THE AMERICAN MUSTANG


nication). Chemical vasectomy is a minimally in- vasive procedure, and although it would more than likely be considered a sterilant if performed success- fully, it may be an attractive alternative to limiting foal production in select areas where free-ranging horses are present and stallions thought to be a limiting factor. Development of chemical vasec- tomy, if proven successful and applied, will inevita- bly raise issues of stallion-targeted fertility control such as which and how many stallions will be re- quired to have an effect on large-scale population dynamics and whether long-term effect on behavior or genetics will occur.


3. Stallion Versus Mares as Targets for Contraception


Controversy exists as to whether the male should be targeted rather than the female for large-scale pop- ulation reduction. It is the author’s opinion and speculation that the development of a chemical agent of fertility control in stallions is more than likely to have an effect on sexual behavior over time and is dependent upon the temperament and expe- rience of the stallion. Any effect on sexual behavior in free-ranging dominant stallions could/would be an invitation for bachelor and subordinate stallions to meet the challenge and/or for mares of the af- fected stallion of a herd to wander during estrus. Additionally, a single fertile stallion (dominant, sub- ordinate, or bachelor) may be capable of impregnat- ing approximately 40 mares per year, although this scenario of access to mares would be unlikely in free-ranging horse herds. Regardless, in view of the number of bachelors available in free-ranging horse herds, it would require treating the majority of stallions (dominant, subordinate, and bachelors) in a given territory to have any effect on reducing foal crops for a given year. Would this have an effect on genetic diversity? What percentage of the stallions would be required to be treated to have an effect on population growth? Is this strategy feasible from a management perspective? These are a few of the many questions that require further investigation. Modeling studies may help answer questions of whether inhibition of fertility to favor males or fe- males, or a combination of both, would be the best measurements of limiting foal production on a yearly basis. However, accurate data on popula- tion demographics and dynamics is required to in- form and validate these models before they are truly useful.


4. Fertility Control in the Female


A number of agents for use as contraceptives in female equids have been studied. The agents com- prise of steroids, GnRH agonists, immunocontracep- tives, and intrauterine devices.


Intrauterine Devices


Several forms of intrauterine devices have been in- vestigated in the horse. Presumptively, the mech- anism of action in horses is to incite a low grade


426 2014  Vol. 60  AAEP PROCEEDINGS


endometritis, with the devices serving as a nidus, stimulating persistent inflammation within the uterine lumen as long as the device remains in the uterus. Persistent low grade endometritis repre- sents a hostile environment within the uterus and is incompatible with survival of the embryo. Flexible, silastic O-ring devices were inserted into the uterus in six domestic mares.17 The mares cycled nor- mally when monitored by palpation and ultrasound. None of the mares conceived throughout one breed- ing season but all conceived after removal of the intrauterine devices. Another study examined the effectiveness of a copper T shaped intrauterine de- vice in 15 feral mares over a three year period.18 The mares were exposed to fertile stallions for 60 days and found 20 percent of the treated mares to have conceived compared with 75 percent of the controls in the first year of the study. However, 75% to 80% of the treated mares were found to be pregnant over the next two years with no evidence of the intrauterine device being present in the preg- nant mares. The authors speculated that the intra- uterine devices were spontaneously expelled from the uterus of treated mares after the first year. As mentioned previously, the insertion of glass mar- bles and water-filled propylene balls into the uterus of mares may also be considered as intrauterine devices, and they may also have an effect on fertility. However, the objective of these two investigations was aimed at estrous suppression and not long-term contraception. Subsequent and consistent losses over time of glass marbles inserted into mares when housed with stallions suggests that intrauterine de- vices may not serve effectively as a long-term con- traceptive strategy for large-scale population growth reduction.


Estrogens and Progesterone


In several earlier studies, the use of native estrogen and progesterone as long-term contraception agents in horses proved not to be as successful as ex- pected.19,20 It is speculated that its failure to achieve a desired effectiveness was due to the rapid metabolism and enzymatic degradation of the native steroids and that significantly and impractical higher doses were required for a desired effect. These earlier studies led to the successful use of synthetic estrogens and progesterone or its combi- nation thereof for contraception in mares. Formu- lations of the synthetic steroid ethinyl estradiol and natural progesterone into silastic implants for long acting effects have been reported.21,22 The authors report that contraception effect from intraperitone- al-inserted implants in treated mares was 85% to 100% effective for 2 to 3 years. In another study by one of the investigators, ethinyl estradiol implants alone, when inserted into the peritoneal cavity of mares at concentrations of 1.5 g, 3.0 g, and 8.0 g, yielded a 75% to 100% contraception rate based on the rate of decline of plasma ethinyl estradiol.23 Despite the successful outcome and relative inva-


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