NON LINEAR GROWTH CURVES FOR WEIGHT AND HEIGHT IN FOUR GENETIC GROUPS OF HORSES

Abstract

Hanoverian, Brazilian Showjumper, English Thoroughbred and Crossbred horses reared by the Brazilian Army were weighed and measured from six months of age to adult. In total 4,860 measurements on 1,445 horses were available. General curves were estimated as a function of time by the Gompertz, Brody, Logistic, Weibull and Richards curves, using PROC NLIN procedures of SAS ®. The Richards Curve did not converge for weight or height of any of the genetic groups or sexes. The logistic curve did not converge for any of the weight traits while the Gompertz also did not converge for height in several groups. R2 varied between 0.55 for weight in females of the crossbred group to 0.92 for males of the same group. For the height traits the highest R2 (0.66) was found for female Hanovarian horses and lowest for males of the same breed (0.12). In general the curves estimated similar values for asymptotic height and weight, except for Logistic curve, which also showed lowest R2 and highest error. Results for the Weibull and Brody curves were similar in all cases so where possible the Brody curve was selected as the best curve as it had less parameters. The Gompertz curve tended to underestimate mature weights and height. Estimates for both weight and height were in general higher in males than for females. In most cases the b parameter was shown to account for < 0.0001% of the variation in the curve shape. The k parameters, which indicate maturity, were of similar magnitude for the Brody, Gompertz and Weibull curves, for both height and weight within breed. This parameter indicated that there is little difference in maturation rates between males and females.

KEY WORDS: Brody, Gompertz, Logistic, Richards, Weibull.