Читать книгу Bovine Reproduction - Группа авторов - Страница 98

In general terms, evaluation of bulls for breeding soundness involves a general physical examination, a specific examination of the reproductive system (including measurement of SC), and examination of the semen. According to the guidelines of the Society for Theriogenology (SFT), bulls must be free of any physical abnormalities, have a minimum SC of 30–34 cm depending on age, have 30% or more progressively motile sperm, and have 70% or more morphologically normal sperm in order to be classified as satisfactory potential breeders [105]. Since developing bulls experience significant changes in SC and semen quality over relatively short periods of time, the effect of age and the interaction of age with breed need to be considered for breeding soundness evaluation. The SFT guidelines list the breeding soundness category “classification deferred,” which is used for bulls that the veterinarian considers likely to improve with time after reflecting on the bull's signalment, history, test results, and nature of the deficiencies. Most bulls classified in this category are young animals that may not have reached the suggested minimum for SC and semen quality by the time of the examination.

The proportion of yearling B. taurus beef bulls classified as satisfactory potential breeders increases from approximately 55% at 10 months of age, to 72% at 11 months, and 78% at 12 months of age as more bulls reach minimum requirements for SC and semen quality [106, 107]. Since there are breed differences in sexual development, a larger proportion of bulls from early‐maturing breeds are expected to obtain satisfactory classification at a younger age than bulls from late‐maturing breeds. An extreme example was found with tropically adapted Senepol bulls; the proportion of satisfactory yearling bulls was only 25–50%, but this increased to 70–85% at approximately 24 months of age [108, 109]. It has also been reported that the proportion of satisfactory bulls decreases after five to six years of age as bulls become more prone to develop physical and reproductive problems [110, 111].

Special consideration must also be given to B. indicus bulls, since these animals in general attain puberty at considerably older ages than B. taurus bulls. Although SC in selected mature B. indicus and B. taurus bulls does not differ [46], using SC guidelines for young bulls devised for the latter unjustifiably penalizes the former. Satisfactory semen quality is also observed at older ages in B. indicus bulls. In Florida, Brahman and Brahman × Nelore bulls only had greater than 70% normal sperm after 24 months of age [108]; these observations are consistent with those in Nelore bulls in Brazil [10]. In addition, the proportion of total sperm defects was approximately 57% in Nelore bulls 12–18 months old [112] and 20% of Nelore bulls were still considered immature at two years of age based on the spermiogram [113]. Less than 5% of B. indicus bulls were considered satisfactory potential breeders before 15 months of age and the proportion increased to only 20–50% between 18 and 22 months of age [108]. Accordingly, it might be prudent to delay breeding soundness evaluation for B. indicus bulls until two years of age.

In breeds that selection pressure has demonstrably resulted in significant increases in the mean SC, suggestions have been made for veterinarians and breed associations to consider updating minimum SC requirements accordingly. Minimum requirements designed to cull the bottom 5–10% of the population should help maintain adequate selection pressure [35].