Effect of postruminal protein supply on the response to ruminal protein supplementation in beef steers fed a low-quality grass hay*1

Abstract

An experiment was conducted to evaluate effects of infusing protein postruminally on the utilization of low-quality grass hay by beef steers concurrently ruminally infused with increasing levels of protein. Twelve ruminally fistulated steers (361±8.2 kg BW) were used in a two-treatment, two-period cross-over design nested within a six treatment completely randomized experiment. Steers were allowed ad libitum access to a low-quality grass hay (53 g/kg CP, 717 g/kg NDF) for 17 days before and throughout the experiment. Each period of the experiment had five intervals: (1) 10 days adaptation to treatments; (2) 7 days measurement of intake and digestibility; (3) 3 days ruminal sampling; (4) 10 days ad libitum access to hay only; (5) 7 days measurement of intake. Steers were supplemented in a 6×2 factorial arrangement of treatments. For the first factor, steers were dosed ruminally once daily with one of six levels of sodium caseinate (0, 0.29, 0.58, 0.87, 1.16, and 1.45 g/kg of initial BW) to simulate ruminally degradable protein supplementation. For the second factor, steers were infused postruminally once daily with one of two levels of sodium caseinate (0 and 0.87 g/kg of initial BW) via infusion lines anchored in the abomasum to simulate ruminally undegradable protein supplementation. Provision of protein ruminally resulted in a linear (P≤0.03) increase in hay OM intake, total digestible OM intake (TDOMI), and OM digestion. Infusion of protein postruminally tended (P=0.06) to increase hay OM intake, and increased TDOMI, and OM digestion (P≤0.01). A trend to a ruminal protein level×postruminal protein interaction (P=0.07) was observed for TDOMI, which was due to a lower response to the first increment of ruminal protein supplementation when steers received postruminal protein. A quadratic increase (P=0.03) in NDF digestion occurred with increasing ruminal protein although, postruminal protein did not affect NDF digestion. Ruminal and postruminal protein supplementation improved forage utilization, but the response to direct ruminal protein provision was higher. Improvements in forage utilization in response to postruminal protein supply may be due to N recycling, or other effects related to improved N status of the animal.