The effects of the ratio of grass silage to concentrates in the diet and restricted dry matter intake on the performance and carcass composition of beef cattle

The effects of the proportion of concentrates in grass silage-based diets and of restricted dry matter (DM) intake were examined using 156 Simmental cross Friesian steers which were initially 389 kg live weight. The animals were reared on high-forage diets until 14 months of age and were then given grass silage ad libitum and supplemented with rolled barley which constituted (1) 0, (2) 120, (3) 240 and (4) 360 g/kg total DM intake. Diet 5 consisted of grass silage only until 510 kg live weight and diet 4 thereafter, while diet 6 consisted of 640 g silage DM and 360 g barley /kg DM and was offered at 80% of ad libitum intake. Representative groups of animals were slaughtered initially and serially for each diet at 510, 560 and 610 kg live weight. Increasing the proportion of concentrates in the diet increased metabolisable energy intake and carcass gain by 0.042 (S.E. 0.007) MJ/day and 0.728 (S.E. 0.036) g /day per g /kg increase in the proportion of concentrates in the diet but did not significantly affect carcass fat content. Restricting dry matter intake by 21% reduced carcass, lean and fat gains by 31, 29 and 37%, respectively, and carcass fat content by 13 g/kg. The response in carcass gain per MJ reduction in ME intake was 60% greater when ME intake was reduced by reducing the proportion of concentrates in the diet rather than restricting DM intake. Increasing slaughter weight increased carcass fat content by 39 g/kg per 100 kg increase in slaughter weight. It is concluded that reducing energy intake by restricting DM intake or reducing the proportion of concentrates in the diet reduced the efficiency of carcass and lean gains. All strategies used to reduce energy intake had only small to modest effects on carcass fat content and hence the effect of reducing energy intake on carcass fat content were equivalent to those of reducing slaughter weight by only 18 to 45 kg. Consequently for cattle reared on high-forage diets reducing slaughter weight is likely to be a more effective approach to reducing carcass fat content than reducing energy intake during the finishing period.  2000 Elsevier Science B.V. All rights reserved.