Relationships Among Herd Life, Milk Production, and Calving Interval

Whether to select for longer life in the herd or shorter calving interval depends on the genetic and phenotypic relationships among herd life, amount of milk in first lactation, and calving interval. Herd life and average calving intmwal are essentially phenotypically uncorrelated. The phenotypic and genetic correlations of herd life and milk in first lactation ranged from .19 to .25 and from .54 to .77, respectively. The phenotypic correlation of average calving interval and milk in first lactation ranged from .19 to .21. Estimates of the heritability of herd life ranged from .05 to .14. This heritability was reduced by 30% when the influence o£ milk in first lactation was eliminated. Selection of females for longer herd life or shorter calving interval would not be advantageous, but some consideration of the herd life of half-sisters and close ancestors could be of value in choosing young sires for testing. The early studies of longevity or herd life were made using records from one or a few herds, and inferences were limited by specific management and disease conditions and by small numbers of animals. Rendel and Robertson (6) pointed out that with only a moderate margin between salvage value and cost of replacement the main economic advantage to be gained from a long productive life arises from the increase in selection intensity for milk production and, conversely, that the major economic loss due to a shortened herd life results from the inability to cull low producers. Van Vleck (10) studied Dai~T Herd Improvement Association (Dt t IA) records in New York and found that cows with high milk production in their first lactations not only produced more than their herdmates in later lactations, but also had a higher survival rate in each of the first four lactations. White and Nichols (11) studied daughters of artificial insemination (AI) sires in Pennsylvania and found the heritability of number of lactations completed to be Received for publication March 6, 1967. .13, but found little increase in herd life after production exceeded 5,000 kg in the first lactation. The current study was initiated to investigate the importance of longevity in selection of dairy cattle.