PSIX-23 Effect of restricted forage intake and protein supplementation on low-quality forage digestibility and rumen fermentation in Brahman steers.

Abstract A deficiency in ruminally available N, caused by a basal diet of low-quality forage (LQF), results in reduced forage utilization. These effects are further exacerbated during times of limited forage availability such as drought. Supplemental protein ameliorates the negative effects of LQF; however, the response to protein supplementation when forage intake is restricted is not well characterized especially in Bos taurus indicus. Therefore, our objective was to determine the effect of restricted forage intake and protein supplementation on LQF digestibility and rumen fermentation in Bos taurus indicus steers. Six ruminally-cannulated Brahman steers (BW 427 ± 53 kg) were used in a 6 × 4 incomplete Latin square with four treatments arranged as a 2×2 factorial. Level of forage intake was the first factor (ad libitum or 1% of BW; A or R, respectively) and the second factor consisted of two levels of protein supplementation (0 or 130 mg N/ kg BW; CON or PRO, respectively). Hay was KR Bluestem (5.4% CP and 76.6% NDF) and supplemental protein was provided as range cubes (45.3% CP). Four 14-d periods were conducted with 9d for adaptation, 4 d to measure intake and digestion, and 1d for rumen fermentation. There was a tendency (P ≥ 0.06) for an intake × supplementation interaction for all measures of intake, except for supplement intake, as per project design. When steers were afforded ad libitum access to hay, supplementation increased forage organic matter intake (FOMI) from 1.39 to 1.58% of BW. In contrast, FOMI was 0.81 and 0.79% of BW for restricted steers with and without supplementation, respectively. Supplement OM intake was, as per experimental design, increased from 0.00 to 0.17% of BW for CON and PRO, respectively. There were no main effects or interaction effects on OM and NDF digestibility (P ≥ 0.15). When intake and digestion were combined as total digestible organic matter intake (TDOMI) there was a significant intake × supplementation interaction (P = 0.05). Protein supplementation resulted in greater TDOMI for RPRO than RCON (0.75 vs. 0.60% of BW), a response driven by the supplement itself and not an improvement in forage digestion. Without supplementation (ACON) steers were able to access more TDOMI (1.19% of BW) than either of the restricted treatments but still less than APRO (1.54% of BW). There was an intake × hour interaction on ruminal pH (P < 0.01); pH was affected by hour in restricted steers with the highest pH at 0h post-feeding (6.9). Ruminal pH was not affected by hour in ad libitum steers (P > 0.06). Further research investigating the response of protein supplementation at varying levels of forage intake in Bos taurus indicus can lead to more precise supplementation strategies for beef cattle producers.