Abstract
The objective of this study was to analyze the rates of rejection in the different steps of the breeding soundness evaluation of beef bulls in the state of Rio Grande do Sul, Brazil. The breeding soundness of 22,113 young and mature bulls of 14 beef breeds, participating in the Program of Reproductive Evaluation (PARTO) was evaluated in the experiment. Data concerning to the causes of rejection in the four steps of the breeding soundness evaluation (general physical examination, genital tract examination, semen evaluation and sexual behavior assessment) were analyzed by the Chi-square test and univariate regression analysis. According to year, general physical examination, genital tract examination and semen evaluation determined the rejection of 0.9% to 2.5%, 5.1% to 7.7%, 1.8% to 5.3%; and 4.2% to 6.7%, 7.3 to 9.3%, and 2.5% to 5.5% of young and mature bulls, respectively. Mature bulls presented higher chances of rejection than young bulls in the general physical examination in every year evaluated; as to the genital tract examination, their chances of rejection were higher in years I and II, but not in year III. In the semen evaluation, there was no difference between the ages assessed in any of the three years evaluated. Sexual behavior assessment accounted for the rejection of 1.9 to 6.0% of the young bulls and 2.9% to 3.9% of mature bulls, in accordance with the years evaluated; in years I and II, mature bulls presented higher rejection rates than young bulls. These results confirm the importance of performing all steps of the breeding soundness evaluation, including the sexual behavior assessment as a work routine, rather than an additional, optional stage of the evaluation, which should be carried out before the breeding season. The results indicate the relationship between bull age and rejection rate in the breeding soundness evaluation.
Highlights
Because 95% of cows and heifers considered apt for reproduction are subjected to natural service, it is fundamental to develop and apply reliable methods of evaluation of the reproductive potential of bulls in order to select the animals capable of transmitting desirable economic traits, such as fertility and precocity
The results indicate the relationship between bull age and rejection rate in the breeding soundness evaluation
The cumulative rejection rates observed in the present study are according to the expected rates, as in a bull population, 10 to 20% are expected to be rejected due to semen quantity and quality problems, physical defects that prevent mating, and/or absence of libido (Radostits et al, 1994) (Table 1)
Summary
Because 95% of cows and heifers considered apt for reproduction are subjected to natural service, it is fundamental to develop and apply reliable methods of evaluation of the reproductive potential of bulls in order to select the animals capable of transmitting desirable economic traits, such as fertility and precocity. High production costs and management failures account for unacceptable productivity rates in herds of small, medium and large producers. This emphasizes the need for the adoption of genetic, reproductive, nutritional, sanitary and managerial improvement in the breeding farms considered. Menegassi et al (2011) showed the bio-economical effect of breeding soundness evaluation: an increase of 31% in calf production, 13.8 calves/bull/year, an increase of 24 kg of calves/cow/year, and a cost-benefit ratio of US$ 19.37, which raised the income of breeding farms. Sereno et al (2002) showed that it is possible to reduce the bull:cow ratio in the Brazilian Pantanal region without negatively influencing the fertility rate of the herd. Soundness analysis on the bull semen and the duration of the breeding season are fundamental for the establishment of the new bull:cow ratio and the incorporation of these recommendations in the extensive production system used in the Brazilian Pantanal may generate an economy of 28.88%, if the bull:cow ratio changes from 1:10 to 1:25, or 35.55%, if changed to a 1:40 ratio
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