Estimates of genetic correlations between testicular measurements and female reproductive traits in cattle.

Data from 528 male and 645 female progeny of 63 sires were used to estimate genetic correlations between female and male reproductive traits. Data were from two Hereford herds involved in a long-term selection program of the North Carolina Agricultural Experiment Service. Testicular measurements of circumference, diameter, length and volume were obtained on bulls at 205 and 365 d. Testicular growth measures were defined as differences between 205-and 365-d measurements. Heifers were placed in the breeding herd as yearlings and given two breeding seasons to produce a calf. Traits utilized from females were three age-at-first-breeding traits, two age-at-first-calving traits, two pregnancy rate traits, rebreeding interval and calving interval. Genetic correlations were estimated from half-sib and from sire-daughter analyses. Seventy-five percent or more of the correlations of testicular measurements with pregnancy rats, age at first breeding and age at first calving were in the favorable direction. Average correlations were .62, -.55 and -.66, respectively. For each of the remaining female traits, approximately 50% of the correlations were favorable and the average correlations were small. Correlations were summarized by testicular measurement with favorable correlations given a negative sign. Testicular diameter had more favorable correlations (80%) than length, volume or circumference (70%). However, average correlations were similar (-.31, -.30, -.34 and -.26, respectively). Testicular measurements taken at either 205 or 365 d had the same percentage of favorable correlations (72%), while testicular growth measurements had a slightly higher percentage of favorable correlations (78%). Average correlations of 365-d measures were higher (-.38) than either 205-d or growth measures (-.25 and -.28, respectively). Heritabilities for testicular measurements tended to be moderate to high, while those for female reproduction tended to be low to moderate. These results suggest that selection for increased testicular size would lead to improvement in female reproduction, particularly an increase in calving rate and a decrease in age at first breeding.