Scrotal circumference Many purebred breeders have been collecting yearling scrotal circumference data for some time. In fact, several breed associations have developed yearling scrotal circumference (SC) EPDs. Even though a large amount of research has been dedicated to evaluating relationships between scrotal circumference and male fertility traits, could there be any existing relationships with female fertility traits? Brinks and co-workers (1978) estimated a genetic correlation of 2.36 between yearling scrotal circumference and age at puberty in females, indicating that bulls that have the genes for larger scrotal circumference will also have paternal sibs or daughters that reach puberty earlier. More recently, Morris and others (1993) estimated a genetic correlation of 2.64 between the same traits in Angus cattle, indicating even a stronger relationship than in the first study. The presence of such a genetic correlation could provide a beneficial "correlated response" in age at puberty in heifers by selecting sires with genes for larger scrotal circumference. The low heritability of female reproductive traits in general would result in a slow response in female reproductive performance through direct selection for those traits. Considering the recent research indicating that scrotal circumference is moderately heritable, we may be able to select indirectly for age at puberty by using bulls with genes for larger scrotal circumference.

     Although many research studies have evaluated the relationships between scrotal circumference and female reproductive traits, there have been only limited reports of selection studies based on sire scrotal circumference. Morris and co-workers (1993) suggest that advantages in age of puberty in females would occur by selecting bulls with larger scrotal circumference. Based on direct selection for scrotal circumference in 141 Angus sires, these researchers were able to reduce age at first estrus in daughters by 21 days relative to a randomly mated control line, while increasing scrotal circumference by 1.6 cm in male progeny. Additionally, average calving day was reduced by almost five days.

     Many seedstock breeders collect yearling scrotal circumference data and as a result, several breed associations now compute SC EPDs. However, limited information is available on the selection response based on SC EPDs. Based on only one year's data, a study using Hereford cattle at Auburn University showed that daughters of bulls from a high SC EPD line reached puberty 62 days earlier than daughters from bulls of a low SC EPD line (Hough, 1991; Hough, American Hereford Association, Kansas City, Mo., personal communication).

     Summary Reproductive performance plays an important role in the profitability of any beef operation. Designed crossbreeding systems that take advantage of maternal heterosis should always be considered. However, other considerations for selecting sires within-breed may enhance reproductive performance in the herd, both in cows to which those bulls are mated and in their resulting female progeny. Attention to both birth weight and scrotal circumference EPDs in sire selection may provide beneficial correlated responses in the postpartum interval and age at puberty.

     As your knowledge about "Beef Cattle Genetics And Reproduction" continues to grow, you will begin to see how genetics and reproduction fits into the overall scheme of things. Knowing how something relates to the rest of the world is important too.

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