Influence of season of birth on weight for age in two genetic groups of cattle.

Body weights from birth to 756 days of age for calves of Mpwapwa cattle (12% Bos taurus inheritance) and backcrosses (34% Bos taurus) born in the wet or the dry season were analysed. The calves were weaned at 75 days of age and after that fed entirely on pasture. Backcrosses were heavier than Mpwapwa at birth and at 252 days of age but not at subsequent ages. Calves born in the dry season had slightly larger birth weights than calves born in the wet season. Weight differences were reversed from 252 to 504 days reflecting the seasonal fluctuations in grass growth. The effect of season of birth on long term rate of gain was negligible. No significant interaction between genetic group (Mpwapwa vs backcrosses) and season of birth (wet vs dry) was observed at any of the ages. The study thus did not confirm the hypothesis that backcrosses because of their higher proportion of Bos taurus genes were more sensitive to the nutritional stress of the dry season than were pure Mpwapwa calves.