Provision of post-ruminally available urea impacts ruminal ammonia concentration and fermentation

The objective of this work was to evaluate the effect of urea sources on feed intake, N balance and rumen parameters of beef steers fed a high fibrous diet. Rumen-fistulated steers were used in a replicated 3x3 Latin square design. Wheat straw was offered ad libitum and a compound feed (3 g/kg BW) was delivered daily into the rumen via a cannula. Treatments were feed grade Urea, slow release urea (SRU - Optigen (R)) and post-ruminally available urea (PRU) mixed into the compound feed at 0.3 g of urea equivalent per kg BW. Urine and faeces were quantitatively collected for 4 days and serial blood and rumen fluid samples were taken. Urea resulted in the highest NH3 concentration, followed by SRU and PRU. Straw intake tended to be most stimulated by PRU, followed by SRU and Urea. Nitrogen retention was similar across treatments, indicating similar N use efficiency. Total VFA concentration and acetate proportions were higher with PRU. Post-ruminal urea is a viable source of N for ruminants. Results indicate that lower ruminal NH3 concentration stimulates voluntary intake and increases ruminal VFA concentration.