COMPENSATORY GROWTH AND BODY-COMPOSITION CHANGES IN GRAZING STEERS FOLLOWING SILAGE FEEDING WITHOUT OR WITH A SUPPLEMENT OF FISH-MEAL AND MONENSIN

The effects of fish meal and monensin supplementation of silage on the growth and composition of Friesian steers was examined in winter and subsequently during grazing. Four-month-old steers were given grass silage ad libitum, alone (U) or mixed with fish meal and monensin at 100 and 0.005 g/kg dry matter (DM) of silage (S) for 17 weeks. Empty body weight (EBW) and carcass weight (CW) gains were significantly increased by supplementation (810 v. 495, and 553 v. 332 g/day, respectively). Although treatment S steers were heavier, their composition at turn-out was similar to that of the treatment U animals: mean fat, crude protein (CP), water, ash and energy contents were 121, 187, 640, 51 g/kg and 9.10 MJ/kg EBW, respectively. During the following 20 weeks, six animals from each treatment grazed a perennial ryegrass/white clover sward unsupplemented (UU and SU), or supplemented with a fish meal/maize/molassine meal (50:35:15) pelleted diet and monensin at 4 g/kg live weight (LW) per day and 0.1 mg/kg LW per day, respectively, (UMF and SMF). A further group of six steers from treatment S received the pelleted diet without monensin during grazing (SF). Steers from treatment U showed significant compensation in EBW and in carcass growth at pasture, irrespective of whether they received a supplement, which led to a marked reduction of the differences in EBW and CW by final slaughter. Composition at final slaughter showed no significant effect of winter nutrition, with the exception of fat content (g/kg EBW), which was significantly lower in animals from treatment SU. Steers from treatments UMF and SMF had slightly higher rates of fat, CP, water and energy deposition, which resulted in higher fat and lower water contents (g/kg EBW) than those from treatments UU and SU, although these differences were not significant. Steers on treatment SF had gains and final compositions intermediate between those on treatments SU and SMF. Supplementation was cost effective in winter, but marginal in summer.