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Sex Ratio Management

One way to potentially slow population growth and extend the time between gathers in wild horse herds is to adjust herd sex ratios to include more males than females. BLM rangeland managers can use this option following a gather by releasing more stallions or geldings than mares back to the range. The larger proportion of males means there will be fewer mares in the breeding population, resulting in fewer births.

Sex ratio adjustment is mostly applicable to larger Herd Management Areas (HMAs) and is also most practical after appropriate management level (AML) has been achieved.

Rangeland managers can monitor the impact of sex ratio adjustments to gauge their effectiveness in helping to maintain AML. This includes monitoring how “bachelor” stallions or geldings may or may not interact with breeding mares; determining whether the size of horse bands changes with a greater proportion of males on the range; monitoring changes in the distribution of animals on the range, if any; and monitoring competition for food or water among the animals.

Geldings released back to the range may form or join bachelor bands with other geldings or bachelor stallions, or they may live in family bands with mares, a dominant stallion, and young horses. Typically, geldings will more easily maintain good body condition than most mares or breeding stallions because there are fewer demands on their energy reserves and they are not sexually active.

As BLM personnel monitor the results of adjusting sex ratios, they can use the information to decide if it would be best to continue releasing more males than females following the next gather, to remove some or all of the extra stallions or geldings, or to release a larger proportion of mares back to the range.

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