Fish meal and monensin, as supplements to grass silage, were offered either separately or together to Friesian steers from 5 to 9 months of age (period 1). At 9 months, six steers from each of the supplemented treatments were slaughtered to determine body composition. Of the remaining steers, from 9 months of age to slaughter at 490 kg live weight (LW) (period 2), six continued on each of the same treatments, whilst six received silage unsupplemented. A further group of six steers which had received fish meal and monensin during period 1, received monensin only as a supplement to their silage.
During period 1 fish-meal supplementation increased LW, empty body and carcass weight gains, and tissues gains, while monensin had no apparent effect. During period 2, continued supplementation with both fish meal and monensin significantly increased LW, empty body and carcass gains compared with the continuously unsupplemented steers, whilst fish meal and monensin separately produced intermediate increased gains. All three supplement regimes significantly reduced the time taken to reach slaughter.
The only significant effect on body composition was that steers receiving fish meal and monensin had a higher crude protein concentration in the empty body than those receiving only fish meal as a supplement. They also had the highest crude protein concentration in the carcass, although this was not significant. Steers which had their supplements withdrawn after 9 months of age generally maintained gains intermediate between those continuing to receive supplements and those steers unsupplemented throughout. Only those not continuing to receive fish meal and monensin together appeared to perform appreciably worse. Nevertheless, although time taken to reach slaughter from 4 months of age varied by as much as 96 days, there were no other significant differences in body or carcass composition, hi terms of cost effectiveness, use of monensin showed distinct advantages over fish meal.