Abstract:
In North America, range constraints due to burgeoning development increasingly encroach
on wild horse habitat and necessitate effective but humane reproductive management. The
largest free-roaming wild horse fertility control program by population (>3500) and territory size
(≈300,000 acres) is located within Nevada’s Virginia Range. Data from a field study investigated
porcine zona pellucida (pZP) immunocontraception via remote dart delivery to mares in this population.
Analyses aimed to measure efficacy by treatment effects on annual birth rates and population
demographics and to evaluate treatment frequency and season against these variables. Analyses
included mares’ monthly data (January 2019–December 2022; 48 months), characterized by cumulative
vaccination numbers subset into four classifications considering the vaccine as having no loss
of efficacy or a loss within a 6-, 12-, and 18-month period post vaccination; from foaling data, the
likelihood of being in foal and of conceiving in that month; and from age, as mature or immature
(<1 years-old). A downward foaling rate and trend in the numbers of mature mares, descriptively
presented at monthly intervals, showed markedly declining annual seasonal breeding peaks, with no
observed change in foaling season or duration. Within four years, population coverage surpassed
70% and was associated with a 58% reduction in foaling, with only a 10% conception rate. Vaccinated
mares increased proportionally: assuming a 12-month decay rate, the system reached stability at an
average ≈1.0 vaccination/mare/year, providing a robust recommendation for treatment frequency
contributing to best management practices.
