PSII-3 Effect of Protein Supplementation Level and Degradability on Forage Utilization by Steers Consuming Low-Quality Forage: II. Bos taurus indicus

Abstract

Abstract A deficiency of ruminally available N (RAN) caused by a basal diet of low-quality forage may depress forage utilization. Supplemental protein improves utilization by addressing the RAN therefore increasing intake and potentially improving digestibility. The objective of this study was to determine the effect of ruminal degradability and level of protein supplementation on forage utilization by Bos taurus indicus steers. Accordingly, five ruminally cannulated Brahman steers (BW 369 ± 51 kg) were used in a 5 × 5 Latin square to evaluate the interaction between level and degradability of protein supplementation when consuming a basal diet of King Ranch Bluestem hay (3.5% CP, 71.0% NDF). Treatments were arranged as a 2 × 2 factorial with steers receiving protein supplements (43% CP) having two levels of protein degradability (35 or 70% RDP) fed at two levels of supplementation, 1.26 or 2.53 g/kg BW (LOW and HIGH). A control treatment providing no supplemental protein was also included (CON). Five 14-d periods were conducted, consisting of 9 d adaptation, 4 d to measure intake and digestion, and 1 d for ruminal fermentation. No degradability × level interactions were observed for any measures of intake or digestion (P > 0.10). Protein supplementation linearly increased (P < 0.01) total digestible OM intake (TDOMI), NDF intake (P = 0.01), and total digestible NDF intake (DNDFI; P < 0.03). Total digestible OMI was greater in steers receiving 70% RDP than the 35% RDP (P = 0.08). This response resulted from the linear increase in FOMI with 75% RDP (P = 0.03), that was not observed with 35% RDP. Despite differences in FOMI, TDOMI increased linearly (P ≤ 0.03) for both 35 and 70% RDP, partially the result of providing a highly digestible supplement. Forage OMI differences also resulted in NDFI being greater for 70% RDP than 35% RDP (P = 0.06) and DNDFI increasing linearly with the addition of both supplements (P ≤ 0.03). Organic matter digestibility (OMD) increased linearly with supplementation as a result of supplement provision. In contrast to observations previously reported in Bos taurus steers, provision of 35% RDP supplement did not improve FOMI. Supplementation with 70% RDP improved FOMI 19 and 20% compared to the basal diet at LOW and HIGH levels, respectively. A portion of this relatively modest response to protein supplementation could be attributed to greater intake in the unsupplemented controls.