When tall fescue first arrived in the United States during the 1940s, it was considered a wonder grass. It was easy to establish, it yielded a good amount of forage, it was tolerant of a wide range of management regimens, and it could handle a variety of climatic conditions as well as sustain heavy foot traffic. Farmers and ranchers embraced the new grass with great enthusiasm and, in a relatively short span of time, some 35 million acres of land in the United States was planted with tall fescue.

Then, the honeymoon ended. Horse breeders noticed that they were having foaling problems with some mares which were grazing tall fescue grass or being fed tall fescue hay. Cattle producers had problems, too. The cattle grazing on the grass during the late summer months were unthrifty, and there was a drop in milk production from dairy cows.

Agriculture turned to research to find out why these things were happening. In the late 1970s, it was discovered that much of the tall fescue contained a fungal endophyte that can have highly deleterious effects on animals consuming it. The scientific name for this fungal endophyte is Acremonium coenophialum.

Its effect on pregnant mares can be particularly severe. Mares grazing on infected tall fescue pasture or fed infected tall fescue hay might have prolonged gestation or a tendency to abort, can develop an abnormally thick and/or tough placenta, can retain the placenta after giving birth, often have difficulty in the birthing process (dystocia), and might have no milk (agalactia) for the foal once it is born--if, indeed, the foal is born alive and remains alive after birth.

There were some positives early on in the battle against fescue toxicosis. First, it was discovered that if mares were removed from infected tall fescue several months before foaling, the endophyte's harmful effects dissipated for the most part.

Next, scientists developed a fescue grass that was free of the harmful endophyte. Unfortunately, it was also found that the endophyte-free fescue was not quite as hardy as its infected counterpart. It will be years, if ever, before all of the acres of infected tall fescue can be replaced with that which is noninfected. There still is the problem that some horse owners are unaware that the grass their horses are consuming is infected tall fescue. There also is the incorrect belief that the problem exists only in the South and Southeast when, in reality, infected tall fescue can be found all across the country.

The most dramatic weapon in this ongoing battle is the drug domperidone. It is providing a margin of victory on the scientific front. Domperidone has proven to be successful in reversing the harmful effects that occur when pregnant mares ingest endophyte-infested fescue hay or grass.

To understand how domperidone works, we must first comprehend what is going on within the mare's system and what happens when she ingests infected tall fescue.

Inside The Body

The body of a pregnant mare produces a biochemical known as dopamine. It occurs naturally throughout the body and affects the functions of glands, organs, muscles, and nerves.

Within the endophyte fungus are substances that, in a sense, imprison the dopamine receptors in the mare's system so that they can't function normally. The result is decreased production of both prolactin and progesterone. Prolactin is the hormone that stimulates milk production. Progesterone is the key hormone involved in maintaining a normal pregnancy.

When the substances in the fungus effectively reduce the normal supplies of these two hormones, the mare fails to produce milk and things go awry within her reproductive system. However, domperidone, when administered, doesn't allow the substances within the fungus to imprison dopamine, and the mare's body is able to produce the appropriate amounts of prolactin and progesterone.

Use of this potent weapon and the effects it has had on reproducing mares was a focal point of discussion at a session on reproduction at this past year's AAEP meeting in Albuquerque, N.M.

One of the key speakers was Dee Cross, PhD, of Clemson University. Cross was the leading researcher in the development of domperidone for use in pregnant mares.

For many years, he told his listeners, veterinarians and mare owners reported reproductive problems in mares grazing tall fescue grass. Those problems included increased gestation lengths, agalactia, foal and mare mortality, tough and thickened placentas, and weak and dysmature foals.

Once it was found that domperidone negated the harmful effects of the endophyte, he said, the next questions to be answered concerned just how effective the drug would be in widespread usage outside the lab, as well as the most advantageous time to administer it.

To find answers to those questions, Cross and his colleagues at Clemson studied the use of domperidone in field trials from 1993 through 1997.

Each veterinarian or horse owner participating in the study was asked to complete a Clinical Observation Form. A total of 1,423 forms were returned from 33 states.

Following is a summary of information contained on the returned forms:

Gestation Length
Mares which were dosed with domperidone 16 to 30 days before the expected foaling date foaled an average of 2.97 days before the expected foaling date. Mares which were dosed from 10 to 15 days before the expected foaling date foaled an average of 3.26 days after the expected foaling date. Mares which were dosed from one to nine days before the expected foaling date foaled an average of 6.59 days after the expected foaling date. (In another study, pregnant mares grazing endophyte-infected tall fescue had gestation lengths 27 days longer than mares grazing endophyte-free fescue. None of the mares in that study were treated with domperidone.)

Udder Development Before Treatment
Udder development prior to domperidone therapy was observed and recorded for 1,211 mares. Approximately 88% of those mares had little to no udder development prior to the initiation of domperidone therapy. (In another study, 87.5% of non-domperidone treated mares grazing endophyte-infected fescue were agalactic compared to 12.5% of mares grazing uninfected fescue.)

Udder Development During Treatment
Udder development during domperidone therapy was observed and recorded for 1,238 mares. Approximately 89% of the mares had normally or rapidly developing udders after the initiation of domperidone therapy, with an additional 13% having a "good" udder at foaling (93% total). "These data," Cross told the group, "suggest a marked improvement in udder development after initiation of domperidone therapy when compared to udder development before domperidone therapy."

Mares' Lactation After Foaling
Mare lactation after foaling was observed and recorded for 1,340 mares. "It should be noted," Cross said, "that not all of these mares received treatment before foaling. Two hundred sixty-one did not begin treatment until after the actual foaling date. A majority (76.4%) of mares were reported to have normal lactation after foaling, while 23.28% of mares did not have normal lactation after foaling." Most of the mares which did not have normal lactation after foaling had not received previous domperidone therapy. More than 83% of veterinarians and/or mare owners reporting abnormal lactation post-foaling found domperidone therapy to be effective in the treatment and prevention of fescue toxicosis.

Foal Viability And Mortality
Data on foal births was submitted for 1,369 mares. More than 98% of mares (1,343) treated with domperidone had live foal births. (In another study, 86% of foals delivered on endophyte-infected tall fescue pasture died. Their dams were not treated with domperidone.) "When grouped by initiation of treatment relative to expected foaling date," Cross said, "a higher rate of stillborn foals was observed in mares which were treated after their expected foaling date than in mares treated before their expected foaling date."

Placental Retention
Data on placental retention was submitted for 1,322 mares. Approximately 93% of the mares (1,236) treated with domperidone did not have retained placentas. As was the case with mortality, domperidone therapy was most effective when administered prior to the expected foaling date.

Prevention Effectiveness
Domperidone therapy was felt to be effective in the prevention of fescue toxicosis in 95.2% of mares treated.

"This study," Cross summarized, "provides evidence that domperidone is safe and efficacious for the prevention and treatment of fescue toxicosis in (pregnant) mares under field conditions."

Studies Of Reserpine

Following Cross to the podium during the AAEP program was Tim J. Evans, DVM, MS, of the University of Missouri-Columbia. The importance of finding an effective treatment for fescue toxicosis, he said, is underlined by the fact that more than 35 million acres of tall fescue pasture (with more than 75% of it infected) exists in the United States and is grazed by more than 688,000 horses.

Evans said that he and his colleagues conducted research on reserpine, which has some of the same properties as domperidone. The studies were aimed at determining whether reserpine also could be effective against fescue toxicosis.

The research demonstrated, he said, that reserpine was not effective and that there were side effects of drowsiness and diarrhea. The one positive benefit from reserpine administration, Evans said, was resolution of agalactia.

NAHMS Numbers

Brian McCluskey, MS, DVM, of Colorado State University, reported on a survey conducted by the National Animal Health Monitoring System's (NAHMS) Equine '98 Study to determine where fescue exists in the United States. In some areas, McCluskey said, tall fescue is the dominant grass species. In a number of areas, he told the group, the rugged fescue grass has replaced established forages.

In the past, it was felt that the prime problems with infected tall fescue were in the South and Southeast. The NAHMS study indicated otherwise. It found that infected tall fescue exists all over the country.

States involved in the study were Alabama, California, Colorado, Florida, Georgia, Illinois, Indiana, Kansas, Kentucky, Louisiana, Maryland, Michigan, Minnesota, Missouri, Montana, New Jersey, New Mexico, New York, Ohio, Oklahoma, Oregon, Pennsylvania, Tennessee, Texas, Virginia, Washington, Wisconsin, and Wyoming.

Endophyte was identified on more than half of the operations participating in all four regions, McCluskey said, indicating that endophyte-infected pastures are located in horse operations in all geographic areas of the United States.

"Identification of endophyte-infected pasture and its associated toxin in all geographic regions surveyed emphasizes the widespread nature of this potential problem," he said. "Practitioners should be familiar with signs associated with fescue toxicity and with appropriate prevention and treatment strategies to use if endophyte-infected fescue is identified in their clients' pastures."

Persons who desire to have plants from their pastures tested should contact the National Veterinary Services Laboratory at 515/663-7526.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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