INFLUENCE OF DIETARY MAGNESIUM LEVEL ON METABOLIC AND GROWTH-PERFORMANCE RESPONSES OF FEEDLOT CATTLE TO LAIDLOMYCIN PROPIONATE

A metabolism trial and a growth-performance trial were conducted to ev aluate the interaction of dietary magnesium level (.18 vs .32%) and la idlomycin propionate (LP) (0 vs 11 ppm, air-dry basis) on utilization of a high-energy finishing diet by feedlot steers. There were no treat ment effects (P >.10) on ruminal and total tract digestion of OM and A DF. However, there was an interaction (P <.05) between magnesium level and LP on ruminal starch digestion. With LP, magnesium level had no e ffect on ruminal starch digestion. Without LP, increasing dietary magn esium decreased ruminal starch digestion. Laidlomycin propionate decre ased (P <.10) ruminal degradation of feed N (13.9%) and microbial effi ciency (8.9%) and increased (P <.10) total tract N digestibility. Ther e were no treatment interactions (P >.10) on site and extent of magnes ium digestion. Magnesium absorption decreased with increased dietary m agnesium (11.6%, P <.05) and LP (16.9%, P <.01). There was an interact ion (P <.10) of treatments on postabomasal calcium absorption With the low magnesium level LP increased calcium absorption, whereas with til e 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 tile lower magnesium level and decreased wi th LP supplementation at the higher magnesium level. There were intera ctions (P <.05) between magnesium level and LP on ruminal VFA molar pr oportions and estimated methane production. At the lower magnesium lev el, the effects of LP on VFA molar proportions were small. At the high er 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 in take. At the lower magnesium level, LP increased (3.9%) feed intake; a t the higher magnesium level LP decreased (4.4%) intake. Increasing di etary magnesium enhanced ADG (6%, P <.10). Both magnesium and LP enhan ced (2.8%, P <.10! diet NE, and this effect was strictly additive. The re were no effects (P >.10) of magnesium level or LP on plasma magnesi um concentrations. Plasma calcium concentrations were decreased by sup plemental magnesium (15.2%, P < .05) and increased by LP (16.0%, P <.0 1). We conclude that dietary magnesium levels modulate the metabolic a nd performance responses of feedlot steers to supplemental LP.