|PZP-22 and SpayVac®a||Research and application in both captive and free-ranging horses||Capture needed for hand injection of PZP-22|
|Allows estrous cycles to continue so natural behaviors are maintained||Extended breeding season requires males to defend females longer|
|High efficacy||With repeated use, return to fertility becomes less predictable|
|Can be administered during pregnancy or lactation||Out-of-season births are possible|
|Chemical Vasectomy||Simpler than surgical vasectomy Permanent||Requires handling and light anesthesia Permanent|
|No side effects expected||Only surgical vasectomy has been studied in horses, so side effects of the chemical agent are unknown|
|Normal male behaviors maintained||Extended breeding season requires males to defend females longer and may result in late-season foals if remaining fertile males mate|
|Should have high efficacy||Only surgical vasectomy has been studied in horses, so efficacy rate is unknown|
|GonaCon™ for Females||Capture may be needed for hand injection of initial vaccine and any boosters|
|Effective for multiple years||Lower efficacy than PZP-vaccine products, especially after first year|
|Sexual behavior exhibited||Sexual behavior may not be cyclic, inasmuch as ovulation appears to be blocked|
|Social behaviors not affected in the single field study||Should not be administered during early pregnancy because abortion could occur Few data on horses|
|aPZP-22 and SpayVac® are formulated for longer efficacy and require further documentation of continued efficacy and of rate of unexpected effects.
SOURCE: Asa et al. (1980b), Kirkpatrick et al. (1990), Thompson (2000), Kirkpatrick and Turner (2002, 2003, 2008), Stout and Colenbrander (2004), Imboden et al. (2006), Turner et al. (2007), Killian et al. (2008a), Gray (2009), Nuñez et al. (2009, 2010), Gray et al. (2010, 2011), Powers et al. (2011), Ransom (2012).
might be more appropriate in populations in which a relatively large percentage of males could be treated. The strategy of treating only dominant stallions should be avoided.
Late-season births could occur in mares treated with one of the vaccine products if reversal occurred during the breeding season, but because most free-ranging mares give birth every other year rather than yearly, conceptions and births should become re- established in spring or early summer. For mares that are able to maintain a pregnancy and give birth annually, reversal late in the season could have long-term consequences for all her future foals in that the 11-month gestation and the one or two ovulatory cycles needed to conceive can result in an about 12-month repeating cycle (see Garrott and Siniff, 1992).