ABSTRACT
In any environment, increased production can be achieved by improving environmental conditions. Over a wide range of production systems in environments of moderate stress, the most productive animal has been found to be one with a proportion of its genes from adapted cattle and a proportion from potentially high-producing cattle that are generally of European Bos taurus origin. Selection among breeds for high growth rate would identify a different breed in each environment. In tropical Australia, breed substitution has been highly successful. The heritability of fertility of the Bos taurus breeds in low-stress environments is almost zero, that is, the additive genetic variance in fertility of these breeds has been exhausted and no further genetic progress can be expected to result from selection for fertility. The resulting rate of genetic improvement of productivity is generally slow but is long lasting; whereas improvement made through environmental modification is immediate but requires continual inputs to maintain the improvements.
