BIRTH WEIGHT IN CALVES AND ITS RELATION TO GROWTH RATES FROM BIRTH TO WEANING AND WEANING TO SLAUGHTER

Abstract

Holstein steers and Hereford and Angus bulls were separated from their dams at 2 d of age and supplied fixed amounts of energy in relation to live weight between birth and 100 kg liveweight. Within individuals, relative weight gain (g kg−1 d−1) was constant between birth and weaning at 100 kg, and was taken as a measure of inherent growth efficiency. Constancy of relative weight gain indicates that the weight of body substance deposited daily is proportional to the weight of the growing mass. As a consequence, the smaller the birth weight, the lower the absolute daily gain at any given age up to weaning. Variations in birth weight and relative weight gain before weaning were together shown to account for nearly all the variance in absolute weight gain, and variation in birth weight per se was responsible for between 36 and 60% of it. The Hereford and Angus bulls were supplied uniform amounts of energy in relation to liveweight from 100 to 500 kg liveweight, when they were slaughtered. Birth weight was positively correlated with post-weaning daily weight gain and negatively with both energetic efficiency of weight gain and the proportion of fat in the carcass gain. It appeared that these relationships were the result of an association between birth weight and mature weight since, over any fixed weight interval, cattle of large mature size gain faster, and deposit less fat, than cattle of small mature size. Key words: Birth weight, growth, efficiency, cattle