1 kg/day (control). Even after the high‐nutrition diet was changed to a control diet, bulls previously receiving high nutrition continued to have lower semen quality. There was greater deposition of fat around the testicular vascular cone in the scrotal neck in bulls in the high‐nutrition group and the difference between body and testes temperature was reduced in this group compared with bulls in the control group. This difference was still present after the high‐energy diet was changed and the bulls had lost a considerable amount of weight, indicating that fat accumulated in the scrotum is more difficult to lose than other body fat [99].
In another series of experiments, Angus, Hereford, and Simmental bulls were fed high nutrition (80% grain and 20% forage) or medium nutrition (forage only) from approximately 6.5 until 12–24 months of age. In general, bulls receiving high nutrition had greater body weight and backfat, but paired testes weight was not affected by diet. Moreover, bulls receiving high nutrition had lower daily sperm production and epididymal sperm reserves and a greater proportion of sperm abnormalities. The authors indicated that increased dietary energy may adversely affect sperm production and semen quality due to fat deposition in the scrotum, which reduces the amount of heat that can be radiated from the scrotal skin, thereby increasing the temperature of the testes and scrotum [100–103]. Observations from a different study indicated that bulls fed high‐nutrition diets had greater SC and scrotal weight than bulls fed medium‐nutrition diets, but paired testes weight was not different between the two groups [104]. Growth rate between 6 and 16 months of age did not affect sexual development and reproductive function in Angus and Angus × Charolais bulls. However, greater body weight at various ages was associated with reduced age at puberty and maturity and with larger testes at 16 months of age, indicating that improved nutrition might be beneficial, but only when offered before six months of age. Average daily gains of 1–1.6 kg/day did not result in excessive fat accumulation in the scrotum, increased scrotal temperature, or reduction in sperm production and semen quality, and could be considered “safe” targets for growing beef bulls [61].
In summary, low nutrition has adverse effects on sexual development and reproductive function regardless of the bull's age. However, most research indicates that high nutrition is only beneficial during the first six months of life, which presents a challenge to bull producers. Beef bull calves are usually nursing until six to eight months of age and very little attention is paid to their nutrition, whereas nutrition offered to dairy bull calves is often suboptimal. Efforts to obtain maximum weight gain during the first months after birth by offering high‐nutrition diets and adopting management practices like creep‐feeding will be compensated by reduced age at puberty and greater sperm production capacity in adult bulls. It is also clear that although high nutrition diets after six months of age might be associated with greater SC, this effect is likely the result of fat accumulation in the scrotum and not actually greater testicular size. Moreover, sperm production, semen quality, and serving capacity are all compromised in bulls receiving excessive nutrition after this age. Adjusting diets accordingly to maximize growth but to prevent overconditioning after the peripubertal period is advisable.
Implications of Sexual Development and Puberty for Breeding Soundness Evaluation
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].
References
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