You've deliberated upon which sire to use, your mare has been properly dewormed, vaccinated, and pampered, and the last leg of gestation is approaching. You think you've got all the bases covered, and then someone suggests offhandedly that your mare may be at risk for fescue toxicosis-that the enemy is lurking in the hay you feed and the grass they graze. Can this be true?
Equine fescue toxicosis can be a very real threat to your mare and unborn foal and is caused by ingestion of endophyte (Acremonium coenopbialum)-infected tall fescue grass (Festuca arundinacea Schreb). Brood mares that graze endophyte-infected fescue are at an increased risk for decreased pregnancy rates and early embryonic death. Mares in late gestation (usually the last trimester) that consume infected fescue may also suffer from prolonged gestation, late term abortion, retained or thickened placentas, decreased pregnancy hormone levels and agalactia (lack of milk supply).
The significance of feeding or grazing this popular forage by brood mares cannot be understated, but how common is it? Most tall fescue pastures originate from a Kentucky 31 variety cultivated and named for the state and year in which it was discovered. Admired for its hardiness, drought and insect resistance and ease of maintenance, tall fescue now occupies over thirty-five million acres in the United States.
Although Kentucky 31 fescue grass has been popular since its introduction, it wasn't until the late 1970's that scientists discovered that it was infected with an endophyte-fungus. This fungus lives in a symbiotic relationship within the intracellular spaces of the plant and provides its host with the increased hardiness and insect resistance that made it so popular.
Most fescue pastures are infected with the endophyte to varying degrees, and any tall fescue should be considered infected unless an endophyte-free variety was purposely planted. Non-endophyte fescue has been cultivated, but these grasses have been labeled as difficult to establish and maintain. Endophyte-free pastures are susceptible to infection from nearby endophyte-infected fescue.
Mares that consume endophyte-infected fescue during the last 60-90 days of gestation are commonly overdue by 20-27 days. The foal continues to grow during this prolonged gestation, and the foal's increased size makes the tight transition through the birth canal dangerous for both involved. These larger than normal foals are often malpositioned as well, further complicating birth.
Foals that survive are often weak and exhibit signs of abnormal maturation (overgrown hooves, long fine hair coats, poor muscle mass). Foals may be hypothyroid, show signs of incoordination and have poor suckling reflex. Foals with these symptoms are commonly referred to as "dummy foals".
In addition, mares are often agalactic (failure to secrete milk) and have little or no colostrum to offer their newborn. Because of this, the foals run a greater risk of failure of passive transfer of colostral antibodies. As foals are born immunologically naive, they are dependent upon the mare's colostrum to provide them with immunoglobulins for a fully functional immune system. Those with inadequate protection have an increased risk of infectious disease and death. Lack of adequate nursing by your newborn foal should be considered a medical emergency, and your vet should be notified at once.
How do you protect your brood mares when fescue is so prevalent? There are several ways to combat fescue toxicosis:
1. Send several samples to the local extension agent to have the forage tested. Do this prior to the beginning of the breeding season.
2. Deny access to all infected pasture.
3. Remove all infected hay 90 days prior to foaling. This simple step can prevent the effects of the endophyte. Thirty days is a minimum, however, this may not be protective enough for some mares.
4. Substitute another hay or cubed (processed) hay.
5. Mares on fescue may not produce adequate colostrum, therefore foals must be examined by your veterinarian for failure of passive transfer (FPT) with the first 24 hours of birth to insure they are starting life with a strong immune system.
6. If you are unable to completely remove access to endophyte-infected fescue, inform your veterinarian. Your veterinarian can obtain the necessary medications to manage and treat your mare and foal. More about this later.
7. Eliminate the infected fescue and replant with another grass to prevent future toxicosis. This is harder and more costly than can be justified by most horse men. It may be easier to move the mare and prevent access.
Next week I will discuss treatment options for both the mare and foal. You'll get a glimpse into how complicated some of these cases can become and why prevention is the best option.
