The influence of supplementary feeding on growth and fertility of beef heifers grazing natural pastures

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This paper reports an experiment examining the effect of age and protein supplementation on growth and fertility of Shorthorn-Devon cross heifers grazing natural pastures in the summer rainfall, subtropical environment of the Upper Clarence valley, north-eastern New South Wales. Heifers born over the period October to January 1963-64 were weaned in June 1964 and allotted at random within age groups (by month of birth) to a supplemented or non-supplemented treatment. The supplemented heifers were given linseed meal during the dry winter period each year from 1964 to 1966 inclusive (May-June to September-October). They were weighed each month and joined with bulls in the early summer of 1965 (2 years) and 1966 (3 years). They were examined for pregnancy in the autumn, and their calves were identified at birth. Supplemented heifers gained weight throughout and non-supplemented heifers generally lost weight in the winter periods. At two years fertility was closely related to liveweight at joining in supplemented heifers, but not in others. Heifers born in January gave the poorest reproductive performance, as the two supplemented ones that became pregnant in 1965 died at calving, and two-thirds of the non-supplemented ones were still empty after joining as 3-year-olds. The advantage of high 2-year-old fertility gained by heifers born in October and November, due to their greater size at joining, was partly offset by low fertility when lactating the following year. It is concluded that a decision as to whether a heifer should be supplemented during a period of nutritional stress prior to joining ought to be based on the probability of her reaching a prescribed target weight at joining as a result of receiving the supplement. The target weight for lactating heifers was at least 100 Ib higher than for dry hdfers. The results also indicate specific areas for further research.