– K. M. Cammack M. G. Thomas and R. M. Enns, published in The Professional Animal Scientist 25 ( 2009 ):517–528, and condensed from the original manuscript by Steve Boyles, OSU Beef Extension Specialist
Male Reproductive Measures
Measures of fertility need to be considered not only in the female, but also in the male. Natural service has historically been, and continues to be, used in most beef cattle operations; therefore, acceptable bull fertility is critical to the success of these operations. Bull management is highly intertwined with female fertility, bull fertility, and cow management. Bull fertility may be affected by the number of cows a particular bull is expected to service and, in natural mating, the length of the mating period, as well as the serving capacity of the bulls. Fertility measures may be superficially increased in bulls exposed to a small number of cows during a lengthy breeding season. However, determining the proper bull-to-cow ratio is challenging because these influences are in addition to issues of pasture size and topography, multiple water sources within a pasture, and behavior
Scrotal Circumference
Scrotal circumference is used to predict the quality and quantity of spermatozoa-producing tissue and age at puberty. A scrotal circumference measurement of 28 to 30 cm is generally associated with onset of puberty; specifically, 52 and 97% of males are pubertal when scrotal circumference is 28 and 30 cm, respectively. Increased scrotal circumference has been associated with increased sperm production but decreased semen quality. Reported heritability estimates for scrotal circumference were generally high and ranged from 0.29 to 0.78. Because female fertility traits in general have low heritability, correlated or indicator traits that are more highly heritable are often used to produce more effective selection. Yearling bull scrotal circumference is commonly used to improve female fertility because it is genetically correlated with age at puberty in heifers. Although consistently favorable, the magnitude of this correlation was variable across field and experimental populations, ranging from equal to or less than −0.80 to equal to or greater than −0.15. In general, scrotal circumference as an indicator trait is easy and inexpensive to measure. However, scrotal circumference has also been shown to be positively genetically correlated with growth traits. Because of these findings and the fact that the trait is easily recorded, genetic selection programs have been successful at improving scrotal circumference; thus, the statistical association between scrotal circumference and heifer fertility trait measures is not as easily detected in data sets from more recent times relative to data collected before 1990.
Breeding Soundness
The lack of feasible methods for determining male puberty has made selection and management of young, unproven bulls difficult in regard to fertility. Currently, a breeding soundness exam, or evaluation, is the most practical means to assess bull fertility and consists of a physical examination, scrotal circumference measurement, and semen evaluation. Sex drive and mating ability are not typically included in this evaluation, although they are obviously essential to the successful impregnation of females. No heritability estimates for libido were found in the literature. To our knowledge, there have been no estimates of heritability of the probability of passing a breeding soundness exam.