common weight and switched to an 85% concentrate diet for finishing. During finishing, steers fed low energy diets in the growing period consumed more (P less than .05) feed and had increased (P less than .05) feed:gain compared with those fed high energy diets. Growing lambs were fed the same diets as steers. At 10.5% CP, lambs fed high energy diets had higher (P less than .05) digestibilities of dry matter (DM), organic matter (OM), nitrogen (N) and fiber components, and retained more (P less than .05) N. For lambs on 12% CP, high energy diets had higher (P less than .05) DM and OM digestibilities and lower (P less than .05) N digestibilities. At 12% CP, energy level had no effect (P greater than .05) on N retained. Protein source had no effect (P greater than .05) on N retention. There appeared to be no advantage in supplementing with ruminally undegradable proteins, i.e. CGM, in these experiments.     

Effect of nitrogen supplementation and zilpaterol-HCl on urea kinetics in steers consuming corn-based diets

We studied effects of zilpaterol-HCl on steers consuming corn-based diets with nitrogen (N) supplementation provided by dried distillers grains with solubles (DDGS) or urea. Two sets of six steers (approximately 350 kg) were used in two replicates of similarly designed trials. Within each replicate, three steers were fed 60 mg/day of zilpaterol-HCl throughout the trial and three steers received no zilpaterol-HCl. Within zilpaterol treatment, three corn-based dietary N treatments were offered in Latin square designs: control (9.6% crude protein), urea (UREA; 12.4% crude protein) or DDGS (13.7% crude protein). Total feed intake was unexpectedly greater (p < 0.01) with zilpaterol feeding but was not affected by dietary N (p = 0.76). Nitrogen intake was greater (p < 0.01) when zilpaterol was fed and was greater (p < 0.05) for DDGS and UREA than for control. Despite greater N intake, zilpaterol did not affect urea entry rate (p = 0.80) or urea-N recycled to the gastrointestinal tract (GER; p = 0.94). As a percentage of N intake, urea entry rate (p = 0.19) tended to be less when zilpaterol was fed (91 vs. 123% of N intake), and GER was numerically (p = 0.34) less (72 vs. 92% of N intake). Microbial N flow was greater (p = 0.02) for zilpaterol than for control but did not differ (p = 0.78) among dietary N treatments. As a percentage of N intake, microbial N flow was unaffected by zilpaterol (p = 0.97), but was greater (p < 0.05) for control than DDGS or UREA. The lack of change in urea entry and GER in response to zilpaterol, despite greater N intake, as well as lower urea entry and GER when expressed as proportions of N intake provide some evidence that the amount of N available for urea production and recycling was reduced by zilpaterol.     

The effect of dietary nitrogen and protein on feed intake, nutrient digestibility, and nitrogen flux across the portal-drained viscera and liver of sheep consuming high-concentrate diets ad libitum

Our objectives were to determine the influences of supplemental nonprotein N or protein on feed intake, digestibility, and postabsorptive N metabolism in sheep fed a high-concentrate diet for ad libitum consumption. Nine Romanov-sired, crossbred wethers (13 mo old; 52 kg) were fitted with catheters in a mesenteric artery, mesenteric vein, portal vein, and hepatic vein. Wethers consumed a 95% concentrate diet ad libitum. Treatments consisted of control (no supplemental N; 6.6% CP) or supplemental urea (11.4% CP), soybean meal (SBM; 11.2% CP) or ruminally undegradable protein (BFM; 11.2% CP; 50:50 blood meal and feather meal). Intake or apparently digested intake of DM, OM, and energy did not differ between control and N-supplemented (P > 0.40), or between urea- and protein-supplemented (P > 0.40), but were greater (P < 0.05) in SBM- than in BFM-supplemented wethers. Intake and apparently digested intake of N were less (P < 0.01) in wethers fed the control diet than in those receiving N supplementation but were less (P = 0.03) in BFM- than in SBM-supplemented wethers. Neither portal nor hepatic venous blood flows differed (P > 0.15) among treatments. Net portal release and hepatic uptake of alpha-amino N and ammonia N and hepatic release of urea N were greater (P < 0.05) in wethers supplemented with N than in controls, but portal-drained viscera (PDV) uptake of urea N did not differ (P > 0.40) among diets. Splanchnic release of a-amino N and ammonia N did not differ from 0 or among diets (P > 0.10), but net release of urea N was less (P = 0.05) for control than for sheep receiving N supplementation. No differences (P > 0.10) in blood concentration within vessel or net flux across PDV, hepatic, or splanchnic tissues of alpha-amino N, ammonia N, or urea N were observed among wethers receiving supplemental N. Net uptake of oxygen by the PDV did not differ among diets, but hepatic uptake was less (P < 0.05) in control and urea-supplemented sheep than in sheep receiving SBM or BFM. These observations suggest that the source of supplemental N had no large effects on the overall N economy of the animals used in this study.     

Influence of oscillating dietary crude protein concentration on performance, acid-base balance, and nitrogen excretion of steers

Decreasing dietary N inputs into beef cattle feeding operations could potentially decrease environmental concerns relating to air and water quality. Previous studies with sheep suggest that oscillating dietary CP concentrations may improve N use efficiency and thereby decrease dietary N requirements. Therefore, two studies were conducted to determine the effects of oscillating dietary CP concentrations on performance, acid-base balance, and manure characteristics of steers fed high-concentrate diets. Steers were fed to a constant backfat thickness in both studies. In the first trial, 92 steers (mean BW = 408 +/- 2.8 kg; four pens/treatment) were fed the following diets: 1) constant 12% CP, 2) constant 14% CP, and 3) 10 and 14% CP oscillated at 2-d intervals. Steer performance and carcass characteristics were measured. In the second trial, 27 steers were individually fed the same three experimental dietary regimens (nine steers/treatment). Animal performance, arterial acid-base balance, plasma metabolites, and fecal characteristics were measured. In both trials, steers fed the 14% CP diet tended (P < 0.10) to have greater ADG and gain:feed than steers fed the 12% CP diet. Steers fed the oscillating CP regimen had intermediate performance. In Trial 1, steers fed the 14% CP diet tended (P = 0.09) to have smaller longissimus area and higher quality grades than steers fed the oscillating CP regimen. Protein retentions (g/d) calculated from NRC (2000) equations were greater (P = 0.04) for steers fed the 14% CP diet than steers fed the 12% CP diet. Steers fed the oscillating CP regimen tended (P = 0.08) to have greater calculated protein retention (g/d) than steers fed the 12% CP diet. Steers fed the 14% CP diet had greater (P < 0.05) calculated urinary N excretion than steers fed the 12% CP or oscillating CP regimens. Venous plasma concentrations of urea N were greater (P < 0.001) in steers fed the 14% CP diet than in steers fed the 12% CP diet; steers fed the oscillating CP regimen were intermediate but fluctuated over days. Based on arterial blood gas concentrations, acid-base balance was not significantly affected by dietary CP regimen. Results of these trials suggest that the CP requirement of steers in these studies was greater than 12% of the diet DM, and/or that the degradable CP requirement was greater than 6.3% of diet DM. However, the effects of oscillating dietary CP were minimal.