Improving nitrogen efficiency through enhanced urea-nitrogen recycling in ruminants

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