Milk Production in Hereford Cows II. Heritabilities and Repeatabilities

Heritabilities and repeatabilities of measures of milk yield were estimated from 1,319 lactation records of 528 cows sired by 84 bulls used in two related herds of Hereford cattle. The data were collected over a period of 8 years (1968 to 1975). Estimates of milk yield were obtained at bimonthly intervals by the calf-suckling technique. The three within-lactation observations (M1, M2, and M3), the sum of the three observations (TMY) and a predicted sum of seven monthly observations (PMY) were studied. Estimates of heritability from paternal half-sib correlations were .17±.08, .29±.10, .28±.09, .44±.13 and .45±.13 for M1, M2, M3, TMY and PMY, respectively. Genetic correlations among the various measures of milk yield approximated 1.00. Repeatabilities were estimated by the intraclass correlation method from cows having at least two records. A total of 1,113 lactation records from 322 cows was included. The repeatability estimates for M1, M2, M3, TMY and PMY were .23±.03, .31±.03, .24±.03, .46±.03 and .46±.03, respectively, Repeatabilities for paired records for all possible pairs of records through the sixth lactation were calculated also. Mean repeatability values for adjacent records were .34, .40, .51, .64 and .65 for M1, M2, M3, TMY and PMY, respectively. As degree of adjacency decreased, i.e., as records were more separated by time, the repeatabilities decreased. Where records were separated by 5 years the repeatabilities were .17, .47, .32, .42 and .44 for M1, M2, M3, TMY and PMY, repectively.