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Are There Limits to Genetic Improvement in Dairy Cattle?

Department of Animal Science, North Carolina State University at Raleigh

ABSTRACT

The genetical situation in dairy cattle does not suggest that a selection limit should be anticipated in the near future, due to a complete loss of genetic variability. With many systems, such as the reproductive, endocrine, digestive, circulatory, secretory, and excretory involved in production of milk, it appears likely that an intermediate situation would be present if a plateau were reached. In terms of current population means, we appear to be far from a physiological limit on the secretory and digestive systems, if a high-concentrate ration can be continued. If we must switch, to rely on roughage alone as a ration for milking cows our levels of yield will be measurably decreased. Nonetheless, progress from selection probably could be expected to continue from this lower yield base, that should be equivalent to a number of genetic standard deviations. The limitations of the digestive system to retrieve enough energy from roughages for high yields would seem to be the major deterrent to further genetic improvement under roughage-feeding systems.

If we eventually realize levels of production that press upon physiological limits, the system responsible for the limitations should be identified. If no reasonable environmental remedy is available, the slower adaptive evolutionary route may be necessary, to allow us to move to the next level of production. This may require a period of relaxed selection and integration of other genetic material into the gene pool, as a prerequisite for further selection to move above the plateau. Availability of several plateaued strains, each of high merit, would be most desirable for crossing, to provide a base to continue the next stage in selection.

If nonadditive gene effects are important, a crossing of strains within breeds or of interbreed strains may be possibilities. Probable reproductive difficulties concomitant with inbreeding make inbred line formation and crossing one of the least attractive approaches. In addition, formation of inbred lines and testing for favorable line crosses implicates breeding programs that may span more than one generation of research workers. The likelihood that such a plan would yield results is measurably decreased by this limitation alone. A modification of reciprocal recurrent selection schemes, rather than inbred line formation, would appear to be a more realtistic approach to utilize nonadditive genetic effects in dairy cattle, when that alternative appears productive.