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Measurement of Genetic Change in Twelve California Dairy Herds¹

Department of Animal Husbandry, University of California, Davis

ABSTRACT

Genetic change in fat-corrected milk (FCM) yield was studied in a population of 11,993 lactation records of 3,900 Jersey cows in 12 herds over a 30-year period. Differences between herds as well as total genetic progress was determined.

The between lactation variance of records age-corrected with factors derived from intra-herd analyses of the lactation means was reduced an average of 91%. Repeatability of the age-corrected FCM records estimated by intraclass correlation within herds ranged from .20 ± .05 to .52 ± .05.

Yearly environmental effects were estimated using a maximum likelihood method developed by Henderson (6). Annual genetic change was expressed as the linear regression of FCM yield on year of records adjusted for yearly environmental effects. The average genetic change for all herds was 74 lb FCM annually. The genetic change within individual herds ranged from –51 ± 52 to 145 ± 21 lb FCM per year.

A pooled intra-sire regression of the progeny's MPPA on generation number within the largest herd yielded an estimate of genetic change agreeing closely with the maximum likelihood estimate.

¹ Data taken from a thesis presented by C. W. Arave in partial fulfillment of the requirements of the Ph.D. degree, University of California, September, 1963.

² Present address: Department of Agriculture, Chico State College, Chico, California.