The effect of avoparcin and monensin on the performance of growing and finishing cattle offered grass silage-based diets

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Abstract
The effect on animal performance of the inclusion in the diet of avoparcin at one or more of the following levels 0, 150, 300 and 450 mg per head daily and 200 mg per head daily monensin was studied i n a series of four trials. Finishing cattle of 428 (s.e. 3·9) kg live weight were used in three trials and yearling heifers (218 (s.e. 4·9) kg live weight) in the fourth. All animals received grass silage ad libitum with 0·5 to 6·0 kg per head daily of a cereal-based supplement. Relative to the control diets with no additive, proportionate increases in live-weight gain of between 0·07 and 0·38 were obtained with the inclusion of avoparcin at 150 mg per head daily in the diet with a small depression in silage dry-matter intake. The effects at higher levels of inclusion of avoparcin and of monensin were more variable. Alteration in the proportion of propionate to acetate in the rumen fluid of the animals was observed only at the highest levels of additive inclusion. There was no effect of additive treatment on carcass weight or dressing proportion.