Effects of lasalocid and monensin on nutrient digestion, metabolism and rumen characteristics of sheep.

Two experiments were conducted to evaluate metabolism and rumen fermentation responses of lambs fed either monensin or lasalocid (33 mg/kg level) in an alfalfa-corn diet. Experiment 1 was a lamb metabolism trial where rumen samples were taken at the end of each excreta collection period. In Exp. 2, ruminally cannulated sheep were used and cotton thread cellulose and soybean nitrogen (N) disappearance were measured by the in situ dacron bag technique. Rumen turnover rates and volatile fatty acid (VFA) and ammonia pool sizes were determined. In Exp. 1, dry matter and fiber (neutral detergent fiber, NDF; acid detergent fiber, ADF and hemicellulose) digestibilities were not influenced by treatment. Absorbed N was increased (P less than .05) with lasalocid, but not with monensin. No direct or adaptive rumen effects on in situ cellulose or N disappearance were noted in Exp. 2 for either additive. In Exp. 1, at 6 h postfeeding, both additives increased (P less than .05) propionate concentration and lowered (P less than .05) the acetate: propionate (A:P) ratio, while only monensin decreased (P less than .05) rumen ammonia levels. However, no differences in ammonia or VFA concentrations were noted at 2 and 4 h postfeeding. In Exp. 2, where pool size was considered, monensin-fed sheep had lower (P less than .05) total quantities of rumen ammonia at 8 and 12 h postfeeding. Both additives decreased A:P ratios at 6 and 12 h. Lasalocid increased (P less than .05) acetate, propionate and total VFA pool size at 12 h.