Master of Science / Department of Animal Sciences and Industry / Evan C. Titgemeyer / Two experiments quantified effects of supplemental protein and energy on forage digestion and urea kinetics in beef cattle. In experiment 1, energy treatments included: control, 600 g glucose dosed ruminally once daily, and 480 g VFA infused ruminally over 8 h daily. Casein was dosed ruminally once daily (120 or 240 g). Cattle (208 kg) had ad libitum access to low-quality hay (5.8% protein). Infusion of VFA decreased forage intake by 27%. Glucose decreased NDF digestibility. Microbial N flow was greater for 240 than for 120 g/d casein, but was not affected by energy. Retained N increased with casein supply. Urea-N entry rate (UER) and gut entry of urea-N (GER) were not affected by energy, casein, or interactions, but GER/UER was less when 240 rather than 120 g/d casein was provided. Compared to VFA, glucose tended to increase GER/UER. Glucose led to more microbial uptake of recycled urea than VFA. In these young calves, changes in N and energy supply did not greatly impact urea kinetics, likely because increased N was largely retained. In experiment 2, treatments included: 0 or 1.2 kg glucose, and 240 or 480 g casein. Cattle (391 kg) were fed low-quality hay (4.7% protein). Glucose reduced forage intake by 18%, whereas casein did not affect it, and depressed fiber digestion. Microbial N flow to the duodenum and retained N increased as casein increased, but neither was affected by glucose. Increasing casein increased UER 50%. Urinary urea-N increased as casein increased; moreover, GER numerically increased 25% as casein increased. GER/UER decreased as casein increased. Glucose decreased urinary urea, but did not change UER or GER. Microbial uptake of recycled urea was least for steers receiving 480 g/d casein with no glucose, reflecting that this treatment exceeded ruminal requirement for N. In these more mature steers, increases in N intake increased UER, reflecting that only small proportions of the increased N intake were retained. Thus, as steer maturity increased, UER and GER increased, likely because less N was retained. These studies demonstrate the influence of urea recycling in meeting N needs of cattle fed low-quality forage.

The objective of this research was to investigate the effects of slow release urea on N metabolism in cattle. The ruminal behavior of Optigen®II and the effect of basal diet on the in situ degradability of urea and Optigen®II were evaluated. The effect of slow release urea and its interaction with degradable intake protein (DIP) level in the diet on N retention and excretion was evaluated utilizing 8 Holstein steers in a 4 x 4 Latin square experiment. In addition, the effect of slow release urea and DIP level on ruminal and systemic urea kinetics was evaluated using stable isotope techniques with 8 Holstein steers in a 4 x 4 Latin square experiment. Finally, slow release urea was evaluated under a practical beef production setting. The performance of slow release urea was compared to regular feed grade urea in a 42 day receiving study (288 Angus cross steers) as well as a 70 day growing study (240 Angus cross steers). High forage diets increased the ruminal degradation rate of both urea and slow release urea an increased the extent of degradation of slow release urea when compared to high concentrate diets. Lower DIP concentrations in the diet reduced systemic urea production, ruminal ammonia and plasma urea concentrations and urinary urea excretion under most circumstances but also led to a reduction in N retention, reduced diet digestibility, lower feed intake, lower growth rate and decreased feed efficiency. High DIP intakes increased N retention, growth rate, diet digestibility and improved feed efficiency but also lead to increased excretion on urea N in the urine. Slow release urea improved N retention and efficiency of N retention in high DIP diets when compared to urea and generally reduced plasma urea and ruminal ammonia concentrations. Compared to urea, slow release urea did not significantly improve the production of receiving cattle. However Optigen®II improved the feed efficiency when compared to urea on high concentrate diets but reduced feed efficiency on high forage diets.