The effect of lucerne silage supplements on the utilization of barley straw by steers

Two experiments were conducted to investigate the effects of supplementation on the utilization of barley straw, a typical low quality feed. Supplements provided extra protein and energy with the primary focus on lucerne silage, given at several levels, with cottonseed meal (CSM) also assessed. Supplements were evaluated by effects on feed intake and growth rate of young cattle, and by measurement of digestibility of the diets. In Experiment 1, 36 Hereford yearling steers were fed barley straw ad libitum for 49 days with daily supplements of either 1.9, 3.9 or 5.7 kg lucerne silage (treatments L2, L4 and L6) or 0.8 kg cottonseed meal (CSM) as the control diet. L2, L4, L6 and CSM diets were estimated to contain 97.5, 144.0, 172.4 and 106.2 g CP/kg DM and ME of 7.1, 8.0, 6.8 and 8.5 MJ/kg DM respectively. There were significant treatment differences in total DM intake (P<0.001), straw intake (P<0.001) and liveweight gain (P=0.018). Total DM intakes were 4.66, 5.80, 6.91 and 4.20 kg/d; straw intakes 8.52, 5.95, 3.49 and 10.57 kg/d; liveweight gain 0.064, 0.227, 0.454 and 0.485 kg/d for L2, L4, L6 and CSM diets respectively. NDF intakes were similar between diets. Liveweight gain and intake both increased (P<0.05), and straw intake decreased (P<0.05) in response to increasing level of silage supplement. Liveweight gain on CSM was higher (P<0.05) than L2 and L4 and considerably higher than predicted from feeding standards. In Experiment 2, conducted concurrently, 12 steers from the same source were used to determine digestibility of the L2, L4, L6 and CSM diets, plus barley straw (BS) and lucerne silage (LS) in a partial changeover design; 3 estimates per steer and 6 estimates per diet. The L2, L4, L6 and CSM diets were formulated to have the same composition as predicted consumption in Experiment 1, and were fed at a restricted rate (16.5g DM/kg liveweight) during measurement of digestibility. Significant treatment differences were found for DMD (P= 0.002), OMD (P= 0.007) and DOMD (P= 0.009). The LS diet had the highest digestibility and BS the lowest. Digestibility estimates for L2, L4, L6 and CSM were intermediate and similar to each other. Experiment 1 established that both total DM intake and growth rate were enhanced by supplementation with lucerne silage, with a clear trend for improvement with increasing rates of the silage. Total intake was apparently limited by NDF intake. The growth rate on CSM was higher than expected, based on diet quality and intake, suggesting a better utilization of nutrients. Experiment 2 confirmed the large difference in digestibility between barley straw and lucerne silage. Supplementation with silage increased total diet digestibility, but there was no evidence of an associative effect on the utilization of straw. CSM supplementation also increased total diet digestibility, however the improvement was greater than expected, indicating improved utilization of the straw component. These experiments have shown the potential for supplementation of barley straw with lucerne silage or CSM to improve livestock production, thereby providing practical opportunities to effectively utilize such forage.