Evaluation of four mathematical functions to describe scrotal circumference maturation in Nelore bulls

摘要

532 Nelore bulls born between September and December 1992 to 1994 were studied. Scrotum circumference (SC) measurements were made on fixed dates in January, April, July and October of each year. At the time of SC measurements, the ages of the bullsranged from 200 to 1300 days. The functions used to describe the data were: Brody, SC = A (1 - B exp-kt); Logistic, SC = A / (1 + B exp-kt); Gompertz, SC = A exp(-B exp-kt) and Richards SC = A (1 + B exp-kt)m, where SC is the scrotal circumference at t days of age, A is the estimated SC at maturity, B is the integration constant established by the initial values of SC and t, k is the maturity constant, which equals the ratio between the maximum rate of growth and SC at maturity; m is the inflection point parameter for the Richards' function, which does not converge. The Brody, Gompertz and Logistic functions fitted the data in a similar fashion, with similar values for error mean squares and R2, and they reached convergence with similar computational costs. The Logistic function gave the best pattern of average prediction error, and was therefore selected for biological interpretation. For the Logistic function, estimated SC at maturity (A) was 37.95 cm at 72 months of age. The maturity index (k) was0.11 cm, and the inflection point (time of maximum growth) was reached at 13.09 months of age at an average SC of 18.97 cm.