The correlation between first lactation milk production and longevity in dairy cattle

Abstract

1. An analysis has been made of the relationship between first lactation progeny test for milk yield and the survival of a bull's daughters to different ages using data for British Friesian, Ayrshire and Holstein-Friesian cows. It was found that the relative effect on survival of differences between bulls in milk yield increased by about 10% per 100 gallons for each lactation survived, e.g. for two bulls differing in heifer progeny test by 100 gallons, that with the higher figure will have 10% more daughters surviving to have a second lactation, 20% more having a third and so on. The probable effect on total length of life is 1·2 lactations per 100 gallons.2. No evidence was found that selection on first lactation progeny test would reduce productive life in the dairy herd or even that it would increase the proportion culled for reasons other than yield in later lactations.3. The genetic effects of culling in the first lactation on yield were in agreement with those expected from multiplying the phenotypic selection differential by the heritability. On the other hand, the selection differential in heifer yield produced by culling in later life considerably under-estimated its genetic effect. The effect of culling was to increase the genetic potential of animals for yield in the first lactation by roughly 10 gallons for each lactation survived, so that the average genetic superiority over their heifer contemporaries of cows in the herd at any time would be 30 gallons.4. The heritability of survival to different ages was significantly different from zero in all analyses. The theoretical prediction that this would have a maximum at the fourth lactation was borne out in one sample but not in the other.5. The genetic correlation of first lactation yield with survival was lowest in all samples for survival to have a second lactation. For survival to the third or later lactations it was higher than 0·7 in all analyses. As the genetic correlation between first and later yields in these data averaged about 0·8, this can only mean that milk yield is by far the most important genetic determinant of survival at all ages.