A Note On Diet Digestibility, Feed Conversion Efficiency And Growth In Steers Offered Ad-Libitum Or Restricted Allowances Of Concentrates

Restricting the feed allowance to feed-lot cattle has been shown to reduce live-weight gain but to improve the efficiency of conversion of dietary dry matter (DM) to carcass weight when compared to ad-libitum feeding. The objective of this experiment was to quantify the effects on diet digestibility, feed conversion efficiency and continental crossbred steer performance of restricting the allowance of concentrates formulated from ingredients typically used in Ireland. Twenty-four steers were assigned to two treatments 1) concentrates ad libitum or 2) concentrates restricted to proportionally 0.7 of expected ad-libitum intake. All animals were also offered 1 kg of hay daily and were slaughtered after an 85-day experimental period. Twelve Friesian steers were assigned to two DM allowances which were equivalent to both the DM intake and the hay to concentrate ratios of the steers in both the growth study treatments, respectively, and complete diet digestibility was calculated by total faecal collection. Increasing concentrate intake increased live-weight gain (P < 0.001), carcass gain (P < 0.01), fat score (P < 0.05) and internal fat proportions (P < 0.001) and decreased feed conversion efficiency (P < 0.05). There was no significant effect of concentrate allowance on diet digestibility. The reduced feed conversion efficiency in the animals offered the ad-libitum concentrates could only be attributed in part to a difference in digestibility; the major part was attributed to higher rates of fat deposition and urea synthesis in steers offered the ad-libitum concentrates. The efficiency of utilization of energy for growth in this study was substantially lower than predicted using published standards.