Effect of nitrogen intake on nitrogen recycling and urea transporter abundance in lambs

Urea recycling in ruminants has been studied extensively in the past, but the mechanisms regulating the amount of urea recycled or excreted remain obscure. To elucidate the role of urea transporters (UT) in N recycling, nine Dorset-Finn ewe lambs (20.8 +/- 0.8 kg) were fed diets containing 15.5, 28.4, and 41.3 g of N/kg of DM for 25 d. Nitrogen balance and urea N kinetics were measured during the last 3 d of the period. Animals were then slaughtered and mucosa samples from the rumen, duodenum, ileum, and cecum, as well as kidney medulla and liver, were collected. Increasing N intake tended to increase N balance quadratically (1.5, 5.1, and 4.4 +/- 0.86 g of N/d, P < 0.09), and linearly increased urinary N excretion (2.4, 10, and 16.5 +/- 0.86 g N/d, P < 0.001) and plasma urea N concentration (4.3, 20.3, and 28.4 +/- 2.62 mg of urea N/dL, P < 0.001), but did not affect fecal N excretion (5.0 +/- 0.5 g of N/d; P < 0.94). Urea N production (2.4, 11.8, and 19.2 +/- 0.83 g of N/d; P < 0.001) and urinary urea N excretion (0.7, 7.0, and 13.4 +/- 0.73 g N/d; P < 0.001) increased linearly with N intake, as well as with the urea N recycled to the gastrointestinal tract (1.8, 4.8, and 5.8 +/- 0.40 g of N/d, P < 0.001). No changes due to N intake were observed for creatinine excretion (518 +/- 82.4 mg/d; P < 0.69) and clearance (46 +/- 10.7 mL/min; P < 0.56), but urea N clearance increased linearly with N intake (14.9, 24.4, and 34.9 +/- 5.9 mL/min; P < 0.04). Urea N reabsorption by the kidney tended to decrease (66.3, 38.5, 29.1 +/- 12.6%; P < 0.06) with increasing N content of the diet. Increasing the level of N intake increased linearly the weight of the liver as a proportion of BW (1.73, 1.88, and 2.22 +/- 0.15%, P < 0.03) but only tended to increase the weight of the kidneys (0.36, 0.37, and 0.50 +/- 0.05%, P < 0.08). Urea transporter B was present in all the tissues analyzed, but UT-A was detected only in kidney medulla, liver, and duodenum. Among animals on the three diets, no differences (P > 0.10) in UT abundance, quantified by densitometry, were found. Ruminal-wall urease activity decreased linearly (P < 0.02) with increasing level of N intake. Urease activity in duodenal, ileal, and cecal mucosa did not differ from zero (P > 0.10) in lambs on the high-protein diet. In the present experiment, urea transporter abundance in the kidney medulla and the gastrointestinal tract did not reflect the increase in urea-N reabsorption by the kidney and transferred into the gut.     

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.     

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.     

Influence of dietary urea level on digestive function and growth performance of cattle fed steam-flaked barley-based finishing diets

Four Holstein steers (282 kg) with cannulas in the rumen and proximal duodenum were used in a 4 x 4 Latin square experiment to evaluate the influence of dietary urea level (0, 0.4, 0.8, and 1.2%, DM basis) in a steam-flaked barley-based finishing diet on digestive function. There were no treatment effects (P > 0.20) on ruminal digestion of OM and ADF. Increasing dietary urea level increased (linear, P < 0.01) ruminal starch digestion. Ruminal degradability of protein in the basal diet (no supplemental urea) was 60%. Increasing dietary urea level did not increase (P > 0.20) ruminal microbial protein synthesis or nonammonia N flow to the small intestine. There were no treatment effects (P > 0.20) on total-tract ADF digestion. Total tract digestion of OM (quadratic, P < 0.01) and starch (linear, P < 0.05) increased slightly with increasing urea level. Urea supplementation increased (linear, P < 0.01) ruminal pH 1 h after feeding; however, by 3 h after feeding, ruminal pH was lower (cubic, P < 0.05) with urea-supplemented diets. Urea supplementation did not affect (P > 0.20) ruminal molar proportions of acetate and propionate. One hundred twenty crossbred steers (252 kg; approximately 25% Brahman breeding) were used in an 84-d feeding trial (five pens per treatment) to evaluate treatment effects on growth performance. Daily weight gain increased (linear, P = 0.01) with increasing urea level, tending to be maximal (1.53 kg/d; quadratic, P = 0.13) at the 0.8% level of urea supplementation. Improvements in ADG were due to treatment effects (linear, P < 0.01) on DMI. Urea supplementation did not affect (P > 0.20) the NE value of the diet for maintenance and gain. Observed dietary NE values, based on growth performance, were in close agreement with expected based on tabular values for individual feed ingredients, averaging 100.4%. We conclude that with steam-flaked barely-based finishing diets, ruminal and total-tract digestion of OM and ruminal microbial protein synthesis may not be increased by urea supplementation. In contrast, ADG was optimized by dietary inclusion of 0.8% urea. Urea supplementation may not enhance the net energy value of steam-flaked barely-based finishing diets when degradable intake protein is greater than 85% of microbial protein synthesis.     

Synchrony of nutrient supply to the rumen and dietary energy source and their effects on the growth and metabolism of lambs

The objective of the current series of experiments was to assess the effects of dietary synchrony of OM and N supply to the rumen, achieved by altering the sequence of feeding individual ingredients and in diets with different energy sources, on the metabolism and performance of growing lambs. In Exp. 1, the in situ degradability coefficients of OM and N were determined for five feed ingredients and subsequently was used to formulate two diets, based either on barley or sugar beet pulp, to have a similar predicted nutrient content. Within each diet, specific ingredients were shifted between the 0900 and 1600 feeding to provide either a synchronous, intermediate, or asynchronous supply of OM and N to the rumen. In Exp. 2, these diets were fed at a restricted level to 48 growing lambs with an initial live weight of 25.1 +/- 4.22 kg and a slaughter weight of 41.4 +/- 1.94 kg. There was no significant effect of dietary treatment on live weight gain or feed conversion efficiency. Lambs fed the synchronous diets deposited more kidney knob and channel fat than lambs on the asynchronous or intermediate diets (P < 0.05), whereas lambs fed the barley-based diets deposited more carcass (P < 0.05) and noncarcass (P < 0.001) fat than lambs on the sugar beet-based diets. Lambs fed the asynchronous diets retained less energy over the course of the experiment than lambs on the intermediate or synchronous diets (P < 0.05), and had a lower energy efficiency (0.079, 0.097, and 0.093 MJ retained/ MJ of intake, respectively, P < 0.05). Lambs fed the barley-based diets retained more energy than lambs on the sugar beet-based (P < 0.001) and had a higher energy balance (0.095 vs. 0.084 MJ retained/MJ intake, respectively; P < 0.01). Plasma ammonia concentrations mirrored ruminal ammonia concentrations on the barley-based diets, but not sugar beet-based diets. In Exp. 3, lambs fed the sugar beet-based diets had a higher digestibility of OM and NDF (P < 0.001). By contrast, lambs on the barley-based diets had a higher level of purine derivative excretion and microbial N production (P < 0.001). The results indicate that neither dietary synchrony nor energy source significantly influenced growth rate. However, both the asynchronous and sugar beet pulp-based diets resulted in a lower efficiency of dietary energy use, and the avoidance of asynchronous patterns of nutrient release within the rumen can improve energy efficiency in growing lambs.     

Effects of increasing ruminally degraded nitrogen and abomasal casein infusion on net portal flux of nutrients in yearling heifers consuming a high-grain diet.

Seven Meat Animal Research Center (MARC) III heifers (410+/-25 kg) fitted with hepatic portal, mesenteric venous, carotid catheters, and an abomasal cannula were used in a 7 x 5 incomplete Latin square design experiment. The objective was to evaluate the effects of increasing levels of ruminally degradable N (RDN) with or without the addition of abomasally infused casein on portal-drained visceral (PDV) flux of nutrients. Treatments consisted of dietary CP percentage levels of 9.5 (control), control plus .72% dietary urea (11.5U), control plus 1.44% dietary urea (13.5U), control plus abomasally infused casein (250 g/d; 11.5C), or control plus .72% dietary urea and abomasally infused casein (250 g/d; 13.5UC). All diets contained (DM basis) 80% ground corn, 15% corn silage, and 5% dry supplement and were provided for ad libitum consumption. Nitrogen intake increased (linear, P < .001) as CP increased from 9.5 to 13.5%. Portal-drained visceral release of ammonia N increased (linear, P < .10) as RDN increased, and was greater (P < .05) when protein was fed compared with heifers fed control (P < .10). Urea N removal by PDV was not affected ( P > . 10) by level of RDN but was greatest when 11.5C was fed and least when 13.5UC was fed. Net alpha-amino N (AAN) release by PDV was greatest when 13.5UC was fed (309 mmol/h), least when 9.5% CP was fed (112 mmol/h), and intermediate for the other groups (205 to 252 mmol/h). These data suggest that removal of N by the PDV may promote microbial protein synthesis when dietary RDN is low. When RDN needs have been met and amino acids are deficient for the host, escape protein should be fed to increase amino acid absorption.     

Digesta flows in sheep fed poor-quality hay supplemented with urea and carbohydrates

Two metabolism trials were conducted with 12 yearling crossbred wethers per trial (34 and 38 kg for trials 1 and 2, respectively). The wethers, equipped with ruminal, abomasal and ileal cannulae, were randomly allotted for each trial to the following treatments: 1) hay alone or hay supplemented with 2) .9% urea, 3) 1% urea and 6.5% molasses or 4) 1% urea and 5.2% corn. Two digestive flow markers were used: Cr2O3 powder and Co-ethylenediaminetetraacetic acid (Co-EDTA). Urea and Co-EDTA were infused continuously into the rumen via cannula. Daily dry matter (DM) intake averaged 517 g. Urea supplementation improved N retention (P less than .01). Apparent digestibility of DM, acid detergent fiber (ADF) and energy was not affected by treatment. Urea and carbohydrate supplementation increased ruminal propionic acid molar proportions (P less than .05). Apparent ruminal DM digestion accounted for 41% of the total DM degraded, whereas 77.4% of the digestible ADF was degraded in the rumen. Urea supplementation increased ADF digestion in the large intestine (P less than .01). Urea and carbohydrate supplementation resulted in a stepwise increase in N flowing with the liquid phase at the abomasum. Mean retention times of the solid and liquid phases of digestive contents were similar across treatments. Overall, benefits of supplementation of poor-quality fescue hay diets by small amounts of urea and readily available carbohydrates remain questionable for sheep fed at a fixed level of intake below maintenance.     

Effects of adding fiber sources to reduced-crude protein,amino acid-supplemented diets on nitrogen excretion,growth performance,and carcass traits of finishing pigs

Two experiments were conducted to evaluate the effects of adding fiber sources to reduced-crude protein (CP), amino acid-supplemented diets on N excretion, growth performance, and carcass traits of growing-finishing pigs. In Exp. 1, six sets of four littermate barrows (initial weight = 36.3 kg) were allotted randomly to four dietary treatments to determine N balance and slurry composition. Dietary treatments were: 1) fortified corn-soybean meal, control, 2) as fortified corn-soybean meal with CP lowered by 4 percentage units and supplemented with lysine, threonine, methionine, tryptophan, isoleucine, and valine (LPAA), 3) same as Diet 2 plus 10% soybean hulls, and 4) same as Diet 2 with 10% dried beet pulp. Nitrogen intake, absorption, and retention (g/d) were reduced (P < 0.04) in pigs fed the low- protein diets, but they were not affected (P > 0.10) by addition of fiber sources to the LPAA diet. However, N absorption, as a percentage of intake, was not affected (P > 0.10) by dietary treatment. Nitrogen retention, expressed as a percentage of N intake, was increased (P < 0.02) in pigs fed the low-protein diets, but it was not affected by fiber addition to the LPAA diet. Urinary and total N excretion was reduced (P < 0.01) by 50 and 40%, respectively, in pigs fed the low- protein diets, but it was not affected (P > 0.10) by fiber addition. However, fiber addition to the LPAA diet tended to result in a greater proportion of N excreted in the feces than in the urine. Slurry pH, ammonium N content, and urinary urea N excretion were reduced (P < 0.10) in pigs fed LPAA, and a further reduction (P < 0.06) in slurry ammonium N content and urinary urea N was observed with fiber addition. Also, fiber addition to the LPAA diet increased (P < 0.02) slurry VFA concentrations. In Exp. 2, 72 pigs were blocked by body weight and sex and allotted randomly to three dietary treatments that were similar to those in Exp. 1, with a corn-soybean meal control diet, LPAA diet, and a LPAA diet with 10% soybean hulls. Pigs were fed the diets from 28.6 to 115 kg, and all pigs were killed for collection of carcass data. Growth performance and most carcass traits were not affected (P > 0.10) by dietary treatment. These data suggest that reducing CP with amino acid supplementation markedly decreased N excretion without influencing growth performance. Fiber addition to a LPAA diet had little effect on overall N balance or growth performance, but tended to further reduce slurry ammonium N concentration and increase volatile fatty acid concentrations.     

Effects of high ambient temperature on urea‐nitrogen recycling in lactating dairy cows

Effects of exposure to hot environment on urea metabolism were studied in lactating Holstein cows. Four cows were fed ad libitum a total mixed ration and housed in a temperature‐controlled chamber at constant moderate (18°C) or high (28°C) ambient temperatures in a cross‐over design. Urea nitrogen (N) kinetics was measured by determining urea isotopomer in urine after single injection of [¹⁵N₂]urea into the jugular vein. Both dry matter intake and milk yield were decreased under high ambient temperature. Intakes of total N and digestible N were decreased under high ambient temperature but urinary urea‐N excretion was increased. The ratio of urea‐N production to digestible N was increased, whereas the proportion of gut urea‐N entry to urea‐N production tended to be decreased under high ambient temperature. Neither return to the ornithine cycle, anabolic use nor fecal excretion of urea‐N recycled to the gut was affected by ambient temperature. Under high ambient temperature, renal clearance of plasma urea was not affected but the gut clearance was decreased. Increase of urea‐N production and reduction of gut urea‐N entry, in relative terms, were associated with increased urinary urea‐N excretion of lactating dairy cows in higher thermal environments.     

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.     

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.     

Prevention of goitre in new-born lambs from kale-fed ewes

Milk urea concentrations in dairy cattle. There has been increased use of milk urea concentration as an indicator of dietary protein intake and protein metabolism in dairy cattle over recent years. The value of milk urea content data in predicting dietary composition, particularly for pasture-fed cattle, has not been well described.

Protein metabolism and urea synthesis. Many factors influence the degradation of dietary proteins in the rumen, post-ruminal protein metabolism and urea synthesis in cattle. Strong positive correlations between nitrogenous fertiliser use and the crude protein content of pastures were identified by use of meta-analysis. Similar strong positive correlations were noted between dietary protein intake, rumen ammonia and plasma urea concentrations. The costs of urea synthesis include energy losses, and importantly, the loss of endogenous amino acids, which are deaminated in the synthesis of urea.

Milk urea as an indictor of protein metabolism. Urea concentrations in blood, plasma and milk are strongly correlated. Milk is an adequate indicator of blood and plasma urea content, but non-nutritional factors may significantly influence milk urea concentrations. Recommendations for dietary protein management based on milk urea concentrations must be undertaken with care.     

Effects of protein sources and inclusion levels on nitrogen metabolism and urea kinetics of Nellore feedlot steers fed concentrate-based diets

Urea recycling occurs in all mammalian species and represents an important source of ruminal nitrogen (N) for ruminants fed protein-restricted diets. However, its importance for cattle fed adequate amounts of protein and energy remains unclear. Six Nellore feedlot steers fed concentrate-based diets were used in a 6 × 6 Latin square design with a 3 × 2 factorial arrangement of treatments to evaluate ruminal fermentation, urea kinetics, and N excretion. Treatments consisted of 3 protein sources (PS: soybean meal plus urea [SU], corn gluten meal [CGM], and dry distillers grains [DDG]) and 2 inclusion levels (PL; 11% and 14%). Steers were adapted to the diets for 14 d followed by 8 d of sample collection. Feed intake, fecal output, and urine production were measured from day 18 to day 22 of each period. Blood samples were collected every 6 h on day 18. [¹⁵N-¹⁵N]-urea was infused into the jugular vein for 82 h over day 19 to day 22, and measurement of ¹⁵N in background (day 18) and enriched feces and urine (day 21) were used to evaluate urea kinetics. To evaluate the incorporation of recycled urea N into microbial protein (MICP), ruminal and duodenal fluid were collected on day 22. Steers fed SU diets had lower (P < 0.05) nitrogen use efficiency (NUE), greater (P < 0.05) urea-N entry rate (UER), and tended (P < 0.10) to have greater gastrointestinal entry rate of urea-N (GER) compared with those fed CGM or DDG. In addition, steers fed SU had greater (P < 0.05) urea-N returned to ornithine cycle (ROC) compared with those fed CGM or DDG. Increasing PL tended (P < 0.10) to increase UER. The proportion of total microbial N from recycled urea-N was greater (P < 0.05) for steers fed CGM compared with those fed SU and also greater for steers fed diets with 11% CP than for those fed with 14% CP. Diets with 11% CP can be used for Nellore feedlot cattle fed concentrate-based diets without negatively affecting intake, digestibility, and ruminal fermentation. Moreover, diets containing rumen undegradable protein (RUP) feed sources (CGM or DDG) compared with diets with SU markedly increased NUE, while maintaining microbial protein (MICP) synthesis. Results from this study suggest that the equation adopted by NASEM (NASEM. 2016. Nutrient requirements of beef cattle. 8th revised ed. Washington, DC: The National Academies Press) was not accurate in estimating the urea-N used for anabolism (UUA) in Nellore feedlot cattle fed concentrate-based diets.     

日粮添加尿素对瘤胃上皮细胞增殖、凋亡以及吸收转运能力的影响

本研究探讨了在日粮中添加尿素替代部分豆粕对山羊瘤胃发酵、上皮细胞增殖、凋亡和吸收转运能力的影响。将18 只波杂山羊随机分为3 组（n=6）,分别饲喂3 种日粮：LC组（纯粗料）、MC组（30%精料）以及Urea组（1%DM缓释尿素替代部分豆粕+30%精料）。饲喂Urea组和MC组的山羊瘤胃中短链脂肪酸（SCFA）浓度、pH值均显著高于LC组,而该两组之间无显著差异;但Urea组在MC组的基础上进一步显著提高了瘤胃NH₃和血浆尿素氮（BUN）浓度。因此日粮添加尿素对于瘤胃上皮中受瘤胃SCFA浓度、pH值调节的上皮生长、细胞周期、增殖凋亡相关基因和SCFA转运载体mRNA表达的影响与MC组相似,但对受瘤胃NH₃调节的尿素转运、细胞内pH（pHi）调节相关蛋白mRNA表达则有显著的抑制效果,即显著高于LC组,但低于MC组。     

Effects of energy level and protein source on nitrogen kinetics in steers fed wheat straw-based diets.

A 4 x 4 Latin square metabolism trial with a 2 x 2 factorial arrangement of treatments was conducted to determine N kinetics in steers. Steers were fed either untreated (UNT-WS) or alkaline hydrogen peroxide-treated wheat straw (AHP-WS) based diets supplemented with soybean meal (SBM) or blood meal (BM). Single doses of (15NH4)2SO4 were infused into ruminal pools to determine N kinetics. Ruminal NH3N concentrations (main effects) were 3.81, 1.65, 3.18, and 2.28 mg/dL in steers when fed diets that contained UNT-WS, AHP-WS, SBM, and BM, respectively. Ruminal N pool size was greater (P < .05) for UNT-WS than for AHP-WS diets and also was greater (P < .10) for SBM than for BM diets. Nitrogen flux rate into the rumen was not affected (P > .10) by diet. However, production rate of N from the ruminal pool was greater (P < .05) for UNT-WS than for AHP-WS diets and greater (P < .10) for SBM than for BM diets. Nitrogen recycled into the rumen was 33% greater (P < .05) for AHP-WS than for UNT-WS diets and 26% greater (P < .05) for BM than for SBM diets. Nitrogen recycling (percentage of N intake) was 33, 56, 36, and 49% for UNT-WS, AHP-WS, SBM, and BM diets, respectively. The blood urea N (BUN) concentrations were 10.23, 4.58, 7.15, and 7.65 mg/dL for UNT-WS, AHP-WS, SBM, and BM diets, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prediction of nitrogen excretion in feces and urine of beef cattle offered diets containing grass silage

Data from 286 beef cattle, obtained in total diet digestibility assessments, were used to examine effects of dietary and animal factors on N excretion in feces and urine and to develop prediction equations for N excretion in beef cattle. The animals used were mainly from beef breeds, at various ages (from growth to finishing) and live BW (153 to 580 kg), and offered diets containing grass silage at production feeding levels. Dietary forage proportion ranged from 199 to 1,000 g/kg of DM and dietary CP concentration from 108 to 217 g/kg of DM. Linear and multiple regression techniques were used to examine relationships between the efficiency of N utilization and dietary and animal variables with the experimental effects removed. The statistical analysis indicated that N excretion was related positively (P < 0.001) to live BW and intakes of DM, N, and ME, and negatively (P < 0.001) to dietary forage proportion. The prediction equation for N excretion, developed using N intake alone, produced a large r2 (0.898) and a small SE (12.3). Addition of live BW and forage proportion as supporting predictors to this relationship only marginally increased R2 to 0.915 and reduced SE to 11.2. Nitrogen excretion was less well related to live BW (r2 = 0.771, SE = 18.5) than to N intake. Addition of N intake as a proportion of DMI or ME intake to the relationship between live BW and N excretion increased R2 to 0.824 and reduced SE to 16.2. The internal validation of these equations revealed that using N intake as the primary predictor produced a very accurate prediction of N excretion. In situations where data on N intake are not available, prediction equations based on live BW and dietary N concentration together can produce a relatively accurate assessment of N excretion. A number of mitigation strategies to reduce N excretion in feces and urine in beef cattle are discussed, including manipulation of dietary N concentration, diet quality, and level of feeding. The prediction equations and mitigation strategies developed in the current study provide an approach for beef producers to quantify N excretion against production and to develop their own mitigation strategies to reduce N excretion.     

Feeding behavior and ruminal acidosis in beef cattle offered a total mixed ration or dietary components separately

Eighty continental crossbred beef heifers (414.9 ± 37.9 kg of BW), 16 of which were ruminally cannulated, were used in a 52-d experiment with a generalized randomized block design, to assess if self-selection of dietary ingredients modulates ruminal pH and improves rumen function of feedlot finishing cattle. Treatments were total mixed ration [TMR; 85% barley grain (BG), 10% corn silage (CS), 5% supplement]; or free-choice (self-selection; FC) diets of barley grain and corn silage (BGCS), barley grain and wheat distillers grain (BGDG), or corn silage and wheat distillers grain (CSDG). Heifers were housed in groups of 10 in 8 pens equipped with the GrowSafe System (Airdrie, AB, Canada) enabling feed intake and feeding behavior to be continuously monitored. Two cannulated heifers were randomly assigned to each pen and equipped with indwelling pH probes for continuous measurement of ruminal pH during 4 periods (d 1 to 4, d 7 to 14, d 21 to 28, and d 42 to 49). Rumen fluid samples were collected from cannulated heifers on d 7 and 42 before feed delivery, and on d 4 and 49 at 2 h post-feed delivery for determination of VFA. Heifers fed the TMR had shorter (P = 0.01) and smaller (P = 0.03) meals than those fed the FC diets. Cattle fed BGCS and BGDG increased (P < 0.01) intake of BG over time by up to 80 and 70%, respectively. Increased consumption of BG arose from an increase (P < 0.01) in eating rate over the same (P > 0.10) feeding time, which was accompanied by an increase (P < 0.05) in eating rate but a decrease (P < 0.05) in feeding time of either CS or DG. Even with increased BG consumption, ruminal pH and VFA profiles were not different (P > 0.10) among FC diets or compared with the TMR. Cattle fed FC CSDG consumed DG at 60% of dietary DM over the trial, resulting in greater (P < 0.05) mean ruminal pH and acetate-to-propionate ratio and less (P < 0.05) area under the curve than those given the other FC diets or the TMR. Finishing feedlot cattle fed FC diets containing BG self-regulate intake of diets that have a similar composition, intake level, and ruminal fermentation profile to those fed a TMR.     

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.     

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.     

Effect of dietary protein on intake,nutrients utilization,nitrogen balance,blood metabolites,growth and puberty in growing Bhadawari buffalo (Bubalus bubalis) heifers

Fifteen Bhadawari buffalo heifers of 207?±?9.78 kg mean body weight were randomly distributed into three dietary groups to evaluate the effect of protein level on nutrient utilization, nitrogen (N) balance, growth rate, blood metabolites, and puberty. All animals were offered wheat straw-berseem diets supplemented with concentrate mixtures of similar energy (2.7 Mcal/kg) and different protein levels (14.3–22 %). Animals of standard-protein group (SPG) were offered protein and energy as per requirement, while animals of low-protein group (LPG) and high-protein group (HPG) were fed 20 % less and 20 % more protein, respectively, than SPG. Feed dry matter (DM) and metabolizable energy (ME) intake (% body wt. and g/kg w^0.75) were similar for all three diets; however, the crude protein (CP) and digestible crude protein (DCP) intake on percent body weight and per kilogram metabolic weight was higher (P?<?0.05) in HPG than in SPG or LPG. Digestibility of CP, cellulose, and hemicellulose was higher (P?<?0.05) in HPG versus LPG. Fecal N excretion was similar, while urinary N excretion was highest (P?<?0.05) in HPG (74.83 g/day) compared with SPG (50.03 g/day) and LPG (47.88 g/day), which resulted in lower N retention in HPG than in the other dietary groups. Level of dietary N had no effect on blood metabolites viz. glucose, urea, and N. Digestible energy (DE) and ME contents of diets were identical, while DCP contents were higher (P?<?0.05) in HPG than in LPG. Feed and nutrient (CP and ME) conversion efficiency to produce a unit kilogram weight gain was identical among the dietary groups. Dietary protein level had no effect on the heifer’s weight and age at puberty. The mean growth rate of heifers at 240 days was higher (P?>?0.05) in SPG (330.8 g/day) than in LPG (296.7 g/day), while the animals gained more weight in January to March months and the lowest weight in May to July months. Protein level had no effect on conception rate of heifers. Results revealed that 20 % higher or less protein than the ICAR requirement had no significant (P?>?0.05) on feed intake, nutrient conversion efficiency for weight gain, heifer growth, and puberty; however, 20 % more protein increased urinary N loss.