PSII-2 Effect of Protein Supplementation Level and Degradability on Forage Utilization by Steers Consuming Low-Quality Forage: I. Bos taurus taurus

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 increasing 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 taurus steers. Accordingly, six ruminally cannulated Angus steers (BW 294 ± 37.7 kg) were used in a 5×5 Latin square to evaluate the interaction between level and degradability of protein supplementation when consuming 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) with two levels of protein degradability (35 or 70% RDP) fed at two levels of supplementation, providing 1.26 or 2.53 g/kg BW (LOW and HIGH). A control treatment (CON) provided no supplemental protein. Five 14-d periods were conducted, consisting of 9 d adaptation and 4 d to measure intake and digestion, and 1 d for ruminal fermentation. No degradability × level interactions were significant for any measures of intake or digestion, although there was a tendency for an interaction (P = 0.09) for total organic matter intake (TOMI), resulting from a tendency for a quadratic (P = 0.08) increase when supplemented. In contrast, the 70% RDP supplement continued to increase TOMI linearly (P < 0.01), suggesting the maximum effective level of supplementation was not reached. As expected, protein supplementation increased all measures of intake such as, forage OMI (FOMI), total digestible OMI (TDOMI), and NDF intake (P < 0.01). In addition, level of supplementation affected all measures of intake (P ≤ 0.01) with HIGH being greater than LOW. Protein supplements with 70% RDP at LOW increased FOMI 28% versus control when supplemented at LOW, and 47% at HIGH. Supplementation with 35% RDP increased FOMI 23% at the LOW and 28% at HIGH indicating the lower degradability supplement was less effective at improving forage utilization that the 70% RDP supplement (P = 0.01). Total NDF intake (NDFI) followed this same pattern with 70% RDP supplementation increasing NDFI 29 and 52% with LOW and HIGH supplementation, respectfully; 28 and 37% when 35% RDP supplementation was provided (P = 0.10). Despite the effect of protein supplementation on intake, OM digestion (OMD) and NDF digestion (NDFD) were not affected by protein supplementation regardless of degradability or level (P ≥ 0.23). In conclusion, FOMI and TDOMI were increased with the provision of greater levels and more degradable supplements resulting in better forage utilization.