Effects of Molasses-Based Liquid Feeds Containing Conventional Urea or Commercial Fat-Coated Urea Product on in Vitro Rumen Fermentation, Gas Kinetics, and Substrate Degradability

This study evaluated molasses-based liquid feeds containing conventional urea or a commercial fat-coated urea product marketed as slow-release urea (SRU). The effects on in vitro gas-production kinetics, rumen fermentation, and substrate degradability were examined. Treatments followed a 5 × 3 factorial design. The five formulations were 0% urea, 5% urea, 10% urea, 5% SRU, and 10% SRU. Each formulation was supplied at 0.9%, 1.8%, or 2.7% of the 0.5 g basal substrate dry matter (DM). Four independent incubation runs were conducted. A formulation × inclusion-level interaction was detected for gas-production lag time (p = 0.0046). The shortest lag time occurred with 10% SRU supplied at 2.7%. At 48 h, the 0.9% inclusion level resulted in greater dry matter and organic matter degradability than the 1.8% and 2.7% levels. The 5% SRU formulation had the lowest 24 h degradability, whereas 10% SRU had the greatest 48 h dry matter degradability. Ammonia nitrogen (NH3-N) was analyzed separately at each sampling time. The 10% conventional urea formulation had the greatest NH3-N concentration at 1 and 2 h. The 10% SRU formulation had the greatest concentration at 4, 6, and 12 h. These within-time differences do not demonstrate different ammonia-release patterns over time. At 12 h, the 10% SRU formulation had the greatest total volatile fatty acid concentration and propionate molar proportion. Protozoal counts were unaffected. The results reflect differences among the complete liquid-feed formulations. They do not provide direct evidence of controlled ammonia release or improved nitrogen–carbohydrate synchronization. Further in vivo studies using compositionally balanced formulations are required.