Influence of dietary magnesium level on metabolic and growth-performance responses of feedlot cattle to laidlomycin propionate.

Abstract

A metabolism trial and a growth-performance trial were conducted to evaluate the interaction of dietary magnesium level (.18 vs .32%) and laidlomycin propionate (LP) (0 vs 11 ppm, airdry basis) on utilization of a high-energy finishing diet by feedlot steers. There were no treatment effects (P > .10) on ruminal and total tract digestion of OM and ADF. However, there was an interaction (P < .05) between magnesium level and LP on ruminal starch digestion. With LP, magnesium level had no effect on ruminal starch digestion. Without LP, increasing dietary magnesium decreased ruminal starch digestion. Laidlomycin propionate decreased (P < .10) ruminal degradation of feed N (13.9%) and microbial efficiency (8.9%) and increased (P < .10) total tract N digestibility. There were no treatment interactions (P > .10) on site and extent of magnesium digestion. Magnesium absorption decreased with increased dietary magnesium (11.6%, P < .05) and LP (16.9%, P < .01). There was an interaction (P < .10) of treatments on postabomasal calcium absorption. With the low magnesium level LP increased calcium absorption, whereas with the higher magnesium level LP decreased calcium absorption. There was an interaction between magnesium level and LP on ruminal pH at .5 h (P < .05) and 8 h (P < .10) after feeding. In general, ruminal pH increased with LP supplementation at the lower magnesium level and decreased with LP supplementation at the higher magnesium level. There were interactions (P < .05) between magnesium level and LP on ruminal VFA molar proportions and estimated methane production. At the lower magnesium level, the effects of LP on VFA molar proportions were small. At the higher magnesium level, however, LP decreased (13.2%) molar proportions of acetate and increased (26.5%) molar proportions of propionate. There was an interaction (P < .05) between magnesium level and LP on feed intake. At the lower magnesium level, LP increased (3.9%) feed intake; at the higher magnesium level LP decreased (4.4%) intake. Increasing dietary magnesium enhanced ADG (6%, P < .10). Both magnesium and LP enhanced (2.8%, P < .10) diet NE, and this effect was strictly additive. There were no effects (P > .10) of magnesium level or LP on plasma magnesium concentrations. Plasma calcium concentrations were decreased by supplemental magnesium (15.2%, P < .05) and increased by LP (16.0%, P < .01). We conclude that dietary magnesium levels modulate the metabolic and performance responses of feedlot steers to supplemental LP.