Trends and asymptotic limits for racing speed in standardbred trotters

Racing data on 44,372 Standardbred trotters in Sweden, spanning a period of nineteen years, were analyzed. Racing speed of any i-th horse was measured as the best average racing time (k(1) = sec/km) obtained in a trotting rac e of a length greater than or equal to 1640 m, as a 3- to 5-year-old. The d istribution of the best average racing time records was found to be asymmet ric within the population. A scaled logarithmic function of best average ra cing time of male horses (y(1) = ln(k(i) - 68.2)) was found to be normally distributed (zero skewness and low kurtosis). This means that best average racing time records can be expressed as: k(i) = e(y(1)) + x, where the cons tant x = 68.2 sec/km was interpreted as the asymptotic limit for trotting s peed in the population of male Standardbred trotters. An equation Kbirth-ye ar = x(1 + e(-pt)) was fitted to the data for estimating trends in average, minimum and maximum best average racing time records at time t = (birth-ye ar - z) in Swedish Standardbred trotters. The constants p and z were estima ted by least-squares grid search. The trend in the average of best average racing time in male trotters can be predicted by the following expression: AverK(birth-year) = 68.2(1 + e((-0.015 (birth-year-1861)))). The correspond ing prediction of the fastest racing time records is: MinK(birth-year) = 68 .2(1 + e((-0.019 (birth-year - 1853)))). The log linear scale effects may b e interpreted as successive reductions in marginal substitution effects of genes and environmental factors affecting racing time as measured on the or iginal scale. (C) 2001 Elsevier Science B.V. All rights reserved.