Breeding Management of the Older Stallion With Declining Testicular Function


Chapter 152

Breeding Management of the Older Stallion With Declining Testicular Function



Terry L. Blanchard, Dickson D. Varner, James P. Morehead


Many stallions achieve good pregnancy rates when bred beyond 20 years of age, but eventually fertility begins to decline. A myriad of causes can contribute to this declining fertility (e.g., physical soundness, body condition, mating ability, ejaculatory dysfunction, breeding injuries, and testicular dysfunction). This chapter addresses age-related decline in testicular function.


Clinical experience garnered from annual or more frequent examinations of aging stallions over several years before and during the period when they are undergoing declining testicular function suggests the following course of progression:



1. Early in the course of progression of testicular dysfunction in aged stallions, a period of decreasing sperm output occurs, without remarkable change in testicular size. The diminished sperm output results in lowered spermatogenic efficiency. Pregnancy rates may not be noticeably affected early in this period unless the stallion is breeding a large number of mares.


2. As spermatogenic efficiency continues to decline yet before testicular size is noticeably decreased, mare pregnancy rates per cycle begin decreasing. This period is often associated with a decrease in percentage of morphologically normal and progressively motile sperm in ejaculates, yet sperm chromatin structure assays do not usually reveal prominent changes in sperm DNA integrity.


3. Circulating hormone concentrations usually do not differ from reported normal ranges until after spermatogenic efficiency has declined to 6 to 8 million sperm/mL of testicular parenchyma, total number of sperm in ejaculates at daily sperm output are 2 billion or less, and pregnancy rates per cycle have diminished to 40% or less (in Thoroughbreds). Sperm motility and morphology values typically, but not always, have deteriorated from those found when the horse was still achieving normal pregnancy rates per cycle. Although low circulating estrogen concentration may be noted as the first endocrine abnormality detected at this level of testicular dysfunction, we also commonly find a low circulating concentration of testosterone (i.e., 100 to 300 pg/mL), and administration of gonadotropin-releasing hormone may not elicit greater than or equal to 500 pg/mL testosterone within 2 hours.


4. Late in the course of progression of testicular dysfunction (as more severe subfertility develops), sperm DNA integrity begins to deteriorate. During this late stage, testicular size has usually decreased dramatically to 60% or less of the size when the stallion was several years younger. At this late stage of testicular dysfunction, variations from normal endocrine values often include low circulating concentrations of estrogen, testosterone, and inhibin and finally high circulating concentrations of follicle-stimulating hormone and luteinizing hormone.



Breeding Management Options


Breeding management of stallions ejaculating low numbers of normal, motile sperm is typically directed toward (1) increasing the number of sperm that access the uterus and perhaps oviducts, and (2) decreasing the interval from mating to ovulation because the longevity of sperm motility is often constrained in semen from aged stallions. Optimal reproductive management is necessary to ensure that mares are bred near the time of ovulation, thus resulting in a low percentage of mares in the book requiring multiple covers or inseminations per estrus.


Dietary supplementation with omega-3 fatty acids may increase the number of progressively motile, morphologically normal sperm in ejaculates of some stallions and may increase longevity of progressive sperm motility when semen is cooled or frozen (presumably by improving sperm membrane stability). We are aware of no scientific evidence that feeding omega-3 fatty acids will improve semen quality or fertility of aging stallions with declining testicular function, but no significant problems have been observed when diets are supplemented with omega-3 fatty acids.



Thoroughbred Stallions (Natural Cover)


As output of normal sperm in ejaculates declines and pregnancy rates decrease, one approach that may benefit pregnancy rates in an aging Thoroughbred stallion is to diminish the mating frequency (i.e., reduce the number of covers allowed per day and thus the size of the mare book). Evaluating breeding records to compare pregnancy rates among days with differing numbers of covers may provide guidelines for how much to limit mating frequency.


Another management strategy sometimes employed with natural service mating programs involving stallions with lowered fertility is the routine use of reinforcement breeding. Reinforcement breeding is a process wherein the dismount semen sample (or aspiration of semen pooled in the vagina, or both) is collected following natural cover and is infused into the uterus of the mare just covered. The dismount semen sample should be filtered to remove as much debris as possible and immediately mixed with a suitable volume (5 to 20 mL) of prewarmed extender before infusion. Alternatively, extender can be mixed with the dismount sample (or semen sample aspirated from the vagina) before filtering and infusion into the uterus. The process results in an increased number of sperm being placed directly in the uterus, and the added extender may provide some protection to sperm and improve the longevity of motility until they access the oviduct. Pregnancy rates can be compared between “reinforced” and “nonreinforced” mares, and if fertility seems to be improved, reinforcement breeding may be justified as a standard protocol for that stallion. In aged Thoroughbred stallions with declining testicular function and reduced pregnancy rates (i.e., pregnancy rates per cycle of 20% to 40%), reinforcement breeding often results in a 1.2-fold to twofold increase in pregnancy rates per cycle.


Older stallions with declining testicular function may also ejaculate a low number of normal motile sperm of short longevity (i.e., sperm motility rapidly declines in vitro). Managing each mare in the stallion’s book with two to three covers in the period 12 to 36 hours after the administration of an ovulation-inducing drug may improve pregnancy rates. This strategy is directed toward maximizing the number of live normal sperm that will have colonized the oviduct as near to the time of expected fertilization as possible. By necessity, the mare book number must be reduced because of the increase in mating frequency required for each mare bred.

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Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Breeding Management of the Older Stallion With Declining Testicular Function

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