Effect of salinomycin or monensin on performance and feeding behavior of cattle fed wheat- or barley-based diets

Feeding behavior and growth performance of cattle fed diets containing monensin or salinomycin were assessed in two trials. In trial 1, 36 Hereford x Angus steers (267.7 +/- 4.3 kg) were individually fed (n = 12) wheat-based transition and finishing diets containing no ionophore (control, C), 26 mg monensin (M) or 13 mg salinomycin (S) per kg of dietary dry matter (DM). Cattle fed M consumed less than those fed C or S, and their intake was more stable during the transition to the finishing diet. Overall, steers fed M exhibited lower dry matter intake (DMI) (8.0 vs. 9.2 and 9.2 kg d(-1)) and rates of gain (1.21 vs. 1.62 and 1.56 kg d(-1)) than those fed C or S. Cattle fed S required fewer days (93.3) to reach the targeted finish (5 mm backfat) than those fed C or M (105.8 d). Monensin reduced slaughter weight and carcass weights, relative to controls (414.3 vs. 480.5 kg, and 231.2 vs. 245.8 kg, respectively). In trial 2, M (25 ppm) or S (13 ppm) were included in barley-based diets for 72 yearling steers placed in four pens equipped with radio frequency identification systems. Individual bunk attendance patterns were monitored during transition to a finishing diet, during 11 d of limit feeding the finishing diet twice daily (LF2/d), 13 d of limit feeding once daily (LF1/d), and 21 d of feeding once daily to ad libitum intake (AL1/d). Ionophore type did not affect (P > 0.10) DMI, rate of gain or efficiency of feed conversion. Bunk visits were more frequent (P < 0.05) with M than with S during transition and limit-feeding. With M, total daily attendance (TDA) at the bunk during LF1/d and AL1/d, was higher (P < 0.05) than with S, and variability in TDA was lower (P < 0.05) during LF1/d. In the present study, there was no performance advantage in providing S or M in wheat-based finishing diets. Monensin moderated feeding intensity, but this effect may have been strong enough to suppress intake and even reduce gain on the wheat-based diet.