Nonlinear Models to Describe the Testicular Size Growth Curve of Mangalarga Marchador Stallions

This study aimed to describe the testicular growth curve in Mangalarga Marchador stallions fitting nonlinear models to data of total scrotal width (TSW) and testicular volume (TV). A cross-sectional study was conducted with 120 stallions, aging from 1.75 to 21.93 years. The parameters of nonlinear models (Brody, Gompertz, Hill, Logistic I and II, Meloun I and II, Michaelis-Menten, Mitscherlich, and von-Bertalanffy) were estimated by Gauss-Newton iterative process. Goodness of fit was evaluated by the corrected Akaike (AICc) and Bayesian (BIC) information criteria, adjusted R2, error sum of square, mean absolute deviation and average prediction error. Based on the goodness of fit, the Logistic I model fitted better for TSW while Logistic II fitted better for TV growth curve. The inflection point (IP) of TSW growth was estimated in 5.23 cm at 0.76 years; the IP of TV growth was estimated in 139.36 cm3 at 2.57 years. The TV growth curve reached a plateau later than TSW growth curve, mainly because testicular height maintains growth for a longer period. In addition, there were moderate and positive correlations between whiter height and TSW and TV (r = 0.51 and r = 0.53, respectively). Findings suggest that TV is more associated with sexual maturity than sexual precocity. In addition, the moderate and positive correlations between whiter height and TSW and TV indicate that testicular traits can be included in selection programs of Mangalarga Marchador stallions with the same relevance as the morphometric traits.