Managing Sub-fertile Stallions

The decision to manage a sub-fertile stallion non-conventionally is "based on the condition of farm economics or despair, or more appropriately, the last hope before retiring the stallion," said Irwin K.M. Liu, DVM, PhD, a professor in the Department of Population Health and Reproduction in the University of California, Davis, School of Veterinary Medicine. Liu brought to the table suggestions for managing stallions with declined fertility at the Hagyard Bluegrass Equine Symposium, held Oct. 20-23, 2004, in Lexington, Ky.

"Most of you realize that our understanding and management of sub-fertile mares has substantially increased," he said. "Assisted reproduction technology has made a significant impact on pregnancy rates in sub-fertile mares," and has the potential to help sub-fertile stallions as well.  But not many strategies have helped the sub-fertile stallion under natural cover conditions and requirements.

Liu explained that testicular degeneration is often--but not always--associated with age and declining pregnancy rates, and it is highly correlated with low sperm concentrations and abnormal sperm pathology. Testicular degeneration varies from stud to stud. "Clearly each management strategy is based on the individual cause of sub-fertility," said Liu.

The normal sperm of young sub-fertile stallions  appear to be more fertile than the normal sperm of older sub-fertile stallions, so managing an older stallion will likely be more intensive.

Stallions can have progressive sperm motility, but show fertility rates that are far below standards--a situation that can be frustrating, but manageable. Liu described several studs whose pregnancy rates with a single breeding prior to the mare's ovulation were unimpressive (10-20%) before they underwent unconventional management. The problems were solved by adjusting the stallions'  breeding schedule: Each stallion bred each mare twice at four- to six- hour intervals in the late afternoon and early evening the day before the mare's anticipated time of ovulation. Liu emphasized that the validity of the suggested strategy was not scientifically documented.

After the first year of management, the stallions' pregnancy rates increased dramatically, although their books of mares were kept small. More frequent matings allow a greater number of normal sperm to be available at the fertilization site, but can deplete the stallion's reserves, so the smaller book might be necessary.

It's important to remember the mare plays a key role in sperm transport, and her reproductive condition will help determine the stud's success. Cilia in the normal mare's uterus and sperm have close interactions and communications that result in only normal sperm reaching the fertilization site. In a chronically infected uterus, adverse conditions prevent healthy sperm from reaching the highly selective oviduct, which selects out sperm with abnormal morphology.

Liu suggested veterinarians should flush the mare's uterus four hours after each mating to a sub-fertile stallion, if the mare is a persistent accumulator of fluid and debris. "The rationale is based on our current knowledge of sperm transport in the mare," he said. "Billions of sperm are ejaculated in a normal stallion, and only a few hundred gain access to the fertility site. Four hours after mating, one can lavage the uterus of all its contents (to decrease chance of infection that could hamper the pregnancy) without disturbing the pregnancy.

"Once you know you're dealing with a sub-fertile mare, you know she will always be susceptible to infection after she is bred," he added, so make sure this step is taken.

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“Sex is truly a luxury in the body--you’ve got to be productive before you can be reproductive,” began David Pugh, DVM, MS, Dipl. ACT, Dipl. ACVN, a professor of reproduction at Auburn University, in his presentation “Nutrition and Its Effects on Reproduction” at the Hagyard-Davidson-McGee Bluegrass Equine Reproduction Symposium October 23-26 in Lexington, Ky. He stated that broodmares with a body condition score (BCS) of less than 4 on a nine-point scale exhibit:

• Increased length of spring transition (to proper estrous cycling);

• Increased time from foaling to ovulation;

• Increased number of cycles to conception;

• Decreased conception rates;

• Increased fetal wastage (fetal loss); and

• Decreased milk production.

Pugh discussed several other points to remember when considering nutrition relative to equine reproduction, including the following.

Feeding Broodmares

• Underfeeding might delay the date of first ovulation in maturing mares.

• Moderate mare body weight gain or loss during gestation doesn’t appear to greatly affect foal weight.

• More than 60% of the foal’s growth occurs in the last trimester of the pregnancy. Most non-draft horses gain 150-200 pounds during gestation.

• Feeding fat beginning at nine months of gestation will increase the mare’s milk fat percentage, but not birth weight of the foal.

• Pugh recommends a diet with 8-9% protein until nine months of gestation, then a diet with 9-10% protein for months 10-11. Inadequate protein can decrease foal birth weight, even in mares in good body condition. But too much protein can be problematic--mares with a history of stress and consumption of high-protein or legume-rich diets may have placental edema or premature separation of the placenta, particularly with diets high in some estrogen-rich legumes. A rising nutritional plane, or feeding for weight gain, in pregnant mares (especially with high protein) is associated with placental edema (indicative of placental inflammation, which can decrease the nutrients going to the foal).

• Supplementing vitamin E near the end of gestation may increase IgA and IgG (types of antibodies) in colostrum.

Mare Problems

• Copper in the diet may decrease the incidence of rupture of the middle uterine artery, utero-ovarian artery, or external iliac artery.

• For mares with ventral edema and potential rupture of the prepubic tendon, rectus abdomen muscle, oblique abdominal, and transverse abdominal muscles (more common in older mares with poor abdominal tone), it might be best to induce parturition. Nutritional treatment includes decreasing salt (which might require cutting out commercial feeds that normally contain salt) in the diet and lowering fiber content to decrease weight of feed in the gastrointestinal tract.

• Eclampsia (convulsions and coma associated with hypertension, edema, and/or excess protein in the urine) can occur two weeks or so after foaling, and is associated with lactation, stress (such as inadequate intake or transport stress). Decreasing high-protein feeds in the diet such as alfalfa in late gestation might help prevent this condition in susceptible mares. Treatment of affected mares involves decreasing dietary calcium two to five weeks before foaling, then adding calcium to the diet after foaling. These mares need a high-protein, high-calcium diet after foaling and throughout lactation, said Pugh.

• Overweight mares are more likely to get laminitis when problems such as retained placenta occur, so maintaining mares in a more normal body condition might help prevent laminitis secondary to other problems.

Feeding Stallions

• A stallion with a heavy breeding schedule might require more than 25% more energy during the breeding season compared to the non-breeding season.

• High-calcium feeds can depress the horse’s absorption of zinc, resulting in a zinc deficiency. This can be a problem in stallions, though it’s rare.

General Feeding Notes

• When trying to balance minerals in any horse’s diet, you should select one designed for your area and soil type. However, trucking in hay or feed grown in other areas can change this; consult an equine nutritionist for help in balancing your horse’s diet.

• A high fat content in the diet can decrease absorption of some minerals.

• Studies indicate that once a diet is balanced for vitamins and minerals, addition of supplements does not enhance fertility. Additionally, one study presented at the American Association of Equine Practitioners convention in 1991 found that many horses that are supplemented have significant dietary imbalances, and no improvement in their rations from the supplements.

• Feed effects might be stronger in horses that already have problems.

• Animals that are prone to problems need a diet analysis to determine if the diet is causing or worsening problems.

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Cooling and freezing equine semen damages cell membranes, which results in loss of motility and general viability known as "cold shock." Most domestic species have high concentrations of polyunsaturated fatty acids (PUFAs) in the semen, but the amounts of these PUFAs, particularly DHA, an omega-3 fatty acid, and DPA, an omega-6 fatty acid, vary by species. These variations influence susceptibility of semen to cold shock. Unfortunately, boar and stallion semen have a low DHA:DPA ratio, which is associated with increased susceptibility to cold shock and decreased fertility.

Animals get the precursors for PUFAs in their diet. The problem is that most commercial horse feeds are high in precursors for DPA, but not DHA. This led researchers from Texas A&M University, including Steven Brinsko, DVM, PhD, Dipl. ACT, in conjunction with Minitube of America, to design an experiment to determine if adding a DHA-rich nutraceutical (supplement) to stallions' diets could increase the DHA:DPA ratio and improve the quality of fresh, cooled, or frozen-thawed semen.

Eight breeding stallions were randomly assigned to treatment or control groups. For 14 weeks the treatment group received 250 grams of a commercial boar nutraceutical designed to increase DHA concentration. Stallions were then examined and semen collected for analysis. After a 14-week "washout" period, horses in each group were switched and the trial was repeated. Semen samples from treatment and control groups were analyzed for DHA and DPA concentrations. The remaining semen from each sample was divided for fresh, cooled, and frozen semen analysis.

The results indicated that stallions fed the nutraceutical had an average sperm concentration that was 1.8 times higher than stallions fed the control diet. The concentration of DHA per billion sperm was almost three times higher in stallions fed the nutraceutical. In addition, when examining the DHA:DPA ratio, stallions fed the nutraceutical experienced a 1.5-fold increase in the ratio compared to control stallions. Finally, stallions fed the nutraceutical experienced significant improvements in sperm motility for fresh, cooled, and frozen semen samples. "Motility, especially progressive motility, is commonly used to assess viable numbers of sperm," explains Brinsko. "The increased percentage of motile sperm in the cooled semen from the stallions fed the nutraceutical means that increased DHA in the semen also increased the number of sperm capable of fertilizing an egg."

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