As long as your stallion is in good weight and is receiving a balanced diet, supplementing additional vitamins and minerals above requirement has not been conclusively proven to have any positive effect on fertility. However, there is a nutrient that stands out as having a positive
impact on the viability and motility of sperm, especially those stallions that battle with sub-par fertility when their semen is cooled or frozen.
Cooling and freezing semen causes cellular level injury in sperm, disrupting membrane lipids. Additionally motility, viability and freezing capacity can be affected. Many livestock species have their semen collected in these ways, and not all species sperm are as impacted by cooling and freezing. This is thought to be due to differences in sperm membrane structure, the major difference being the amount of docosahexaenoic acid (DHA), an omega-3 fatty acid.
Sperm also contain docosapentaenoic acid (DPA), an omega-6 fatty acid. Both DHA and DPA are examples of polyunsaturated acids (PUFA). High DPA to DHA ratios have been linked to infertility with higher DHA being linked to increased fertility, and species with naturally higher levels of DHA handle cooling and freezing with less negative effects.
Research in other species has shown effective transfer of dietary PUFA to sperm. However, due to the types of fats, grains and forages commonly fed to horses, many rations are high in omega-6 fatty acids, precursors for DPA. This may lead to a lowering of the ratio of DHA to DPA in sperm membranes causing a decrease in fertility. Research published in 2005 from Texas A & M looked at the effects of feeding a DHA enriched nutriceutical to 8 stallions to see if they could improve sperm quality. They found that the supplemental DHA resulted in a tripling of semen DHA levels, and a 50% increase in the ratio of DHA to DPA. While motility in fresh semen was unaffected, and total and progressive motility were not impacted after 24hrs of cooled storage, they did see a higher velocity and straighter projection. Furthermore, after 48hrs of cooled storage, improvements were seen in total, progressive and rapid motility.
Perhaps of most interest were the results from a subset of the stallions who had progressive motility of <40% after 24hrs of cooled storage when receiving no DHA supplementation. With supplementation, these same stallions showed a dramatic improvement in progressive motility at 24 and 48 hrs of cooling. Similar results were seen on freeze thawing of semen. For these stallions it was speculated that supplementation could improve semen quality by a factor great enough to make them candidates for cooling and freezing.
More recently researchers from Uruguay performed a similar experiment feeding 30g of DHA per day for 80 days and found an increase in total sperm number per ejaculate, motility, and a reduced percentage of dead and abnormal sperm in comparison with the control treatment. Again, the greatest increase in these parameters, and improvement in morphology was in those stallions whose semen had initially been of poor quality.
For stallions that already have high fertility and handle cooling and freezing, supplementing DHA is likely unnecessary. If you are unsure as to whether your stallion would be a good candidate for DHA supplementation you may wish to ask your veterinarian or stud manager. Good sources of DHA include fish oil and some algal preparations. When choosing a DHA supplement, with the goal of improving the DHA to DPA ratio, choose one that specifically contains DHA and overall contains predominantly omega-3 fatty acids. If there is more omega-6 than omega-3, little impact will be made on the DHA to DPA ratio. It has been speculated that even more dramatic effects may be seen if supplementation is done in conjunction with modifying the main fat content of the diet. A qualified equine nutritionist can help you to determine whether the balance of PUFA's in your stallions diet are optimal, as well as helping you find a suitable DHA supplement.
Lastly a word of caution. If trying to influence sperm quality do not expect immediate results. Spermatogenesis (the cycle of sperm formation) takes approximately 60 days. This means that the beneficial results of omega-3 supplementation may take up to 2 months to be noticeable. Therefore starting supplementation before the beginning of the season is advised.
Brinsko SP, Varner DD, Love CC, Blanchard TL, Day BC, and Wilson ME. Effect of feeding a DHA-enriched nutriceutical on the quality of fresh, cooled and frozen stallion semen. Theriogenology 63 (2005) 1519-1527.
Elhordoy DM, Cazales N, Costa G, and Estevez J. Effect of dietary supplementation with DHA on the quality of fresh, cooled and frozen stallion semen. Animal Reproduction Science 107:3 (2008) 319.