Different Lactations for Estimating Genetic Merit of Dairy Cows

Heritabilities for milk and fat production of Holstein cows were highest at .35 and .33 for first lactation and decreased to .21 and .20 for fifth lactation. Genetic correlations between consecutive lactations were above .9, except for .75 between first and second, and decreased as lactations were farther apart. Relationships between production by cows and their daughters tended to be higher when both animals were in the same herd than when they were in different herds. The cow lactation most valuable in predicting a daughter lactation was generally the one with the same number; this finding suggests that production in different lactations is controlled by some different genes. The third lactation of a cow was most highly related to lifetime production of the daughter. First lactation was not as useful in estimating genetic merit as is accepted generally. The results provide little support for weighting lactations differentially for sequence.