Effects of dietary protein and oathull fiber on nitrogen excretion patterns and postprandial plasma urea profiles in grower pigs

The objectives of this study were: 1) to determine if dietary protein reduction or oathull fiber inclusion would reduce urinary N excretion in grower pigs, 2) to determine if plasma urea could predict urinary N excretion among diets differing in protein and fiber content with an expected range in N excretion patterns, and 3) to determine the postprandial time point to sample blood for the best prediction. Three dietary protein concentrations (high, 19.7; medium, 16.9; low, 13.8%) and two fiber levels (high, 5.0; low, 3.6% crude fiber) were tested in a 3 x 2 factorial arrangement. Diets (wheat, barley, soybean meal; oathulls as fiber source) were formulated to 3.25 Mcal of digestible energy (DE)/kg and 2.2 g of digestible lysine/Mcal DE for low- and medium-protein diets, and 2.4 g/Mcal of DE for high-protein diets, and supplemented with lysine, methionine, tryptophan, threonine, isoleucine, or valine to meet an ideal amino acid profile. Pigs (32 +/- 3.4 kg; n = 42) were housed in metabolism crates for 19 d. On d 10 or 11, catheters were installed by cranial vena cava venipuncture. Daily feeding allowance was adjusted to 3x maintenance (3 x 110 kcal DE/kg body weight(0.75)), and was fed in two equal meals. Feces and urine were collected from d 15 to 19. Five blood samples were collected in 2-h intervals on d 16 and 19. Fecal, urinary, and total N excretion was reduced linearly with a reduction of dietary protein (P < 0.001); the reduction was greater for urinary (48%) and total N excretion (40%) than for fecal N excretion (23%). Similarly, the ratio of urinary to fecal N was reduced linearly with a reduction of dietary protein (P < 0.001). Retention of N (g/d) was reduced linearly, but N retention as a percentage of N intake was increased linearly with a reduction of dietary protein (P < 0.001). The addition of oathulls did not affect N excretion patterns and plasma urea (P > 0.10). Dietary treatments did not affect average daily gain or feed efficiency (P > 0.10). A dietary protein x time interaction affected plasma urea (P < 0.001). For medium- and high-protein diets, plasma urea increased postprandially, peaking 4 h after feeding, and then decreased toward preprandial levels (P < 0.05). Plasma urea did not alter postprandially for the low-protein diet (P > 0.10). Urinary N excretion (g/d) was predicted by 3.03 + 2.14 x plasma urea concentration (mmol/L) at 4 h after feeding (R2 = 0.66). Plasma urea concentration is indicative of daily urinary N excretion and reduction of dietary protein is effective to reduce total and urinary N excretion.     

Effects of dietary protein and fermentable fiber on nitrogen excretion patterns and plasma urea in grower pigs

Effects of dietary protein concentration (high, 18.5; low, 15.7%) and fermentable fiber (control; soyhulls, SH; and sugar beet pulp, SBP) on N excretion patterns and plasma urea were tested in a 2 x 3 factorial arrangement. The objectives were: 1) to determine if reduced dietary protein together with fermentable fiber would reduce urinary N excretion further than a single diet manipulation, 2) to determine if effects of diet manipulations were similar between pigs with restricted and free access of feed, and 3) to further develop predictions of urinary N excretion using plasma urea. Diets were formulated to 3.30 Mcal digestible energy (DE)/kg and 2.4 g of digestible lysine per Mcal DE, and supplemented with lysine, methionine, tryptophan, threonine, isoleucine, leucine, or valine to ensure meeting an ideal AA profile. Pigs (30.5 +/- 3 kg; n = 36) were housed in metabolism crates with restricted access to feed (3 x 110 kcal DE/kg BW(0.75)) from d 1 to 18, and free access from d 19 to 26. Feces and urine were collected from d 15 to 18 and d 23 to 26, and blood was sampled on d 17 and 25. With restricted access to feed, urinary N was reduced 28% and N retention was reduced 12% for the low- compared to high-protein diet (P < 0.01; as g/d). Fecal N was increased 4% units for SH and 6.5% units for SBP (P < 0.01; as % of N intake) and urinary N was reduced 5% units for SH (P < 0.10) and 9% units for SBP (P < 0.05) compared to the control. With free access to feed, urinary N was reduced 27% (P < 0.05; as g/d) and N retention was reduced 7% (P < 0.10) for the low- compared to high-protein diet. Fecal N was increased 5% units for SH and 9% units for SBP (P < 0.001; as % of N intake), and urinary N was reduced 9% units for SH and 10% units for SBP (P < 0.01) compared to the control. For either restricted or free access to feed, fermentable fiber did not affect N retention (P > 0.10). A protein x fiber interaction was not observed for urinary N excretion (P > 0.10), indicating that reducing dietary protein and including fermentable fiber reduced urinary N excretion in an additive manner. Daily urinary N excretion was related positively and linearly with plasma urea in pigs with free access to feed (R2 = 0.71; at 0800). In summary, reduction of dietary protein reduced urine N excretion, and fermentable fiber shifted N excretion from urine to feces. Effects of dietary protein and fermentable fiber on reducing urinary N excretion are additive.     

Using blood urea nitrogen to predict nitrogen excretion and efficiency of nitrogen utilization in cattle, sheep, goats, horses, pigs, and rats

The objectives of this study were to evaluate the potential for using blood urea N concentration to predict urinary N excretion rate, and to develop a mathematical model to estimate important variables of N utilization for several different species of farm animals and for rats. Treatment means (n = 251) from 41 research publications were used to develop mathematical relationships. There was a strong linear relationship between blood urea N concentration (mg/100 mL) and rate of N excretion (g x d(-1) x kg BW(-1)) for all animal species investigated. The N clearance rate of the kidney (L of blood cleared of urea x d(-1) x kg BW(-1)) was greater for pigs and rats than for herbivores (cattle, sheep, goats, horses). A model was developed to estimate parameters of N utilization. Driving variables for the model included blood urea N concentration (mg/100 mL), BW (kg), milk production rate (kg/d), and ADG (kg/d), and response variables included urinary N excretion rate (g/d), fecal N excretion rate (g/d), rate of N intake (g/d), and N utilization efficiency (N in milk and gain per unit of N intake). Prediction errors varied widely depending on the variable and species of animal, with most of the variation attributed to study differences. Blood urea N concentration (mg/100 mL) can be used to predict relative differences in urinary N excretion rate (g/d) for animals of a similar type and stage of production within a study, but is less reliable across animal types or studies. Blood urea N concentration (mg/100 mL) can be further integrated with estimates of N digestibility (g/g) and N retention (g/d) to predict fecal N (g/d), N intake (g/d), and N utilization efficiency (grams of N in milk and meat per gram of N intake). Target values of blood urea N concentration (mg/100 mL) can be backcalculated from required dietary N (g/d) and expected protein digestibility. Blood urea N can be used in various animal species to quantify N utilization and excretion rates.     

Nitrogen metabolism and recycling in Holstein heifers

To study the effect of dietary N level on urea kinetics and recycling, four Holstein heifers (267 +/- 3.6 kg) were used in a Youden square design. Isocaloric diets with a N content of 1.44, 1.89, 2.50, 2.97, and 3.40% were fed at approximately 1.8 times maintenance intake. Increasing the N content of the diet increased urinary N excretion (P < 0.001) and N balance (P < 0.01), but did not affect the fecal N excretion (P = 0.21). Increasing the level of dietary N, increased urea production (P < 0.001) and excretion (P < 0.001), but no effect (P = 0.24) could be detected in the amount of N recycled to the gut. Urea recycled with the saliva, however, increased (P < 0.001) both in absolute and relative terms, with increasing dietary N. No difference could be detected on the amount of recycled N that was used for anabolism or returned to the ornithine cycle, but less (P = 0.001) N originating from urea was excreted in feces as dietary N increased. Ruminal ammonia concentration increased (P < 0.001) with increasing N intake, but total tract neutral detergent fiber digestibility was depressed only on the lowest N intake diet. No difference (P = 0.30) was detected in ruminal microbial yield among diets, but more (P < 0.003) N was derived from blood urea at low N intakes, and the efficiency of use of the recycled N decreased (P < 0.001) with increasing levels of dietary N. Adaptive changes to low-N diets were a decrease (P < 0.003) in the renal clearance of urea and an increase (P < 0.001) in the gastrointestinal clearance of urea. Urea transporters were present in the rumen wall of the heifers and differentially expressed depending on dietary N content, but their role in the transfer of urea into the rumen remains uncertain. Different mechanisms of N salvage and recycling were involved when animals were fed low-N diets that ensured a supply of endogenous N to the gastrointestinal tract and, due to the reduced contribution of dietary N, an increased efficiency of the N recycled was observed.     

Influence of slow-release urea on nitrogen balance and portal-drained visceral nutrient flux in beef steers

Two experiments were conducted to evaluate the effects of slow-release urea (SRU) versus feed-grade urea on portal-drained visceral (PDV) nutrient flux, nutrient digestibility, and total N balance in beef steers. Multi-catheterized steers were used to determine effects of intraruminal dosing (Exp. 1; n = 4; 319 +/- 5 kg of BW) or feeding (Exp. 2; n = 10; 4 Holstein steers 236 +/- 43 kg of BW and 6 Angus steers 367 +/- 46 kg of BW) SRU or urea on PDV nutrient flux and blood variables for 10 h after dosing. Intraruminal dosing of SRU (Exp. 1) prevented the rapid increase in ruminal ammonia concentrations that occurred with urea dosing (treatment x time P = 0.001). Although apparent total tract digestibilities of DM, OM, NDF, and ADF were not affected by treatment (P > 0.53, Exp. 2), SRU increased fecal N excretion (49.6 vs. 45.6 g/d; P = 0.04) and reduced apparent total tract N digestibility (61.7 vs. 66.0%; P = 0.003). Transfer of urea from the blood to the gastrointestinal tract occurred for both treatments in Exp. 1 and 2 at all time points with the exception for 0.5 h after dosing of urea in Exp. 1, when urea was actually transferred from the gastrointestinal tract to the blood. In both Exp. 1 and 2, both urea and SRU treatments increased arterial urea concentrations from 0.5 to 6 h after feeding, but arterial urea concentrations were consistently less with SRU (treatment x time P < 0.001, Exp. 1; P = 0.007, Exp. 2). Net portal ammonia release remained relatively consistent across the entire sampling period with SRU treatment, whereas urea treatment increased portal ammonia release in Exp. 1 and tended to have a similar effect in Exp. 2 (treatment x time P = 0.003 and P = 0.11, respectively). Urea treatment also increased hepatic ammonia uptake within 0.5 h (treatment x time P = 0.02, Exp. 1); however, increased total splanchnic release of ammonia for the 2 h after urea treatment dosing suggests that PDV ammonia flux may have exceeded hepatic capacity for removal. Slow-release urea reduces the rapidity of ammonia-N release and may reduce shifts in N metabolism associated with disposal of ammonia. However, SRU increased fecal N excretion and increased urea transfer to the gastrointestinal tract, possibly by reduced SRU hydrolysis or effects on digestion patterns. Despite this, the ability of SRU to protect against the negative effects of urea feeding may be efficacious in some feeding applications.     

Splanchnic metabolism of nitrogenous compounds and urinary nitrogen excretion in steers fed alfalfa under conditions of increased absorption of ammonia and L-arginine supply across the portal-drained viscera

Effects of increased ammonia and/or arginine absorption across the portal-drained viscera (PDV) on net splanchnic (PDV and liver) metabolism of nitrogenous compounds and urinary N excretion were investigated in six catheterized Hereford x Angus steers (501 +/- 1 kg BW) fed a 75% alfalfa:25% (as-fed basis) corn-soybean meal diet (0.523 MJ of ME/[kg BW(0.75).d]) every 2 h without (27.0 g of N/kg of dietary DM) and with 20 g of urea/kg of dietary DM (35.7 g of N/kg of dietary DM) in a split-plot design. Net splanchnic flux measurements were obtained immediately before beginning and ending a 72-h mesenteric vein infusion of L-arginine (15 mmol/h). For 3 d before and during arginine infusion, daily urine voided was measured and analyzed for N composition. Feeding urea increased PDV absorption (P < 0.01) and hepatic removal (P < 0.01) of ammonia N, accounting for 80% of increased hepatic urea N output (P < 0.01). Numerical increases in net hepatic removal of AA N could account for the remaining portion of increased hepatic urea N output. Arginine infusion increased hepatic arginine removal (P < 0.01) and hepatic urea N output (P < 0.03) and switched hepatic ornithine flux from net uptake to net output (P < 0.01), but numerical changes in net hepatic removal of ammonia and AA N could not account fully for the increase in hepatic urea N output. Increases in urine N excretion equaled quantities of N fed as urea or infused as arginine. Estimated salivary urea N excretion was not changed by either treatment. Urea cycle regulation occurs via a complex interaction of mechanisms and requires N sources other than ammonia, but the effect of increased ammonia absorption on hepatic catabolism of individual AA in the present study was not significant.     

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.     

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 the novel urease inhibitor N-(n-butyl) thiophosphoric triamide on ruminant nitrogen metabolism: II. Ruminal nitrogen metabolism, diet digestibility, and nitrogen balance in lambs

Three lamb metabolism experiments were conducted to investigate the effects of chronic administration of the novel urease inhibitor N (n-butyl) thiophosphoric triamide (NBPT) on ruminal N metabolism, fermentation, and N balance. In Exp. 1, ruminally cannulated wethers (n = 28; 45.0 +/- .9 kg) were administered one of seven doses of NBPT (0 [control], .125, .25, .5, 1, 2, or 4 g of NBPT daily) and fed a common cracked corn/cottonseed hull-based diet twice daily containing 2% urea at 2.5% of initial BW for the duration of the 15-d experiment. Overall, NBPT decreased (linear P < .0001; quadratic P < .001) ruminal urease activity, resulting in linear increases (P < .0001) in ruminal urea and decreases in ruminal NH3 N concentrations. However, the detection of an NBPT x day interaction (d 2 vs 15; P < .01) indicated that this depression in urea degradation diminished as the experiment progressed. Increasing NBPT linearly decreased (P < .01) total VFA concentrations on d 2 of the experiment, but it had no effect (P > .10) on d 15. Increasing NBPT had no effect (P > .10) on DM or ADF digestibilities, but it linearly decreased (P < .01) N digestibility. Supplementing NBPT produced a linear increase (P < .05) in urinary N excretion and a linear decrease (P < .01) in N retention. In Exp. 2, ruminally cannulated wethers (n = 30; 46.8 +/- .6 kg) were fed one of two basal diets (2.0 vs 1.1% dietary urea) at 2.5% of initial BW and dosed with either 0 (control), .25, or 2 g of NBPT daily for the duration of the 15-d experiment. There were no NBPT x dietary urea interactions (P > .10) for Exp. 2. Increasing NBPT depressed (linear and quadratic P < .0001) ruminal urease activity, producing linear (P < .0001) increases in urea N and linear decreases in NH3 N in the rumen. As in Exp. 1, an NBPT x day interaction (P < .05) was noted for urea, NH3 N, and total VFA concentrations; the maximum response to NBPT occurred on d 2 but diminished by d 15 of the experiment. Administration of NBPT did not influence (P > .10) DM, ADF, or N digestibilities in Exp. 2. In Exp. 3, wether lambs (n = 30; 26.4 +/- .7 kg) were subjected to the same treatment regimen as in Exp. 2 for a 14-d N balance experiment. Although several NBPT x dietary urea interactions (P < .05) were noted, increasing NBPT did not affect (P > .10) N digestibility. Administration of NBPT quadratically increased (P < .10) urinary N excretion, producing a linear decrease (P < .05) in N retention. These results suggest that although NBPT is capable of inhibiting ruminal urease short-term, the ruminal microflora may be capable of adapting to chronic NBPT administration, thereby limiting its practical use in improving the utilization of dietary urea.     

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 barley grain processing and dietary ruminally degradable protein on urea nitrogen recycling and nitrogen metabolism in growing lambs

The objective of this study was to determine how interactions between dietary ruminally degradable protein (RDP) level and ruminally fermentable carbohydrate (RFC) alter urea N transfer to the gastrointestinal tract (GIT) and the utilization of this recycled urea N in rapidly growing lambs fed high-N diets. Four Suffolk ram lambs (34.8 +/- 0.5 kg of BW) were used in a 4 x 4 Latin square design with 21-d periods and a 2 x 2 factorial arrangement of dietary treatments. The dietary factors studied were 1) dry-rolled vs. pelleted barley as the principal source of RFC and 2) dietary levels of RDP of 60 vs. 70% (% of CP). All diets contained 28.8 g of N/kg of DM. Experimental diets were composed of 80% concentrate mixture and 20% barley silage (DM basis) and were fed twice daily at 0900 and 1700 as total mixed rations. Nitrogen balance was measured from d 15 to 20, and urea N kinetics were measured from d 15 to 19 using intrajugular infusions of [(15)N(15)N]-urea. Nitrogen intake (P = 0.001) and fecal (P = 0.002) and urinary (P = 0.03) N excretion increased as dietary RDP level increased, but the method of barley processing had no effect. Feeding dry-rolled compared with pelleted barley (P = 0.04) as well as feeding 60% RDP compared with 70% RDP (P = 0.04) resulted in a greater N digestibility. Whole-body N retention was unaffected (P >/= 0.74) by dietary treatment. Dietary treatment had no effect on endogenous production of urea N and its recycling to the GIT; however, across dietary treatments, endogenous production of urea N (45.8 to 50.9 g/d) exceeded N intake (42.3 to 47.9 g/d). Across dietary treatments, 30.6 to 38.5 g/d of urea N were recycled to the GIT, representing 0.67 to 0.74 of endogenous urea N production; however, 0.64 to 0.76 of urea N recycled to the GIT was returned to the ornithine cycle. In summary, although dietary treatment did not alter urea N kinetics, substantial amounts of hepatic urea N output were recycled to the GIT under the dietary conditions used in this study, and additional research is required to determine how this recycled urea N can be efficiently captured by bacteria within the GIT.     

Influence of rumen protein degradability and supplementation frequency on performance and nitrogen use in ruminants consuming low-quality forage: cow performance and efficiency of nitrogen use in wethers

Two studies were conducted to determine the influence of CP degradability and supplementation frequency (SF) on ruminant performance and N efficiency. Treatments included an unsupplemented control (CON) and degradable intake protein (DIP; 82% of CP) or undegradable intake protein (UIP; 60% of CP) provided daily, every 3 d, or every 6 d. Seven wethers (36+/-1 kg BW) were used in the digestibility study with DIP and UIP treatments formulated to meet CP requirements. Eighty-four Angus x Hereford cows (512+/-42 kg BW) in the last third of gestation were used for the performance study. The DIP treatments were calculated to provide 100% of the DIP requirement and UIP treatments were provided on an isonitrogenous basis compared with DIP. Basal diets consisted of low-quality (5% CP) meadow hay. Forage DMI and N intake by lambs decreased (P < 0.05) linearly as SF decreased. Additionally, DMI, OM intake, N retention, N digestibility, and digested N retained were greater (P < 0.01) for supplemented wethers than for controls with no difference due to crude protein degradability. Nitrogen balance, DMI, and OM intake decreased linearly (P < 0.05) as SF decreased. Plasma urea (PU; mM) was measured over a 6-d period and supplemented lambs had increased (P < 0.01) PU compared with CON. Plasma urea linearly decreased (P < 0.01) as SF decreased. Pre- and postcalving (within 14 d and 24 h of calving, respectively) cow weight and body condition score change were more positive (P < 0.05) for supplemented groups than for controls. Results suggest CP supplements consisting of 20 to 60% UIP can be effectively used by ruminants consuming low-quality forage without adversely affecting N efficiency and animal performance, even when provided as infrequently as once every 6 d.     

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.     

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.     

Salvage of blood urea nitrogen in sheep is highly dependent on plasma urea concentration and the efficiency of capture within the digestive tract

The aims of this study were 1) to determine whether transfer of blood urea to the gastrointestinal tract (GIT) or the efficiency of capture of urea N within the GIT is more limiting for urea N salvage, and 2) to establish the relationship between plasma urea concentration and recycling of urea N to the GIT. We used an i.v. urea infusion model in sheep to elevate the urea entry rate and plasma concentrations, thus avoiding direct manipulation of the rumen environment that otherwise occurs when feeding additional N. Four growing sheep (28.1 +/- 0.6 kg of BW) were fed a low-protein (6.8% CP, DM basis) diet and assigned to 4 rates of i.v. urea infusion (0, 3.8, 7.5, or 11.3 g of urea N/d; 10-d periods) in a balanced 4 x 4 Latin square design. Nitrogen retention (d 6 to 9), urea kinetics([(15)N2]urea infusion over 80 h), and plasma AA were determined. Urea infusion increased apparent total tract digestibility of N (29.9 to 41.3%) and DM (47.5 to 58.9%), and N retention (1.45 to 5.46 g/d). The plasma urea N entry rate increased (5.1 to 21.8 g/d) with urea infusion, as did the amount of urea N entering the GIT (4.1 to 13.2 g/d). Urea N transfer to the GIT increased with plasma urea concentration, but the increases were smaller at greater concentrations of plasma urea. Anabolic use of urea N within the GIT also increased with urea infusion (1.43 to 2.98 g/d; P = 0.003), but anabolic use as a proportion of GIT entry was low and decreased (35 to 22%; P = 0.003) with urea infusions. Consequently, much (44 to 67%) of the urea N transferred to the GIT returned to the liver for resynthesis of urea (1.8 to 9.2 g/d; P < 0.05). The present results suggest that transfer of blood urea to the GIT is 1) highly related to blood urea concentration, and 2) less limiting for N retention than is the efficiency of capture of recycled urea N by microbes within the GIT.     

Net portal and hepatic flux of nutrients in growing wethers fed high-concentrate diets with oscillating protein concentrations

We hypothesized that oscillating dietary CP would improve N retention by increasing the uptake of endogenous urea N by portal drained viscera (PDV), compared with static dietary CP regimens. Chronic indwelling catheters were surgically implanted in the abdominal aorta, a mesenteric vein, a hepatic vein, and the portal vein of 18 growing Dorset x Suffolk wethers (44.6 +/- 3.6 kg of BW). Wethers had ad libitum access to the following diets in a completely randomized block design: 1) Low (9.9% CP), 2) Medium (12.5% CP), or 3) Low and High (14.2% CP) diets oscillated on a 48-h interval (Osc). Dry matter intake was greater (P = 0.04) for the Osc diet (1,313 g/d) than the Low diet (987 g/d) and was intermediate for the Medium diet (1,112 g/d). Nitrogen intake was not different between the wethers fed the Osc (25.4 g/d) and Medium diets (22.2 g/d), but was lower (P < 0.01) in wethers fed the Low diet (16.0 g/d). Wethers fed the Osc diet (6.7 g/d) retained more (P < 0.04) N than did those fed the Medium diet (4.0 g/d). Hepatic arterial blood flow was not different (P = 0.81) between wethers fed the Osc (31 L/h) or Medium diet (39 L/h) but was greater (P = 0.05) in wethers fed the Low diet (66 L/h). Net release of alpha-amino N by the PDV did not differ (P = 0.90) between the Low (37.8 mmol/h) and Medium diets (41.5 mmol/h) or between the Osc (53.0 mmol/h) and Medium diets (P = 0.29). Net PDV release of ammonia N was less (P = 0.05) for the Low diet than for the Medium diet, and this was accompanied by a similar decrease (P = 0.04) in hepatic ammonia N uptake. Urea N concentrations tended to be (P = 0.06) less in arterial, portal, and hepatic blood in wethers fed the Low diet compared with those fed the Medium diet. Wethers fed the Osc diet tended (P = 0.06) to have a greater PDV uptake of urea N than did those fed the Medium diet, but there was no difference between the Osc and Medium diets (P = 0.72) in hepatic urea N release. Net PDV uptake of glutamine tended to be greater (P < 0.07) in wethers fed the Low diet (6.7 mmol/h) than those fed the Medium diet (2.7 mmol/h). These data indicate that oscillating dietary protein may improve N retention by increasing endogenous urea N uptake by the gastrointestinal tract.     

Influence of advancing season on dietary composition, intake, site of digestion, and microbial efficiency in beef steers grazing a native range in western North Dakota

Four ruminally and duodenally cannulated beef steers (388 +/- 12 kg) were used to evaluate effects of advancing season on forage quality, intake, site of digestion, and microbial efficiency while grazing mixed-grass prairie in western North Dakota. Five 11-d sample collections were conducted from late June to mid-November. Chromic oxide (8 g) was dosed twice daily at 0700 and 1900 h via gelatin capsule from d 2 to 11 of each collection period, and duodenal and fecal collections were performed on d 7 to 11. Masticate samples were collected for each sampling period. Dietary N declined linearly (P = 0.01), from 1.95% in June to 1.15% in November, whereas NDF increased linearly (P = 0.01), 72.4% in June to 85.1% in November. Total OM intake (g/kg of BW) decreased linearly (P 相似文献   

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.     

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.