PSV-2 - Effect of feeding level during feedlot backgrounding on animal growth and feed efficiency of the winter-spring period

Abstract: This study evaluated the effect of winter backgrounding feeding system (grazing vs. feedlot) and the amount of feed offered in the feedlot (FO) on average daily gain (ADG) and feed-to-gain ratio (FG) during the overall winter-spring period (182 days). Forty Hereford female calves (159 ± 25 kg body weight (BW)) were randomly assigned to one of five treatments during winter (98 days): grazing winter grass pasture at a forage allowance of 5 kg dry matter (DM) per 100 kg BW, or receiving a feedlot total mixed ration (70% concentrate/ 30% roughage) differing in FO (2.2%, 2.5%, 2.9%, and 3.2% BW on a DM basis). During spring (84 days), all treatments grazed a mixed grass-legume pasture (6 kg DM/100 kg BW, 2524 ± 880 kg DM/ha) in weekly plots (n = 2 plots/ treatment). BW was recorded every 14 days. DMI was measured daily at feedlot and estimated on even weeks as the difference between pre-and post-grazing pasture DM biomass during grazing period. Data were analyzed using a completely randomized design, and orthogonal contrasts to compare feeding system effects (feedlot (FL) vs. grazing (GR)) and the linear or quadratic effects of FO in the feedlot. During winter (Table 1), FL animals exhibited greater ADG (1.07 vs. 0.59 kg/d, SE: 0.04, P < 0.0001) and better FG (4.90 vs. 6.42 kg DM/kg BW, SE: 0.14, P = 0.0002) compared to GR. These responses were the opposite during spring, with a worst FG observed for feedlot animals (33.59 vs. 18.80; SE: 1.80, P = 0.0007) due to lower ADG (0.21 vs. 0.48; SE: 0.07, P = 0.0007). Lower ADG in the FL group during spring was primarily explained by significant weight loss during the first two grazing weeks compared to GR (-0.46 vs. 0.44 kg/day, SE: 0.08, P < 0.001). However, over the entire winter-spring period, FL animals demonstrated greater ADG compared to GR (0.68 vs. 0.56 kg/day, SE: 0.05, P = 0.0092) and better FG (10.71 vs. 9.65 kg DM/kg LW, SE: 0.36 P = 0.0508). Within the FL group during the winter-spring period, ADG increased linearly with FO (P = 0.0196), though no significant differences were observed in FG (P = 0.0916). No quadratic significant effects were found for any of the variables analyzed. Results evidence that over the entire winter-spring period, FL animals showed higher overall ADG and FG compared to grazing animals. Additionally, increasing FO linearly improved ADG and final BW by the end of spring.