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Estimating Herd Improvement from Selection Programs

New Zealand Dairy Board, Wellington

ABSTRACT

Estimates for a 15-yr. period have been derived for the mean additive genetic improvement in a herd's milk fat production that is likely to be achieved from average herd selection practices used in New Zealand. The estimates are obtained on the basis of replacing 20% of the herd annually with young stock born from the top 75% of the cows in the herd, 6% of the herd having been culled for low production. Estimates are shown of the improvement to be obtained from culling alone with just average replacements and from culling and selecting replacements together. Additional gains from using sires estimated as 30 lb. of milk fat above average, such as are available in artificial breeding, are also considered and shown to be approximately equal to those to be derived from culling and replacement selection based on the individual cow records provided by herd-testing. Thus, herd-testing and artificial breeding combined give improvements approximately double those derived from herd-testing alone. The upper limits to these gains and the number of years required to achieve them are discussed, all such gains being relative to an initial herd level and not including any gains that may occur in the whole dairy population. Estimates of production improvement are also obtained and compared with actual production increases reported in New Zealand as directly associated with continuous herd-testing. Mathematical expressions for the estimates are shown in an appendix, in terms of the underlying parameters of the selection programs considered.