INFLUENCE OF DIETARY MAGNESIUM LEVEL ON GROWTH-PERFORMANCE AND METABOLIC RESPONSES OF HOLSTEIN STEERS TO LAIDLOMYCIN PROPIONATE

We used 216 Holstein steers (151 kg) in a 262-d trial to evaluate the influence of dietary magnesium level(.19, .25, and .32%) and laidlomyc in propionate (LP; 0 vs 11 ppm, air-dry basis) on growth performance a nd NE value of the diet. During the initial 112 d of the trial, LP inc reased (P < .01) ADG (6.3%) and feed efficiency (4.2%). From d 112 unt il slaughter, LP increased (P < .05) ADG (9.7%) and feed efficiency (4 .5%). Across the 262-d feeding period, LP supplementation enhanced(P < .01) ADG (8.9%) and feed efficiency (6.3%). There was an interaction(P <.05) between dietary Mg and LP on NE value of the diet. The enhancem ent in NE value of the diets owing to LP with.19,.25, and.32% dietary Mg were .5, 3.0, and 5.9%, respectively. Six Holstein steers (302 kg) were used in a 6 x 6 Latin square experiment to evaluate treatment eff ects on characteristics of ruminal and total tract digestion. There we re no treatment interactions (P > .10) on site and extent of digestion of OM, starch, and N. Supplemental Mg increased (quadratic effect, P < .10) ruminal OM digestion. Neither LP nor dietary Mg level affected (P > .10) ruminal digestion of starch and feed N. Supplemental LP decr eased (15%, P < .05) ruminal microbial efficiency. Total tract digesti on of OM and N increased (linear effect, P < .01) with increasing diet ary Mg level. There were interactions between LP and dietary Mg level on ruminal soluble-Mg concentration (linear effect, P < .01) and Mg ab sorption (quadratic effect, P < .05). Apparent total tract Mg digestio n increased owing to LP (P < .01) and dietary Mg level (linear effect, P < .01). There were no treatment effects (P > .10) on ruminal pH. Di etary Mg level did not influence (P > .10) ruminal VFA concentrations or molar proportions. Supplemental LP increased (14%; P < .10) total r uminal VFA concentration but did not affect (P > .10) VFA molar propor tions. We conclude that LP will increase daily weight gain and feed ef ficiency of calf-fed Holstein steers and that this response may be enh anced by increasing dietary Mg level.