Finishing cattle at pasture: concentrate supplementation?
In autumn the diet of grazing cattle is generally unbalanced in terms of energy and protein as there is usually excess degradable protein in autumn grass. Researchers Edward O’Riordan and Mark McGee have some information.
Research at Grange has shown dietary energy rather than protein is the limiting factor and where supplementation occurs, concentrate energy sources are required.
Due to the seasonality of grass growth, herd feed demand usually exceeds supply in the autumn. This is particularly so on beef farms because, due to accumulated animal growth, feed requirements increase as the grazing season progresses, whereas grass growth declines after mid-summer. Furthermore, all else being equal, when compared with spring grass, autumn grass often has a lower feeding value. As grazed grass is considerably cheaper than grass silage (or concentrates), early finishing of cattle at pasture in autumn, before housing becomes necessary, is less costly.
An important factor affecting nutrient intake of cattle when supplementing forage, is substitution rate i.e. the decrease in grass dry matter (DM) intake per unit of concentrate intake. Research at Grange shows substitution rates for finishing cattle grazing autumn pasture supplemented with concentrates ranging from 0 to 0.81, with marginal values at higher levels of supplementation in excess of 1.0 in some studies. In other words, where grass supply was restricted, substitution rates were low, but where grass supply was adequate, increasing the level of concentrates reduced grass intake but usually also increased total DM and energy intake. Carcass growth response to concentrate supplementation while grazing will primarily depend on the availability plus quality of pasture and level of supplemented concentrate; response is higher where grass supply is low and where grass quality is poorer, and declines as concentrate supplementation level increases.
Studies at Grange have shown that at adequate (approx. 20 g DM/kg live weight) grass allowances in autumn, feeding 0.50-0.75 kg of concentrate per 100 kg live weight resulted in carcass growth responses between 30 and 110 g carcass per kg concentrate. In practice, feeding a moderate level of concentrate - 0.5 kg concentrate/100 kg live weight - will likely result in carcass growth responses at the upper end of this range.