PSII-22 Effect of Protein Supplementation Level and Degradability on N Balance By Steers Consuming Low-Quality Forage: I.Bos Taurus Taurus

Abstract

Abstract Globally, cattle convert low-quality forage to beef and milk; therefore, understanding protein utilization in Bos taurus taurus is critical to enhancing sustainability as over and under-supplementation of protein are economically and environmentally costly. Low-quality forage lacks the N necessary for proper utilization and metabolic function. Supplemental protein improves low-quality forage utilization and is expected to increase N absorption and retention. Accordingly, our objective was to determine the effect of ruminal degradability and level of protein supplementation on N balance in Bos taurus taurus steers. To achieve this objective, 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 steers receiving protein supplements (43% CP) with two ruminal degradable protein levels; 35% RDP (35%; 58% corn gluten meal and 42% soybean hulls) and 70% RDP (70%; 100% soybean meal). Supplements were fed at two levels, providing 1.26 or 2.53 g of supplement/kg BW (LOW and HIGH, respectively). A control (CON) provided no supplement. Five, 14-d periods, consisted of 9 d adaptation and 4 d to measure intake, digestion, and N balance, and 1 d for ruminal fermentation. No significant (P ≥ 0.14) degradability × level interactions were detected for any measures of N balance. Although forage organic matter intake increased (P < 0.01) in response to supplementation, forage N intake was not improved. Supplemental N intake increased linearly (P < 0.01) with increased supplementation, as designed, resulting in a significant linear (P < 0.01) increase in total N intake. In addition, 70% RDP supplement resulted in greater supplemental N intake than the 35% RDP supplement. Fecal N excretion increased linearly with supplementation. Supplementation at the HIGH level increased (P < 0.01) fecal N excretion 90% compared with CON. There was no significant (P = 0.94) effect of ruminal degradability. Urinary N excretion was greater (P < 0.01) when the 70% RDP supplement was fed than the 35% RDP. Nitrogen absorption linearly increased with supplementation (P < 0.01). Similarly, nitrogen retention linearly increased (P < 0.01) with level of supplementation but was not significantly affected by degradability (P = 0.87). In conclusion, providing supplemental N to Bos taurus taurus increased N intake, excretion, absorption, and retention. Level of ruminal degradability did not affect N retention or absorption suggesting that protein supplementation regardless of ruminal N degradability improves N balance in Bos taurus taurus consuming low-quality forage.