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Timing of induction of ovulation in mares treated with Ovuplant or Chorulon,☆☆

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Abstract

Reliable induction of timed ovulation is an important managerial tool in any horse-breeding operation. Not only does breeding close to ovulation increase pregnancy rates when using cooled, frozen, or poor-quality semen, but it also reduces the number of inseminations needed per cycle, resulting in a more efficient breeding program. To better predict ovulation time in the long estrus period of the mare, one could increase the frequency of transrectal palpations and ultrasounds and/or implement hormonal therapies to induce ovulations. However, previous studies have been unclear on the exact timing of ovulation of mares treated with human chorionic gonadotropin (Chorulon, Intervet Inc, Millsboro, DE) or deslorelin acetate (Ovuplant, Pharmacia and UpJohn Co, Kalamazoo, MI). This study was designed to determine the timing of ovulation after Ovuplant or Chorulon treatment in normal cycling mares presented to the veterinary clinic. In addition, the pregnancy rates were determined for mares bred when a single insemination, using frozen or chilled semen, was performed at a fixed time (36 hours) after Ovuplant or Chorulon treatment. Thirty-two mares were given a subcutaneous injection of 7.5 mg of prostaglandin F2α (Lutlyse, Ft Dodge Animal Health, Ft Dodge, IA) 5 days after the last ovulation and were examined every 48 hours until estrus was detected based on a dominant follicle and the presence of endometrial edema as determined by ultrasonographic examination. Group 1 (N = 12) was treated intravenously with 2,500 units of Chorulon, and group 2 (N = 20) was treated subcutaneously with Ovuplant as soon as mares were determined to be in estrus. Once treated all mares were examined by rectal palpation and ultrasound at 0, 12, 24, 28, 30, 32, 34, 36, 38, 40, 42, 44, 48, 60, 72, 84, 96, hours or until ovulation was detected. Ovulation rate in response to Chorulon was 83.3% at 48 hours, 91.6% at 72 hours, and 100% at 96 hours. All of the mares in the Ovuplant-treated group had ovulated by 48 hours. Chi-square analysis of the data showed a significant (P < .01) variation in the distribution of ovulation times between mares treated with Chorulon and mares treated with Ovuplant. This study provides enough evidence to support the hypothesis that timing of ovulation is a more reliable event in mares treated with Ovuplant compared with those treated with Chorulon.

Introduction

Although it has been shown that mares bred to fertile stallions can conceive even if ovulation occurs several days after breeding, conception rates increase significantly if the breeding takes place closer to ovulation.1 The reliability of inducing a timed ovulation in relationship to breeding becomes an important managerial tool when (1) mares are bred too highly booked stallions, and only 1 service per cycle is possible; (2) mares are bred to subfertile stallions whose sperm concentration is low or have poor sperm longevity; (3) mares must be bred only once because of delayed uterine clearance or susceptibility to uterine infections so that uterine contamination is reduced; or (4) mares are bred by artificial insemination with cooled transported or frozen semen because the number of shipments or insemination doses are often limited and semen quality is not optimal. Furthermore, limiting or reducing the number of inseminations per cycle will result in a more labor-efficient and cost-effective breeding operation.

Currently, there are 2 products commercially available to induce ovulation in the mare. Human chorionic gonadotropin (hCG) is a large glycoprotein with luteinizing hormone–like activity purified from the urine of pregnant women. Intramuscular or intravenous injection of 1,500 to 5,000 IU of hCG in mares displaying behavioral estrus and a follicle of at least 35 mm has proven effective in inducing ovulation.2 In addition to hCG, the gonadotropin releasing hormone (GnRH) analogue, deslorelin, or Ovuplant, has also been effective in inducing ovulation in mares that are displaying behavioral estrus with a follicle of 30 mm or greater.3, 4 Ovuplant is a small synthetic peptide that when implanted releases the active product over a period of 48 to 72 hours.3

Although hCG is effective in inducing ovulation, recent data indicates that the interval between treatment and ovulation when using hCG ranged between 12 hours to more than 48.5 In contrast, clinical impressions from practitioners have suggested that Ovuplant induces ovulation in what appears to be a very predictable time frame after treatment and that pregnancy rates of mares bred once or twice with frozen semen at fixed times after Ovuplant treatment appear to be comparable with no significant deleterious effect on pregnancy rates.6 Although ovulations induced by Ovuplant appear to occur around 24 to 48 hours, the precise ovulation time is not clear.

This study was designed to determine the precise timing of ovulation after Ovuplant treatment. In addition, we wanted to compare the distribution of the ovulations of normal cycling mares presented to the Veterinary Medicine Teaching Hospital for breeding and treated with Ovuplant or hCG (Chorulon). Pregnancy rates of mares bred with a single insemination of frozen or chilled semen at a fixed time after Ovuplant or hCG were determined. Based on previous observations,6, 7 we hypothesized that Ovuplant would have a reduced variation in the timing of ovulation induction compared with hCG.

Section snippets

Materials and methods

This study was conducted between May and July of 2001, at Kansas State University and complied with all criteria set by the animal use and care committee of Kansas State University.

Twenty-two quarterhorse (QH), 4 warmblood (WB), 4 thoroughbred (TB), and 2 Arabian mares were used for this study and were kept on pasture and supplemented with grass-alfalfa hay and minerals ad libitum or housed at the Veterinary Medical Teaching Hospital at Kansas State University. All mares were monitored for

Results

Average follicle size at the time of treatment for the hCG-treated mares was 40.5 ± 2.4 mm. and was not statistically different than that for the Ovuplant-treated mares (39.1 ± 3.7 mm.) (P > .05). Ovulation rate in response to Chorulon was 83.3% at 48 hours, 91.6% at 72 hours, and 100% at 96 hours. In contrast, all of the Ovuplant-treated mares had ovulated by 42 hours (Fig 1).

. Intervals to ovulation in hours from individual mares treated with Ovuplant (n = 20) or hCG (n = 12).

Although there

Discussion

In brood mare practice, timing ovulations at a specific time is highly desirable and could have many advantages for the veterinarian and breeding farm manager. Benefits of inducing a timed and reliable ovulation include a reduction in the number of services per cycle in highly booked stallions, increased accuracy in the timing of breeding when using cooled shipped or frozen semen or stallions with poor sperm longevity, a reduction in the number of covers or inseminations of problem or difficult

Conclusions

This data shows that Ovuplant induces ovulation in a very narrow window (36-42 hours) when implanted using the criteria set in the present study. Although hCG-induced ovulation reliably in most mares, the distribution was much wider than those ovulations in mares treated with Ovuplant. Because of its predictability in the time it causes ovulation, Ovuplant could become a very important tool in the management of mares breed by artificial insemination with shipped or frozen semen, or by natural

References (12)

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Cited by (27)

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    The equine is a positive phototropic species in which increased luminosity has a positive effect on the pulses of hypothalamic gonadotropin-releasing hormone (GnRH) and gonadotropins secretion [9,10]. Hence, the breeding activity of the mare is mainly influenced by photoperiod, nutrition, and climatic factors [11]. Several hormonal treatments have been used to induce and/or synchronize the ovulation as it facilitates equine reproductive management, reduces animal handling, and increases the successful application of reproductive biotechniques [12].

  • Histrelin acetate-induced ovulation in Brazilian Northeastern jennies (Equus asinus) with different follicle diameters

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    Ovulation induction is a methodology often used in mares [19–22] for predicting the best times for artificial insemination [23], which improves conception rates and enables the use of advanced techniques for assisted reproduction. Ovulation induction in mares is well established [24–26]. However, protocols for inducing ovulation in jennies are not well established yet.

  • Efficacy of Deslorelin Acetate (SucroMate) on Induction of Ovulation in American Quarter Horse Mares

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    Historically, deslorelin acetate has been administered to mares in implant, slow release, aqueous, and lyophilized formulations with the intent to induce a timed ovulation [3-8]. Subcutaneous administration of a sustained release implant containing 2.1 mg of deslorelin acetate (Ovuplant; Fort Dodge Animal Health, Fort Dodge, IA) was reported to induce ovulation in approximately 40 hours [2,9]. However, limited clinical data have been published on the recently approved deslorelin product.

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Address reprint requests to Juan C. Samper, DVM, PhD, Department of Clinical Sciences, 1800 Dennison Avenue, Manhattan, KS 66506.

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