Effect of level of rumen degradable protein and type of supplemental non-fiber carbohydrate on intake and digestion of low-quality grass hay by beef cattle

Abstract

Sixteen ruminally fistulated beef steers (BW=220±16 kg) were used to evaluate the interaction of non-fiber carbohydrate (NFC) type and level of rumen degradable protein (RDP) supplementation on low-quality forage utilization. Steers given ad libitum access to a low-quality grass hay (51 g/kg CP, 762 g/kg NDF) were used in a 15 treatment, two period crossover experiment. Steers were subjected to two 24-day periods that included intervals for measuring intake and total fecal output, ruminal evacuation, and monitoring ruminal fermentation. Treatments were arranged as a 2×7 factorial plus an additional treatment which served as a negative control (i.e., no supplemental NFC or RDP). Treatments within the supplemented groups consisted of feeding one of two sources of NFC (starch or dextrose) at 3.0 g/kg of initial BW in combination with one of seven levels of RDP (casein; 0, 0.15, 0.51, 0.87, 1.23, 1.59, 1.95 g/kg of initial BW). Supplements were ruminally dosed once daily. No NFC type×RDP level interactions occurred for intake or digestion measure. Total digestible OM intake increased with increasing supplemental RDP, but at a decreasing rate (quadratic; P<0.05). Both forage and total OM intake responded linearly ( P<0.01) to increasing supplemental RDP and slight quadratic trends ( P=0.17 and 0.18, respectively) were observed. Type of supplemental NFC did not affect forage, total or digestible OM intake. In response to increasing RDP supplementation, digestion of OM increased linearly ( P=0.03) whereas NDF digestion tended (quadratic; P=0.08) to increase initially but plateaued. When compared with the negative control, digestion of NDF was depressed ( P<0.05) when supplemental NFC was fed in combination with RDP levels of 0.51 g/kg of BW or less. Type of NFC did not affect OM or NDF digestion. Passage rate increased linearly ( P<0.01) with supplemental RDP, but supplemental NFC type did not affect passage rate. Supplemental RDP exerted positive effects on forage intake and digestion and, when supplied in sufficient quantity, was able to overcame the negative effect of supplemental NFC on fiber digestion.