The effects of severe feed restriction on bulls and steers. I. Liveweight loss, behaviour and non-carcass body composition

Abstract

The effects of substantial liveweight loss on the behaviour and body composition of half-sib Hereford bulls and steers were studied by using a serial slaughter technique and starting with nine animals—three bulls and six steers—of similar liveweights (mean 472 kg) but different fatness (the steers having appreciably more fat).

Following the establishment of initial body composition by slaughter of a bull and two steers (group I), two bulls and four steers were given 0.91 kg of poor quality hay per day for 142 days. The six cattle all had similar liveweight losses (mean 170 kg) during this period, but the bulls were extremely emaciated and lethargic by day 142 while the steers were not. Slaughter and comparison of the pair of bulls with one pair of steers (group 2) at this time indicated a considerably greater loss of carcass weight and lesser loss of offal weight by the bulls than by the steers. The remaining pair of steers (group 3) continued to lose weight for a further 57 days, by which time they appeared as emaciated as the group 2 bulls had been, though not so lethargic.

The proportions of feet, head and hide relative to carcass weight were significantly (P < 0.05) greater in bulls than steers, but there were no significant sex differences in the proportions of liver, spleen, heart or kidney. Except for the head, the interaction between weight loss and sexual status had no significant effect on offal proportions; the head percentage increased significantly more in bulls than steers during weight loss. The effect of progressive weight loss in the three groups of steers was a significant (P < 0.05) increase in the feet, head, hide, spleen and heart weights relative to carcass weight.

It is concluded that from a management point of view, behaviour is a better guide to waning fat reserves than appearance, and that steers, because of their greater fat content, would survive a period of feed restriction longer than bulls of the same liveweight.