Three experiments were conducted to study dietary effects on urea-nitrogen (N) recycling as a strategy to improve the efficiency of N utilization in ruminants. Experiment 1 examined the effects of feeding diets containing two levels of dietary crude protein (CP; 10.8 vs. 14.0%) and ruminally-degradable protein (RDP; 73.4 vs. 76.0% of CP) on urea-N recycling to the gastro-intestinal tract (GIT), N balance, and microbial protein production in beef heifers. Feeding the low CP diet decreased N intake (P < 0.01), ruminal ammonia-N (NH3-N) concentration (P < 0.01) and urinary N excretion (P <0.01). Endogenous urea-N production increased (P = 0.03) with increasing dietary CP level, as did urinary urea-N loss (P = 0.04). However, urea-N transfer to the gastro-intestinal tract (GIT) was similar across diets, with most of this N returned to the ornithine cycle. Microbial N supply was unaffected (P > 0.05) by dietary treatment. Experiment 2 examined the effects of feeding diets containing two levels of ruminally-degradable starch (RDS; 28.6 vs. 69.2% of total starch) and RDP (48.0% vs. 55.0% of CP) on urea-N recycling to the GIT, N balance, duodenal nutrient flow, and ruminal microbial protein production in beef heifers fed low CP (10%) diets. Ruminal NH3-N concentration was greater (P = 0.01) in heifers fed high RDP as compared with those fed low RDP, and it was also greater (P = 0.01) in heifers fed low RDS as compared with those fed high RDS. Microbial N flow to the duodenum increased as RDP level increased on the high RDS diet, but was not affected by RDP level on the low RDS diet (interaction; P = 0.04). Urea-N entry rate and urea-N transfer to the gastro-intestinal tract were similar (P > 0.05) across diets. The amount of recycled urea-N that was incorporated into microbial N increased as RDP level increased on the high RDS diet, but the opposite was observed on the low RDS diet (interaction; P = 0.008). Experiment 3 examined the effects of feeding diets containing two levels of CP (14.9 vs. 17.5%) and RDP (63.0 vs. 69.0% of CP) on urea-N recycling to the GIT, microbial protein production, N balance, omasal nutrient flow, and milk production in lactating dairy cows. Nitrogen intake (P < 0.01) and both urinary N (P < 0.01) and urea-N (P < 0.01) output were greater for cows fed the high compared with those fed the low CP diet. Ruminal NH3-N concentration tended to be greater in cows fed the high than those fed the low CP diet (P = 0.06), and was greater in cows fed high RDP as compared with those fed the low RDP diet (P < 0.01). However, N balance, milk yield, and microbial N supply were unaffected (P > 0.05) by dietary treatment. The proportion of endogenous urea-N that was recycled to the GIT (i.e., GER: UER) was greater (P = 0.02) in cows fed the low CP compared with those fed the high CP diet. In summary, the results of this thesis show that reducing dietary CP level in beef and dairy cattle reduces urinary N excretion whilst maintaining microbial N supply. In addition, judicious combinations of RDP and RDS when feeding low CP diets can potentially enhance the efficiency of microbial N production. These data show that through careful dietary manipulation, overall efficiency of N utilization can be improved leading to a reduction in N excretion into the environment.

Orientador: Telma Teresinha Berchielli / Abstract: The use of rumen undegradable protein (RUP) sources in high-energy diets may be an alternative to increase the nitrogen use efficiency (NUE) in ruminants. In this study we had two main objectives: 1) to understand how the protein content of the diet and the rumen degradable protein (RDP) intake can affect urea recycling and its utilization. 2) how the protein content of the diet and the amino acid profile of the metabolizable protein (MP) can affect the efficiency of the use of amino acids (AA) in feedlot Nellore cattle receiving high-energy density diets. Thus, we evaluated the effects of different sources and protein levels in the diet of feedlot Nellore cattle receiving high-concentrate diets. We had two major hypotheses: 1) reducing dietary N associated with RUP sources can increase NUE by reducing ruminal NH3 concentration, urea production in the liver and urinary N excretion, while maintaining N available for microbial protein (MICP) synthesis through urea N recycling. 2) differences in AA use efficiency exists and the gross AA use efficiency is affected by dietary factors such as N sources and levels. These hypotheses were tested using six Nellore steers, cannulated in the rumen, duodenum and ileum with initial body weight (BW) of 354 ± 11.8 kg and 18 months of age. The animals were randomly assigned to receive each diet once over the 6 periods in a 6 × 6 Latin square design. Experimental diets consisted of 80% concentrate and 20% roughage (DM basis), where fresh chopp... (Complete abstract click electronic access below) / Resumo: O uso de fontes de proteína não degradável no rúmen (PNDR) em dietas de alta energia pode ser uma alternativa para aumentar a eficiência do uso de nitrogênio (ENU) em ruminantes. Neste estudo, tivemos dois objetivos principais: 1) compreender como o teor de proteína da dieta e a ingestão de proteína degradável no rúmen (PDR) podem afetar a reciclagem de uréia e sua utilização. 2) como o teor de proteína da dieta e o perfil de aminoácidos da proteína metabolizável (MP) podem afetar a eficiência do uso de aminoácidos (AA) em bovinos Nelore em confinamento recebendo dietas de alta densidade energética. Assim, avaliamos os efeitos de diferentes fontes e níveis proteína na dieta de bovinos Nelore confinados que recebendo dietas de alto concentrado. Tivemos duas hipóteses principais: 1) reduzir o teor de N na dieta associado às fontes de PNDR pode aumentar a ENU por uma redução na concentração de NH3 ruminal, produção de uréia no fígado e excreção urinária de N, enquanto mantém o N disponível para a síntese de proteína microbiana (MICP) através da reciclagem de ureia. 2) existem diferenças na eficiência do uso de AA e a eficiência bruta de uso de AA é afetada por fatores dietéticos, como fontes e níveis de N. Essas hipóteses foram testadas utilizando seis novilhos Nelore, canulados no rúmen, duodeno e íleo com peso corporal inicial (PC) de 354 ± 11,8 kg e 18 meses de idade. Os animais foram distribuídos aleatoriamente para receber cada dieta por uma vez ao longo dos 6 períodos, em ... (Resumo completo, clicar acesso eletrônico abaixo) / Doutor

Master of Science / Department of Animal Sciences and Industry / Evan C. Titgemeyer / Two experiments analyzed effects of zilpaterol-HCl and nitrogen supplementation in the form of either dried distiller’s grains with solubles (DDGS) or urea fed to steers. In Experiment 1, steers were fed corn-based diets: control (CON; 10.2% CP), urea (UREA; 13.3% CP), or DDGS (14.9% CP). Nitrogen intake differed among treatments (99, 151, and 123 g/d for CON, DDGS, and UREA). Urea-N synthesis tended to be greater for DDGS (118 g/d) than for UREA (86 g/d), which tended to be greater than CON (52 g/d). Urinary urea-N excretion was greater ([italics]P[italics]<0.03) for DDGS (35.1 g/d) and UREA (28.6 g/d) than for CON (12.7 g/d). Gut entry of urea-N (GER) was numerically greatest for DDGS (83 g/d), intermediate for UREA (57 g/d), and least for CON (39 g/d). Urea-N returned to the ornithine cycle tended to be greater for DDGS (47 g/d) than for UREA (27 g/d) or CON (16 g/d). The percent of microbial N flow derived from recycled urea-N tended ([italics]P[italics]=0.10) to be greater for DDGS (35%) than for UREA (22%) or CON (17%). The percent of urea production captured by ruminal bacteria was greater ([italics]P[italics]<0.03) for CON (42%) than for DDGS (25%) or UREA (22%). Experiment 2 diets were identical to those used in Experiment 1. In addition, steers were also fed either 0 or 60 mg/d zilpaterol-HCl. Dietary CP was 9.6, 12.4, and 13.7% for CON, UREA, and DDGS, respectively. Zilpaterol increased ([italics]P[italics]<0.01) total DMI and N intake; however, zilpaterol did not affect urea entry rate ([italics]P[italics]=0.80) or GER ([italics]P[italics]=0.94). Urea entry rate and GER were numerically greater for DDGS than CON and UREA. In conclusion, zilpaterol did not influence urea entry rate or GER. This lack of response in the face of greater N intake was interpreted to suggest that zilpaterol may reduce urea production and GER at constant N intake.