Research articleUse of nonlinear models for describing scrotal circumference growth in Guzerat bulls raised under grazing conditions
Introduction
Scrotal circumference (SC) is frequently used in breeding programs, because of its easy measurement, high repeatability, and moderate to high heritability [1], [2], [3], [4]. In addition, SC is favorably associated with physical semen characteristics, age at puberty, sexual precocity, and weight gain [5], [6], [7], [8]. Furthermore, based on the genetic correlation between SC and reproductive characteristics in females, e.g., age at puberty, days to calving, and age at first calving, selection for SC had a positive influence on female reproductive performance [8], [9], [10], [11].
Zebu cattle are older at puberty and have delayed first calving (30–46 months) [12], [13]; these are attributed to genetic and environmental factors [14]. Bulls selected on the basis of larger SC and early puberty usually produce daughters reaching puberty at significantly earlier ages and greater percentages of heifers cycling early in the breeding season, even when used in crossbreeding [8], [10]. This is very important for cattle breeding in tropical countries, where delayed puberty and poor reproductive efficiency are major constraints limiting the cattle industry [15].
The SC at puberty is relatively constant among breeds and across bulls varying widely in age and weight at puberty; therefore, SC might be more useful than other characteristics for predicting age at puberty [1], [16], [17]. Scrotal circumference is more easily obtained than sperm production or behavioral measurements and should be useful in the selection of beef bulls for early sexual maturity [1].
Selection of bulls at the earliest possible age will not only improve reproduction, but also provide economic advantages by decreasing feeding and management costs and improve economic returns to producers [18]. However, Lunstra et al. [1] reported that male offspring of diverse heat-adapted sire breeds had lower postweaning testicular growth rates and were slower to reach puberty than offspring of European sire breeds. Therefore, there is a need for characterization of reproductive traits in bulls representing diverse beef breeds.
Furthermore, nutrition, breed, and environmental factors influence reproductive development and testicular size [19], [20]. Therefore, to achieve higher accuracy in assessing the reproductive capacity of bulls based on testicular measurements, comparisons should be made within the same age group, breed, bull stud, and year-season [21].
One way to describe testicular growth is using nonlinear regression models. The advantage of nonlinear models is that they can accommodate a large number of measurements in some parameters and, thus, permit appropriate biological interpretation. The benefit of these models in animal breeding is using these parameters to identify animals more suited for specific purposes [22]. Furthermore, a growth trait can be improved by using growth curves and their properties. In that regard, Kratochvílová, et al. [23] identified early and late maturing heifers during early phases of growth (6–9 months) using growth curve parameters of nonlinear models.
Given the growing interest in Zebu cattle because of their better adaptation than Bos taurus breeds to tropical conditions, and the need for producers to emphasize growth rate and early pubertal development in genetic improvement programs [17], the objective of this study was to evaluate six nonlinear models to describe SC development in Guzerat bulls and to evaluate environmental effects on growth curve parameters.
Section snippets
Bulls and scrotal circumference measurement
All procedures were approved by the ethics committee of Universidade Federal de Minas Gerais. This study was conducted on three farms located in Brasilândia de Minas (17°00′36″ South and 46°00′32″ West), Carlos Chagas (17°41′30″ South and 40°45′15″ West), and Unaí (16°21′43″ South and 46°54′09″ West), in the state of Minas Gerais, Brazil. The climate classification in this regions is Aw (tropical rainy climate, Köppen classification), with average temperature of 18° in the coldest month. The
Results
The Richards model did not reach the convergence criterion nor did it produce biologically interpretable parameters. For the Brody, Gompertz, Logistic, Von Bertalanffy, and Tanaka models, the values of the parameters estimated and standard error are shown (Table 2). Regarding parameter estimates, the Gompertz and Von Bertalanffy models had close values for SC at maturity (parameters A), and the Brody model estimated the highest value for A and the lowest value for the k parameter compared with
Discussion
The nonconvergence of the Richards model found in this study was also reported by others using this model to assess SC growth in sheep [25], goats [39], and cattle [26]. These results contrasted with those reported by Delgado et al. [40] and Jiménez-Severiano et al. [41] where the Richards model yielded good fit to the data for SC in Retinto bulls and Blackbelly ram lambs, respectively. Several authors warned about the difficulty of convergence of this model, because parameter m (the inflection
Acknowledgments
The authors thank Conselho Nacional de desenvolvimento Científico e Tecnológico (CNPq) for financial support, and Farm São Geraldo (Brasilândia), Farm Guzerá do Rosário (Carlos Chagas), and Farm Palestina (Unaí) for providing the opportunity and the facilities to collect data.
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2019, TheriogenologyCitation Excerpt :It is known that, for scrotal circumference, the IP may be related to puberty since it coincides to the period of rapid increase of testicular parenchyma [7,10]. Nevertheless, the age at IP found in this study is later than the age at IP for scrotal circumference reported in Nellore (13 months of age) [7] and Guzerat (12.5 months of age) [10] bulls. Moreover, it was noted that testicular length increased at higher rate than testicular width, indicating that the testes become longer with increasing age contrary to the findings of Unanian et al. [5] in Nellore bulls.