Effects of ruminal infusion of a slow-release polymer-coated urea or conventional urea on apparent nutrient digestibility, in situ degradability, and rumen parameters in cattle fed low-quality hay

Four ruminally cannulated beef steers were used in a 4 × 4 Latin square design experiment conducted to compare ruminal infusion effects of different sources of non-protein nitrogen (NPN) on intake; digestibility; ruminal dry matter (DM) and neutral detergent fiber (NDF) degradability; ruminal pH; and ammonia nitrogen (N-NH3) of beef cattle fed Brachiaria humidicola cv. Llanero hay [CP = 24.9 g/kg DM; ash-free NDF corrected for contamination of CP (NDFom,cp) = 782.4 g/kg DM]. Treatments were: Control (hay only); U [hay + conventional urea (204.2 g/d)]; SRU [hay + slow-release polymer-coated urea (297.5 g/d)]; U + SRU [hay + a combination of 1:1 by weight of both urea types (130.4 g/d U + 130.4 g/d SRU)]. Supplements were infused into the rumen through the rumen cannula. Treatments SRU and U + SRU had an increasing of hay dry matter intake (DMI) (9.44 and 9.02 kg/d, respectively; P<0.01, compared with 7.62 kg/d for the control), as well as total NDF (P<0.01) and crude protein (CP, P<0.01). The CP digestibility was higher (P<0.01) for all treatments with NPN when compared to control. There were differences on DM and NDF degradability curves (P<0.05) between rumen exposure times, in which the disappearance was increased up to 120 h after incubation. On the first day of collections, the lowest pH values (6.89 and 6.91) were observed on SRU and U + SRU 6 h after the afternoon infusion, and the highest pH was observed immediately after the afternoon infusion of U (7.39). High levels of ammonia-N were kept for 8 h after the last infusion daily of SRU, which resulted in the most constant values of NH3-N in the rumen. In general, our data suggested that SRU had a more adequate concentration of N during the fermentation of low-quality hay, and also had a proper pH for rumen activity.