ESTIMATES OF REPEATABILITY AND HERITABILITY OF HORN FLY RESISTANCE IN BEEF-CATTLE

Horn fly population density on 215 beef cows representing seven breed groups and 51 sires was used to obtain estimates of repeatability (rXX) and heritability (h2) for resistance to the horn fly (Haematobia irritans [L.] Diptera: Muscidae). Total horn fly densities were determined weekly on each cow beginning in May and ending in late October or early November of 1988, 1989, and 1990. No insecticides were used on cattle in this study. Estimates of h2 for horn fly resistance (low horn fly number per cow) were obtained by the paternal half-sib method (4-sigma(S)2) and as twice the intrasire regression of offspring on dam (2b(DD/S)). Variance component estimates were obtained using a completely nested ANOVA that included overall mean, breed, sire/breed, cow/sire, and residual error. One hundred twenty-six daughter-dam pairs were available for regression analysis. In a preliminary analysis, the within-breed regression was nonsignificant (P > .05), implying that the regression was the same for all breeds; therefore, breed was deleted from the model. The regression model included an overall mean, year, and the intrasire regression of daughter on dam (b(DD/S)). The estimate of rxx was .47 +/- .02. Estimates of h2 were .78 +/- .16 and .59 +/- .10 from the 4-sigma(S)2 and 2b(DD/S) methods, respectively. Similar estimates of r(XX) and h2 were obtained when each observation of horn fly number per cow (x) was transformed to both log10 (x) and square-root x. These estimates suggest the possibility of selection procedures as an environmentally safe alternative to the use of chemical control.