TABLE 4-1 Advantages and Disadvantages of the Most Promising Fertility-Control Methods

Method Advantages Disadvantages
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).

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement