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4 Methods and Effects of Fertility Management
Pages 93-142

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From page 93...
... . Research on effective methods of fertility control remains important to the Bureau of Land Management (BLM)
From page 94...
... Reversible contraception and permanent sterilization are achieved by interrupting reproductive processes, and the committee's evaluation of these methods is based in part on understanding their effects on an animal's reproductive physiology and behavior. Accordingly, this chapter starts with two reviews: one on equine social and mating behavior, social relationships, and social structure and a second on reproductive physiology in domestic horses and donkeys, with information on free-ranging horses and burros when available.
From page 95...
... Male domestic horses can produce sperm year round, but the quality declines during winter, the mares' nonbreeding season (Pickett et al., 1975)
From page 96...
... Fertility rates in domestic horses are reported to range from about 80 to 100 percent per breeding season, depending on factors such as breed, age, and reproductive history (reviewed in Ginther, 1979)
From page 97...
... The porcine zona pellucida (PZP) vaccine, an immunocontraceptive, is the most extensively tested method in free-ranging horses and may be the most promising option at present.
From page 98...
... FEMALE-DIRECTED METHODS OF FERTILITY CONTROL Potential methods of fertility control directed at female equids include surgical o ­ variectomy (removal of the ovaries) ; immunocontraceptives, which trigger the animal's immune system to prevent pregnancy; GnRH agonists; steroid hormones; and intrauterine devices.
From page 99...
... No comparable study of the sexual behavior of free-ranging, nonpregnant mares has been conducted during the nonbreeding season. However, if freeranging ovariectomized mares also show estrous behavior and occasionally allow copulation, interest of the stallion would be maintained, and this would foster band cohesion.
From page 100...
... . In both procedures, the product passes through a series of filters of decreasing pore size to remove other ovarian debris, but it is possible that the SpayVac preparation contains more non-zona pellucida ovarian proteins than liquid PZP produced with the Liu et al.
From page 101...
... For example, in the first study of liquid PZP in domestic mares, Liu et al.
From page 102...
... induced the strongest immune response in domestic horses as measured by antibody titers and exhibited the strongest suppression of progesterone compared with an aqueous preparation of FMA and non–mycobacterium-based adjuvant, but she did not assess pregnancy or foaling. 4  Assateague Island National Seashore is on a barrier island off the coast of Maryland and operated by the U.S.
From page 103...
... (1990) first demonstrated that three of seven free-ranging mares became fertile in the first year after 1 year of liquid-PZP treatment, although foaling rates of treated mares overall were lower after treatment than in control mares.
From page 104...
... Twenty-two mares on the Little Book Cliffs HMA and 38 mares on the Pryor Mountain Wild Horse Range were treated with liquid PZP up to 5 consecutive years. At the McCullough Peaks HMA, 28 mares were treated with PZP-22.
From page 105...
... Mares that have been treated with liquid PZP for 3-7 consecutive years have been reported to have decreased ovulation rates in successive years of treatment (Kirkpatrick et al., 1992, 1995) ; this suggests that PZP may act at sites other than just the zona pellucida.
From page 106...
... found that foal survival to 1 year is equivalent between untreated mares and mares treated with liquid PZP during pregnancy; female foals born to PZP-treated females also successfully bred and reared offspring. Kirkpatrick and Turner (2003)
From page 107...
... (2010) found no differences in foal survival, birth seasonality, or foal sex ratio between treated and untreated mares.
From page 108...
... Contraceptive effectiveness continues to be high (A. Turner, Assateague Island National Seashore, email communication, February 24, 2013)
From page 109...
... No other studies of PZP contraception in burros have been published. The effects of liquid PZP on harem stability in horses have been studied in Nevada during breeding and nonbreeding seasons by Gray (2009)
From page 110...
... The importance of harem stability to mare well-being is not clear, but considering the relatively large number of free-ranging mares that have been treated with liquid PZP in a variety of ecological settings, the likelihood of serious adverse effects seems low. Side Effects: Demography and Population Processes.
From page 111...
... Many of the behavioral changes associated with fertility control that are discussed in the preceding section are also likely to affect population dynamics. A longer breeding season could affect band stability and would probably extend male sexual activity into months when they normally recover strength and rebuild body condition.
From page 112...
... . Whether that cascade of events will occur in particular horse or burro populations will depend on the magnitude and interaction of three factors: envi­ ronmental harshness in the nonbreeding season, social instability, and improvement of body condition in treated females due to absence of energetic demands of pregnancy and lactation.
From page 113...
... in the middle of the breeding season. By day 35, only 14.5 percent of treated mares showed
From page 114...
... All treated mares showed reduced ovarian activity; by 4 weeks after the booster, ovaries of treated mares resembled those of seasonally ­ novulatory mares. a The efficacy of GnRH vaccines has also been studied in other species.
From page 115...
... In probably the earliest study of a GnRH vaccine, Goodloe (1991) found no differences in birth seasonality between treated and untreated mares on Cumberland Island, a barrier island off the coast of Georgia.
From page 116...
... An additional or alternative explanation might be continued production of adrenal sex steroids in the absence of ovarian steroids; this has been shown to support estrous behavior in domestic horses during the nonbreeding season or after ovariectomy (Asa et al., 1980b)
From page 117...
... GnRH agonists (synthetic versions of GnRH that have activity similar to the natural hormone) are commonly used in many domestic species to stimulate follicle growth, estrus, and ovulation.
From page 118...
... Effects of GnRH agonists on behavior, after the initial stimulation phase when estrous behavior might result, should be similar to those associated with ovariectomy. That is, e ­ strous cycles would be absent, but sporadic expression of estrus supported by adrenal sex steroids might occur.
From page 119...
... . No adverse effects of repeated administration of these GnRH agonists have been reported in the literature over the last 2 decades since its acceptance, and they continue to be used in the manipulation of the estrous cycle in domestic mares (I.K.M.
From page 120...
... However, there were some differences in health and survival of foals born to altrenogest-treated mares in that study. Although progesterone (as the "progestational" hormone)
From page 121...
... Almost all formulations of human birth-control pills contain synthetic estrogen plus progestagen; one contains only progestagen. Treatment of mares with estrogen stimulates estrous behavior (Asa et al., 1984)
From page 122...
... used a flexible, silastic O-ring, fabricated specially for the study, in six domestic mares; when compressed, it could be easily inserted into the cervix and later removed in the same way. During the breeding season after the IUDs were in place, none of the mares conceived, but all conceived after IUD removal during the next 2 years.
From page 123...
... Because castration removes the primary source of androgen production, maletype aggressive and sexual behaviors are usually reduced. Adrenal androgens (such as dehydroepiandrosterone)
From page 124...
... . The probability that subordinate stallions will mate is higher in bands that have a vasectomized dominant stallion because the females continue to have estrous cycles throughout the entire breeding season, whereas females with intact, fertile stallions are likely to conceive in the first month or so of the breeding season.
From page 125...
... reported that mature stallions treated with GnRH vaccines continued to produce sufficient semen to impregnate a mare. 10  Genetic diversity and effective population size are discussed further in Chapter 5.
From page 126...
... during the nonbreeding season. The vaccination protocol involved five shots at intervals of 2-4 weeks.
From page 127...
... The inability of GnRH vaccines to suppress FSH completely, although central to maintenance of sexual behavior in treated females, is not likely to affect males. The possible effects on male behavior are probably limited to suppression of LH, inasmuch as LH alone is needed to support testosterone production.
From page 128...
... are likely to be associated with a prolonged breeding season. That is, mares that are not pregnant continue to undergo estrous cycles until late summer or fall, when day length is decreasing and no longer stimulates cycling (Sharp and Ginther, 1975)
From page 129...
... Although altrenogest, an oral progestagen product, has been used successfully in domestic mares to control estrus, it requires daily dosing during the breeding season. There is no mechanism to assure delivery to mares only, so consumption by stallions, nontargeted wildlife, and domestic grazing livestock could have deleterious effects.
From page 130...
... At the time this report was prepared, liquid PZP and GonaCon were licensed. Application to EPA for licensing pelleted PZP-22 for free-ranging horses was being prepared.
From page 131...
... In contrast, vasectomy and the PZP vaccines result in a prolonged breeding season, with increased sexual interaction, because females continue to undergo estrous cycles but fail to conceive. That is not ideal because a prolonged breeding season can result in more fighting among males over access to females.
From page 132...
... 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-­ stablished in spring e or early summer.
From page 133...
... Many state veterinary licensing agencies require that a vasectomy be performed by a licensed veterinarian, although the surgery is straightforward, but the simpler chemical vasectomy has not been systematically evaluated in horses, so test ing in captive horses would be needed before widespread application in the field. CONCLUSIONS On the basis of the peer-reviewed literature and direct communication with scientists who are studying fertility control in horses and burros, the committee considers the three
From page 134...
... All three methods should preserve the basic social unit and expression of sexual behavior, although there have been conflicting reports on various effects of the vaccines on social interactions and on the cyclicity of estrous behavior. The major effect of the methods is that the typical breeding season would be extended for females that do not conceive (the implications are discussed at length above)
From page 135...
... GonaCon should be examined to evaluate the extent to which treated females continue to exhibit sexual behavior, which is important for maintaining natural social interactions. A study is needed to assess the efficacy and safety of potential agents for chemical vasectomy before it is used in free-ranging stallions during gathers.
From page 136...
... 2004. Economic Analysis of Alternative Fertility Control and Associated Management Techniques for Three BLM Wild Horse Herds.
From page 137...
... 2009. The Influence of Reproduction and Fertility Manipulation on the Social Behavior of Feral Horses (Equus caballus)
From page 138...
... 2008a. Four-year contraception rates of mares treated with single-injection porcine zona pellucida and GnRH vaccines and intrauterine devices.
From page 139...
... 2005. A comparison of Freund's Complete and Freund's Modified Adjuvants used with a contraceptive vaccine in wild horses (Equus caballus)
From page 140...
... 1978. On the Question of Male-Limited Population Growth in Feral Horses (Equus caballus)
From page 141...
... 2011. Foaling rates in feral horses treated with the immunocontraceptive porcine zona pellucida.
From page 142...
... 2004. Suppressing reproductive activity in horses using GnRH vaccines, a ­ ntagonists or agonists.


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