Effect of a wide range in the ratio of supplemental rumen degradable protein to starch on utilization of low-quality, grass hay by beef steers

Hereford×Angus steers were used in a 14-treatment, 2-period, crossover design experiment to examine effects of a wide range in the ratio of supplemental starch to rumen degradable protein (RDP) on low-quality forage utilization and ruminal characteristics. Steers were given ad libitum access to grass hay (4.9% CP, 42.4% ruminally degradable) and supplemented in a 2×7 factorial arrangement of treatments. All supplements were administered directly into the rumen and delivered one of two levels of ruminally degradable starch (cornstarch grits; 0 and 0.3% of initial BW) and one of seven levels of RDP (sodium caseinate; 0, 0.015, 0.051, 0.087, 0.123, 0.159, and 0.195% of initial BW). Supplementation with RDP increased consumption of forage OM, total OM, NDF, and digestible OM in a quadratic (P<0.01) fashion (intake increased and then declined). Starch supplementation depressed (P<0.01) forage OM and NDF intakes. In general, RDP supplementation elicited a positive quadratic response on NDF digestion (P=0.02). However, an interaction between supplemental starch and RDP level was observed (P<0.01) for NDF digestion. At the four lowest levels of supplemental RDP, starch supplementation substantially reduced NDF digestion, although for steers receiving the three highest levels of supplemental RDP, starch supplementation had little effect on NDF digestion. In contrast, neither the starch×RDP interaction nor the starch main effect was significant for OM digestion, which increased linearly (P<0.01) with supplemental RDP. Supplementation with RDP altered passage rate of acid detergent insoluble ash in a quadratic (P=0.05) manner that paralleled the intake response. However, liquid passage rate was not affected significantly. A decline in ruminal pH was associated (P=0.02) with increasing supplemental RDP and tended (P=0.07) to be associated with increasing starch, but it was not requisite for starch-induced depressions in NDF digestion. Ruminal NH3 concentration increased in response to increasing RDP, although the increase when starch was supplemented was less than that observed without starch (P=0.03). Supplemental starch generally elicited negative effects on low-quality forage intake and fiber digestion, but the effects on fiber digestion were overridden by adequate supplemental RDP. Supplemental RDP exerted a highly positive effect on consumption and digestion of this low-quality forage.