The genetic determination of several heat tolerance characteristics and the nature of their correlations to the whole adaptability-productivity complex are still uncertain. A genetic approach for maximising absolute individual reproductive and productive performance under heat stress, expressed by the composite term "productive adaptability" with sufficient genetic foundation and economic value, seerrfs to be most appropriate.
An important determinant of the productive adaptability is body size which has significantly lower optimum under heat stress, also improving the maintenance metabolism and protein muscle turn over rate. Significant improvement can also be expected through specific major gene effects directly improving the efficiency of thermoregulatory mechanism and indirectly the production efficiency as evident in poultry. In addition to the single or additive genetic effects special emphasis should be given to exploitation of heterosis effects expected to be higher under heat stress, whereby maternal heterosis is more important specially for maternal reproductive abilities. In transfering genetic gains from temperate to hot tropical regions significant genotype xe nvironment interactions mustb e expected signifying actual tests under the specific environmental condition. Synthetic line breeding using differentiated genotypes also seems to be promising in securing distinct degree of maternal and individual heterosis, specially for tropical production systems.

P. Horst, P. K Mathur

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XIV. Dairy cattle genetics and breeding, adaptation and conservation., , 286–296, 1990
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