Two Bos taurus breeds, the Belmont Red (AX) and the Belmont Adaptaur (HS), have been selected since 1983 for high EBV for 550-day liveweight, and for both high 550-day liveweight and high resistance to cattle ticks respectively. When selection began, the AX was moderately resistant to the stresses of the tropics while the HS was lowly resistant, particularly to cattle ticks. Changes in liveweights at different ages, tick resistance and calving rates for each breed were measured relative to the unselected populations. In the AX, liveweights at all ages increased markedly, but calving rate, relative growth rate and tick resistance did not change significantly. Selection for high EBV for 550-day liveweight had been selection primarily for the scale components of growth rather than for increased productivity. In the HS, liveweight at 18 months, relative growth rate, tick resistance and calving rates all increased significantly without corresponding increases in birthweight or mature cow weight. Selection for both increased growth rate and tick resistance had increased production efficiency without increasing the scale components of growth. The genetic trends corroborated the observed responses in growth in both breeds. Thus, if an increase in productivity rather than mature size is required, selection must target the components of productivity, not just high growth potential. 

C. J O'Neill, J. E Frisch

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 25: Lactation; growth and efficiency; meat quality; role of exotic breeds in the tropics; design of village breeding programmes;, , 223–226, 1998
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