Management of the growing mustang population on federal lands has been a controversial issue. A suggested method for controlling overpopulation is to sterilize the dominant male in each group. Eagle, Asa, and Garrott et al. (1993) conducted an experiment evaluating the effectiveness of sterilizing the dominant males as a way to reduce foaling (birth) rates for 2 or more years.
The researchers chose two Herd Management Areas (HMAs), Flanigan in northwestern Nevada and Beaty Butte in southeastern Oregon, for this study. In December 1985, they rounded up the horses in bands and counted all individual horses, determined their sex, and estimated their ages by looking at tooth wear. They photographed all horses three years old or older and fitted them with numbered collars to assist in identification throughout the study. They identified the dominant male in each band, vasectomized it, and fitted it with a radio-transmitting collar. Finally, they released the band as a group. Between June 1986 and July 1988 they attempted to locate each sterilized male 3-4 times a year by aerial survey from helicopter. The researchers recorded the number of adults and foals in each group containing a sterilized male (treated groups), and in the groups without a sterilized male (untreated groups).
While the researchers could not record actual birthrates in the bands of horses, the number of foals per 100 adults in each band is a good substitute. Graphical methods are useful for illustrating the difference in foal to adult ratio for the treated and untreated groups. Different multiple regression models may be used to evaluate the effect of the treatment while controlling for other variables such as herd size