The effect of oestrus behaviour on muscle glycogen concentration and dark-cutting in beef heifers

Preliminary observations at an abattoir showed an association between oestrus behaviour and dark-cutting in beef. Two follow-up experiments examined the relationship between oestrus behaviour and muscle glycogen content, carcass bruising and meat ultimate pH. In the first experiment heifers in oestrus were compared with controls in mid-cycle. Oestrus was associated with a loss of glycogen in M. longissimus dorsi. Also plasma creatine kinase activity increased, reflecting bruising or strenuous muscle activity. In the second experiment heifers were slaughtered during oestrus. On the basis of pre-slaughter behaviour they were assigned to active or inactive groups, which subsequently yielded 39% and zero dark-cutting carcasses, respectively. Loss of muscle glycogen was accounted for by the number of times an animal mounted (r = -0·85). Likewise, variation in meat ultimate pH was associated with mounting activity (P < 0·001). Carcass bruising was higher in the active group (P < 0·0.001) and the main cause of bruising was being mounted (r = 0·74). It was concluded that single penning of oestrus heifers to eliminate mounting activity would reduce the incidence of dark-cutting.