Concentrate Supplementation of Pasture for Beef Production

Abstract

* Unsupplemented cattle offered a high grass allowance (18 kg (DM)/head/day), achieved 0.97 of the DM intake of a positive control offered concentrates ad-libitium. At a low grass allowance (6 kg/DM/head/day), there was no effect of supplementary concentrates on grass intake. At a medium (12 kg/DM/head/day), and high grass allowance, supplementary concentrates reduced grass intake by 0.43 and 0.81 kg/DM respectively per kg/DM concentrate offered. * Supplementary concentrates increased complete diet digestibility even though offering supplementary concentrates also increased total DM intake. Complete diet digestibility was higher than the additive values of the grass and concentrates. This would imply that the supplementary concentrates increased the grass DM digestibility. * Increasing the grass allowance increased plasma urea concentration; supplementary concentrates increased total dietary nitrogen intake and reduced plasma urea concentration. These findings suggest that the concentrate supplement enabled greater utilisation by rumen micro-organisms of the degradable nitrogen supplied by the grass. * Supplementing with concentrates increased carcass growth by 116 g/kg concentrate DM eaten whereas increasing the grass allowance increased carcass growth by 38 g/kg/DM grass eaten. The carcass weight response to concentrates of grazing animals was twice that of animals offered concentrates ad-libitum which gained 57 g carcass per kg concentrate DM eaten. * The relationship between carcass gain (Y) (kg/day) and supplementary concentrates (X) (kg/day) was quadratic (P< 0.001) and was best described by the equation: Y = -0.0099X2 + 0.1364X + 0.2459 (R2 = 0.60). The relationship between carcass gain (Y) (kg day-1) and grass intake (X) was also quadratic (P< 0.01) and was best described by the equation: Y = -43X2 + 275X + 133 (R2 = 0.48). Although there was a much larger (double) carcass growth response to supplementary concentrates than to additional grass DM eaten, increasing grass intake significantly increased carcass fat scores whereas offering supplementary concentrates did not. This would imply that relative to concentrates, autumn grass led to a change in the partitioning of energy from muscle towards subcutaneous fat. * As a strategy for increasing the performance of cattle grazing the type of autumn grass used in this study, offering supplementary concentrates offers more scope to improve animal performance than altering grass allowance. * The carbohydrate source of the three concentrates formulated to differ in rate of degradability did not alter rumen fluid pH, volatile fatty acid (VFA) concentration or the rate of grass DM or N degradation when grass supply was considered to be limiting or liberal. The autumn grass was apparently capable of buffering the effects of concentrate DM degradation rate which varied by up to two fold. * The rumen fluid parameters were more influenced by the pat-tern of grass intake than type of concentrate offered. Hence, there was no effect of concentrate type on animal performance.