AGE-SPECIFIC REPRODUCTION IN FERAL HORSES

Two age-specific reproduction schedules were constructed for feral horses (Equus caballus) on the basis of lactation status of 14 788 females captured during herd reduction programs and pregnancy rates of 667 horses determined by serum progesterone assays. The probability of detecting lactation progressively decreased for females captured further from the foaling season, indicating that these data resulted in substantial underestimates of true foaling rates. A third reproductive schedule was, therefore, constructed on the basis of a subsample of 1144 horses captured immediately after the foaling season in June. Characteristics common across all three data sets were first reproduction at age 2, an increase in the proportion of females foaling through age 6, highest foaling rates from 6 to 15 years, and a gradual decrease in foaling rates of females > 15 years. Variability in the proportion of reproductively active females in each age-class detected among populations and among years within a population; however, the general trend was high reproductive rates, with 80-90% of the prime-age females foaling. The reproduction model suggested by L. L. Eberhardt, provided a close mathematical approximation of the observed age-specific changes in foaling rates, providing a useful tool for the construction of reproduction schedules required for age-structured population models.