Impact of supplementing different sources of non-protein nitrogen on ruminal fermentation, nutrient digestibility, and microbial protein synthesis in beef cattle consuming a corn silage-based diet.

Supplementation of low-protein diets with non-protein nitrogen (NPN) increases ruminal degradable protein and improves rumen fermentation and microbial growth. The objective of this experiment was to evaluate the effect of supplementing urea-biuret (UB) and urea-biuret-nitrate (UBN) mixtures relative to urea (U) on rumen fermentation and microbial N outflow in growing steers. Twelve American Aberdeen steers were used in a replicated and balanced 3 × 3 Latin square design (LSD) with 3 periods of 35 d each. Steers were housed in pens and consumed a corn silage-based diet. Steers were stratified by body weight and randomly allocated to 1 of the 3 NPN supplementation treatments. Treatments were supplementation with U, UB, and UBN, adjusted to the amount of N provided by U when included at 1% of the diet on a dry matter (DM) basis. Intake and feeding behavior were individually recorded throughout the experiment. In each period, steers were adapted to increasing levels of NPN during the first 8 days. From days 19 to 23, feed and fecal samples were collected to assess nutrient digestibility. Samples of blood, rumen contents, and omasal digesta were collected on days 20 to 23. On day 24, rumen evacuation was performed, and subsequently steers were dosed with Co-EDTA and YbCl3 to determine the passage rate of digesta flow. Rumen fluid collection was conducted on days 24 and 25. Steers did not receive NPN supplementation from days 26 to 35 during the washout period. Microbial N flow was estimated for each animal within periods. Intake, digestibility, digesta flow, and microbial N flow were analyzed using a 3 × 3 LSD, while blood and rumen fermentation parameters were analyzed using a 3 × 3 LSD with repeated measures. Steers supplemented with UB tended (P < 0.07) to consume more DM and organic matter (OM) than those supplemented with UBN; however, animals supplemented with UBN tended (P = 0.051) to digest more acid detergent fiber in the total tract. Steers supplemented with U and UB showed greater (P < 0.05) DM and OM flow throughout the omasum than those with UBN. Microbial N flow, microbial efficiency, and the concentration of total volatile fatty acids were not different (P > 0.10) among NPN-supplemented treatments. Steers supplemented with UBN showed lesser (P < 0.05) concentration of ammonia than those with U and UB. In conclusion, novel NPN mixtures have the potential to modify ruminal fermentation without affecting microbial protein outflow.