Nutrition's role in stallion fertility

Obesity may be Enemy No. 1 when it comes to a stallion's libido and fertility during the breeding season — a key reason for equine practitioners to counsel their clients on proper equine nutrition, experts say.

An important indicator of a stallion's nutritional status is body condition score (BCS), a measure of body weight proportional to stature that should be taken prior to and throughout the breeding season. A stallion that is either too thin or too fat may not have the stamina to breed to a full book of mares.

A BCS of 5 to 7 generally is considered a good target, though each horse should be fed based on its specific temperament, activity level and "ease of keeping." One that tends to lose body weight during the breeding season, for example, might do better a bit heavier, perhaps around BCS 7, at the start of the season. It is thought that obesity may lead to poor performance, cardiovascular problems and/or decreased libido in stallions.

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"To me the main thing in feeding the stallion is maintaining appropriate condition," says Steve Jackson, PhD., a consultant to Bluegrass Equine Nutrition in Lexington, Ky. "More times than not, stallions are fed too much and many of the problems that we see are related to their being too fat."

Breeding stallions typically require about 25 percent more nutrition than a mature stallion's intake during the off-season, about the same as a horse performing light work, according to Pete Gibbs, PhD., professor and horse specialist for Texas AgriLife Extension Service at Texas A&M University's Department of Animal Science.

"Stallions need to be in acceptable body condition long before the breeding season starts," Gibbs says. "It's not good to try to manipulate body condition during a breeding season."

"Probably one of the big things you'd be concerned about is managing them to maintain body condition, so you don't have excessive body condition, and so they don't lose a whole lot during the breeding, because they certainly can," concurs Paul Siciliano, PhD, associate professor of equine nutrition at North Carolina State University's Animal Science Department.

"Being overfed absolutely is a problem with many stallions I have seen," says Carey Williams, PhD, equine Extension specialist at the Rutgers University Equine Science Center. "There are a lot of stallions that go into the breeding season obese. It decreases their libido and decreases their fertility."

Stallions that enter the breeding season obese should get regular exercise, increased hay and less grain, Williams says.

"Limit their access to high sugar/starch sweet feed."

One caution Gibbs mentions is that some stallions, if working hard, and depending on their "book," may lose weight if not fed sufficiently, especially Thoroughbreds booked for up to 120 mares. What's ideal? "The stallion should be in a moderate to fleshy condition; there may be a slight crease down the topline; ribs may not be seen, but can be felt; some fat slightly evident between the ribs, and the fat over the ribs feels spongy; small amounts of fat along the sides of the withers and behind the shoulders; and fat around the tailhead feels somewhat soft," Gibbs says.

"Nutritional needs may be modified according to the behavior and general activity level of the individual stallion," Gibbs suggests. The overly temperamental stallion that paces and works excessively should be fed accordingly, he adds, pointing out that pasture and/or hay are not sufficient alone, but are an important part of a total nutrition program. He suggests that 1 percent to 2 percent of body weight (BW) of good quality hay and/or quality pasture may furnish all the animal's forage needs, and that a few hours of grazing provides the added benefits of exercise and some relief from the stress of breeding.

The stallion's body condition should be monitored, perhaps weekly during the breeding season, and if weight strays too far from ideal, intake adjustments should be made to keep the animal fit, experts say.

Energy requirements

Stallions generally have higher dietary energy-maintenance requirements than mares or geldings, and the amount required during the breeding season likely will be affected by breeding frequency, according to the National Research Council's (NRC) 2006 report. "Horses that make a few mounts each week in live cover or artificial-insemination programs would be expected to have lower energy requirements than those that make more than a dozen mounts each week in intensively managed live-cover programs," the NRC says.

Limited data on digestible energy (DE) intakes of breeding stallions are available. The DE requirement for breeding stallions in heavy use is estimated to be 20 percent higher than maintenance. Because they are doing work, they need to be fed an increased energy concentration.

In addition to forage needs, a breeding stallion should be fed at least 0.75 percent of BW of good quality grain daily to boost energy. The increased grain should be added gradually, feeding perhaps a half-portion in two meals or more, but not more than 5 pounds of grain per feeding. Body weight should be monitored and the amount of grain adjusted either because of weight changes or temperament. Adding top-dressed vegetable oil can add energy/calories in lieu of some of the grain.

Other nutrient needs

Because dietary energy density should be increased, protein, vitamins and minerals also need to be adjusted to meet a breeding stallion's needs.

During the breeding season, Gibbs suggests most stallions "receive total daily feed in a 50:50 up to 70:30 ratio of roughage to concentrate," depending on individual needs, along with a concentrate of 8 percent or less crude fiber, when a stallion is getting ample roughage from hay or grazing.

Gibbs recommends "40 grams of crude protein per Mcal (megacalorie) of digestible energy, and if horses receive roughage that contains 10 percent or more crude protein, a 10 percent crude-protein concentrate will usually be sufficient." If feeding a low-protein grass hay, a 14 percent crude-protein concentrate will suffice. "During the breeding season, it's very important that stallions receive a balanced nutrient supply from a combination of a well-formulated feed and the right kind of hay," Gibbs says. "That in a nutshell is important."

A salt block provides needed sodium chloride, but avoid "free-choice" salt in the stall or in small dry-lot situations because some stallions will consume excess salt through boredom. Most balanced horse feeds already contain around 0.5 percent salt, which for some horses will be adequate. If additional salt is needed in confined stallions, a consistent and calculated approach is to add some pure salt directly to the feed. Some feed a trace mineralized salt block, which includes some of the necessary copper, zinc, iron, manganese, cobalt and iodine.

Feeding

A good quality grass hay or alfalfa-grass hay mix will supply forage needs for most stallions. Grazing provides not only energy and protein, but some nutrients that are not adequate in hay, such as beta carotene (precursor to vitamin A) and vitamin E, each found in fresh pasture.

"With the breeding stallion, what we're trying to do with turnout is let them graze enough to keep a good attitude and help meet their roughage requirement, but grazing pasture will not satisfy that stallion's energy requirement — that's got to come from properly formulated feed and some top-quality hay," Williams says.

Some stallions are more aggressive during the breeding season and should have their turnout time expanded, while others need less turnout time depending on their exercise level.

"Additionally, when possible the majority of the stallion's nutrient intake should come from forage rather than grain. If horses have access to quality hay and/or pasture, they can meet most of their requirements," Jackson says. "I have farms in Kentucky that are feeding stallions hay and 2 pounds per day of a supplement pellet and the horses look magnificent. This is particularly important if horses are prone to colic. Feed good hay. One of the problems I see in the field with stallions is cecal impaction, which occurs mainly on poor-quality, highly lignified grass hay." Gibbs agrees, citing his collaborative research at Texas A&M, where hay quality was identified as the single biggest dietary factor related to colic.

Special nutrient needs?

Some nutritionists have suggested that certain nutrients might be of special value to stallions and fertility, but there is no data within scientific literature to show such benefit for most nutrients.

"The latest information regarding stallion fertility and nutrition is work done at Texas A&M University with respect to feeding omega-3 fatty acids to stallions," says Peter C. Sheerin, DVM, Dipl. ACT, at Lexington's Rood and Riddle Equine Hospital.

Semen lipids "play a major role in motion characteristics, sensitivity to cold shock (loss of viability of cooled-stored and frozen semen) and fertilizing capacity of sperm," says Steven P. Brinsko, DVM, MS, PhD, Dipl. ACT, associate professor and chief of theriogenology at Texas A&M's Department of Large Animal Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences. "In the stallion, as in other species, feeding precursors of omega-3's enhance tissue levels of DHA (docosahexaenoic acid, 22:6 n-3)," Brinsko suggests.

Dietary lipids, in particular dietary polyunsaturated fatty acids (PUFAs), which must be acquired from precursors in the diet, may play a beneficial role in stallion fertility. Most horse diets contain corn and soybean oil (i.e., high levels of linoleic acid), therefore preferentially the precursor of DPA (docosapentaenoic acid, 22:5 n-6) and other omega-6 fatty acids vs. DHA, and other omega-3 fatty acids.

A typical horse diet, high in omega-6 fatty acids, therefore favors the formation of these fatty acids vs. the omega-3 fatty acids. High omega-6 to omega-3 fatty-acid ratios in semen have been associated with reduced sperm quality and fertility. The concern is that a typical horse ration could have a negative impact on quality of a stallion's semen and its tolerance to cooling and freezing.

Brinsko and colleagues explored the benefits of omega-3 fatty acids to enhance stallion fertility, "to see if we could improve fertility or preservation of semen in stallions by adding a nutraceutical containing omega-3 fatty acids," Brinsko explains. They showed that a "normal" horse diet supplemented with a nutraceutical containing 30 percent omega-3 fatty acids produced a three-fold increase in semen DHA.

Though total sperm numbers and percentage of spermatozoa with normal morphology were unaffected by the treatment, and though sperm motility characteristics of fresh semen were not affected by the increased omega-3 content, in 48-hour cold-stored semen "increases in the percentages of sperm exhibiting total motility, progressive motility and rapid motility were observed when stallions were being fed the nutraceutical. For stallions whose progressive motility was <40 percent after 24 hours of cooled storage, improvements in mean progressive motility of sperm were observed after 24 hours when they were fed the omega-3-containing supplement," Brinsko's study shows.

Though supplementing the stallions' diet with the precursors of DHA (i.e., flaxseed or cod-liver oil) can increase the DHA content of stallion semen, it may not improve semen quality. "In cases of horses that do appear to have some problems in their semen quality, certainly increasing the level of omega-3 fatty acids (in their diet) is something I sure would investigate and try on these horses," Brinsko says.

"If there is no negative effect of omega 3 fatty acids on semen quality, I particularly would consider an omega 3 fat source," Gibbs says. "For stallions getting a lot of exercise, for some that have a heavy book of mares or an older stallion with some joint mobility issues, omega-3 fatty acids could be an important consideration. That is because recent studies point to potential reductions in joint inflammation for horses receiving omega 3 fat supplementation."

The results would suggest that supplementing the diet of highly fertile stallions or those that produce sperm that survive cooling does not appear warranted. But, stallions of marginal fertility and those whose spermatozoa have poor tolerance to cooling and freezing might benefit from being fed supplemental omega-3 fatty acids.

"For horses that are having fertility problems, consuming a diet that is supplemented with pre-formed DHA, and changing the main source of their fat to something that is more favorable to the omega-3 fatty acid, such as flaxseed oil vs. corn oil, would likely be helpful," Brinkso says.

Still, aside from whatever benefits micronutrients may provide, the most important factor guiding nutrition and feeding of stallions is proper body condition, and that's what equine practitioners should focus on when counseling clients, experts say.

"Pretty simple, really," says Jackson. "Look at the horse, feed it like an herbi-vore and avoid snake oil."

Ed Kane, PhD, is a Seattle author,researcher and consultant in animal nutrition, physiology and veterinary medicine, with a background in horses, pets and livestock.