Effect of supplementing barley straw with lucerne silage or cottonseed meal on diet digestibility and growth rate of steers

Abstract

Two experiments, separately examining digestibility (Experiment 1) and growth (Experiment 2), were conducted concurrently to investigate the effects of supplementation with either increasing amounts of lucerne silage (Medicago sativa) or a single amount of cottonseed meal (CSM—Gossypium hirsutum) on the utilization of barley straw (Hordeum vulgare) by yearling Hereford steers (284–342kg liveweight (LW)).In Experiment 1, twelve steers were used in a partial changeover design to determine the digestibility of barley straw alone (BS), lucerne silage alone (LS) and barley straw supplemented with increasing amounts of lucerne silage (L2, L4 and L6) or a single amount of cottonseed meal (BS+CSM). Proportions of straw in the BS+CSM, L2, L4 and L6 diets were 826, 578, 334 and 225g/kg respectively, and total intakes were 10.8, 13.5, 12.9, 15.6, 15.3 and 15.2g DM/kg LW.d for treatments BS, BS+CSM, L2, L4, L6 and LS respectively. There were three estimates of digestibility per steer and six estimates per diet. Digestibility differed between treatments (P<0.01) with LS diet having the highest digestibility and BS the lowest with L2, L4, L6 and BS+CSM intermediate and not different to each other.In Experiment 2, thirty six steers were fed barley straw ad libitum and were supplemented with 1.9 (L2), 3.9 (L4) or 5.7 (L6) kg lucerne silage DM/d, or 0.8kg CSM DM/d for 49 d. Proportions of straw eaten in the BS+CSM, L2, L4 and L6 diets were 811, 586, 326 and 166g/kg respectively. L2, L4, L6 and BS+CSM diets were estimated to contain 97.5, 144.0, 172.4 and 106.2g CP/kg DM and 7.1, 8.0, 8.5 and 6.8MJ ME/kg DM respectively. There were significant treatment effects on total DMI (P<0.001), straw intake (P<0.001) and LW gain (P=0.018). Total DMI was 4.66, 5.80, 6.91 and 4.28kg/d (l.s.d.=0.485); straw intake 2.73, 1.89, 1.15 and 3.47kg/d (l.s.d.=0.493); LW gain 0.06, 0.23, 0.45 and 0.49kg/d (l.s.d.=0.248) for L2, L4, L6 and BS+CSM diets respectively. NDF intake was similar between diets. Straw intake declined (P<0.05) with increasing silage supplementation, but total intake increased (P<0.05). LW gain on BS+CSM was higher (P<0.05) than L2 and L4, and considerably higher than predicted from feeding standards.