Pasture Management and MRLS

In the spring of 2001, hundreds of mares in Central Kentucky lost their pregnancies in peculiar abortions attributed to mare reproductive loss syndrome (MRLS). Since then, horse farm managers have gone back to square one in reviewing their pasture management practices.

One certain result is they have developed a greater awareness of what’s lurking in the grass. These farm managers have learned that tall fescue is not the only grass commonly used in pastures that might contain an endophyte, an internal fungus that lives inside a plant and is species specific. Some perennial rye, the cool season rye many horse farms in the cooler regions of the country include in their pasture seed mix, also can contain an endophyte.

Regardless of what the horse industry is learning about grass, there has been one significant change in pasture management around Kentucky, and it has been directed not to the grass or soil, but to the trees.

Today, Eastern tent caterpillar (ETC) control is the top priority in pasture management. It is the one practice beyond all others that the vast majority of horse farms around Central Kentucky are taking more seriously than ever before.

Exposure to caterpillars is the apparent link to the springtime abortions of the past two seasons, according to findings by scientists on the University of Kentucky’s toxicology committee, the task force that was organized to study the problem.

The most popular host tree for ETC is Prunus serotina, a variety of cherry tree. There are several types of cherry trees, but this is the one commonly found in Kentucky; the one that is alternately called the black cherry tree or the wild cherry tree. It is the tree literally taking the fall for MRLS.

B.G. Hubbs, an arborist and owner of Community Tree Care, a tree removal and pruning company in Lexington, put the life expectancy for wild cherry trees at 80-100 years. Like other tree removal companies around Lexington, Hubbs reported an increase in business in the past couple of years as farm managers try to remove wild cherry trees from around pregnant mare pastures and paddocks.

Wild cherry tree seedlings grow abundantly along pasture and paddock fencerows, springing up from the droppings of birds that eat the cherries, then perch on the fence rails, and dropping the seeds when they defecate. “In fencerows, horse farms are going to have all different age classes,” Hubbs said. “We’re mostly taking out the bigger, older trees. It’s labor intensive. We have bucket trucks, or chippers.”

Robert Courtney Jr., manager of his family’s Crestfield Farm, estimated he has spent about $25,000 to have tree service companies take the bigger cherry trees off his property, and that was after his own farm crew had removed the younger trees. He said from the spring of 2001 to the spring of 2002, his vigilance in keeping pregnant mares away from wild cherry trees, and ETC, has made all the difference in mares on his farm maintaining their pregnancies.

Even with the early reports of fewer ETC egg masses around Central Kentucky, John Williams, owner and manager of Elmwood Farm, is glad he has removed the majority of cherry trees from his property. “I have a few more to cut down and I hate to do it, but I’m sure going to do it, because I can’t afford to lose those pregnancies,” he said.

Some farms have elected to spray cherry trees for ETC instead of removing them. At Central Equipment, a company in Lexington that supplies pasture maintenance  equipment to horse farms, owner Paul Huber said he is continuing to see interest in a tree-spraying machine that was designed with ETC control in mind. Huber added there has also been some interest in a Nicholson paddock cleaner, a machine used on horse farms in Europe for parasite control. Farm managers are interested in adapting the machine for cleaning ETC out of paddocks, Huber said, although, “At $14,000, it’s hard to justify that expense.”

In the face of all the tree removal and spraying, the question of believing, or not believing, in ETC being the cause of MRLS has become an emotional issue. Even where one would expect to find the greatest support for tree removal, there is skepticism.

“It’s funny that it just all of a sudden became a big problem,” Hubbs said, indicating that even while his business thrives, he wonders if removing cherry trees will solve the breeding industry’s troubles. “We’ve always had cherry trees and tent caterpillars, forever. Some farm managers aren’t convinced that it is a problem.”

Adding to the confusion over what could be affecting the reproductive health of mares were the reported increases in middle trimester abortions by mares in Kentucky last fall.

Those abortions are being attributed to fall fetal loss syndrome (FFLS). They had no obvious symptomatic similarities to the spring abortions.

Apart from ETC control, other things in the pasture have come under closer scrutiny for how they might affect the pregnant mare. The other areas of concern are pasture grass health, pasture grass mineral content, and soil mineral content.

Endophyte and Mycotoxin Alert
When one thinks of pasture, one thinks of grass and soil. When it comes to pasture management, one thinks of seeding, fertilizing, and liming. These three applications are necessary for nourishing the soil and freshening the grass supply.

Any spring pasture renovation horse farms have opted to do this year would be winding up by now. The window for reseeding or fertilizing this time of year is late February through mid-April. Not all farms reseed or fertilize in the spring, and some do so only on an as-needed basis. For the past 15 years, most of the reseeding, fertilizing, and liming has been done from August through mid-November.

No major changes in spring or fall applications are likely in the foreseeable future, a couple of Central Kentucky feed companies reported. But in the past couple of years, farm managers have asked a lot of questions about grass seed.

The most popular grass seed mix for pastures around Central Kentucky is Kentucky bluegrass, orchard grass, and perennial rye grass, according to Bob Cleveland Jr., vice president of Woodford Feed in Versailles, Ky. Mark Southworth, manager of Southern States Lexington Cooperative, agreed. Both also noted farm managers were surprised to learn there are endophytes in some perennial rye grasses.

Tall fescue had fallen out of favor as a horse pasture grass long before MRLS, because the “Kentucky 31” variety of tall fescue contained an endophyte. Ergot alkaloid, the toxin produced by the endophyte of Kentucky 31 tall fescue, is highly toxic to mares in the final trimester of pregnancy.

FFLS, which hit mares in the second trimester, once again drew attention to pastures because it has no apparent connection either to ETC presence or what has historically been attributed to endophyte toxicity.

Most farms have not seeded their grazing land with tall fescue since the mid-1980s, but the Kentucky 31 variety that contains the endophyte is still growing in many acres of horse grazing land.

“Horse farm managers were told tall fescue is evil, and to seed with anything but tall fescue,” said Tim Phillips, PhD, a grass forage specialist and an associate professor in the UK agronomy department. “That’s why they thought rye is OK. I don’t blame them for not knowing; they’ve only been told half the story.”

Rye grass is sown mainly as a nurse crop, Cleveland said. It’s a short-lived grass. The horse industry is now asking seed producers to ensure low-endophyte content in rye grass seed that will be used for horse pastures—10% or less.

“We’ve made the rye grass people aware, if it’s going to go to the horse market, they need to specify low- or zero-endophyte rye grass for this market,” acknowledged Jimmy Henning, PhD, a UK agronomist serving on the toxicology committee. He added that no link was found between rye grass endophyte and MRLS. “That doesn’t really have an MRLS angle, but that’s one of the things we’ve learned along the way.”

Farm managers have become more aware of leaf diseases of orchard grass, powdery mildew of bluegrass, and leaf rust, another grass disease that can affect either of those grasses. There are a few fungal diseases that have the potential to produce mycotoxins, Phillips said. Mycotoxins are an external fungus that can affect a lot of different species of grasses or legumes.

At Crestfield, Courtney learned from his father to be wary of how frost might affect pasture grass, a bit of conventional wisdom some farm managers might have first learned about as a result of the unusual drought and frost weather pattern of April 2001.

“One thing my father always told me: if you’ve got a heavy frost in the morning, don’t turn out mares until that heavy frost has burned off,” Courtney said. “We still don’t do it to this day.”

“As far as pasture management, I don’t know that we’re going to do anything different,” Williams said. “We’ll fertilize in the fall, as we have for years. We’ll continue to clip pastures, as we have for years. We’re probably not going to turn mares out on frozen pasture again this year, without any concrete scientific reason, but we’re going to do it out of a fear factor. We’re going to keep those mares up, restrict their time out on that lush grass, without any real scientific evidence to say that was the cause.”

Not Easy Being Green
A distant third of what pasture management practices need to be reviewed concerns soil nutrition, i.e., fertilization and liming. One theory that was forwarded as to what might have caused MRLS relates to overfertilizing soil with nitrogen. Another theory relates to an out-of-whack ratio of potassium to calcium.

Soil testing is a practice most Kentucky horse farms do on a regular basis. The results provide information regarding which acreage on the property needs fertilization, and what the proper balance of nutrients should be. 

In soil testing, the soil’s calcium, magnesium, phosphorous, and potassium content are tested, in addition to the soil’s pH. The pH reading is a measure of the soil’s acidity, measured in terms of pH. The neutral number is 7.

“When pHs of the soil around Central Kentucky are between 6 and 7, we do put out limestone from time to time,” Henning said.

Regarding the potassium to calcium imbalance theory, he said, “We measured that.” Using records of horse farm soils from a private grass and soil analysis company, “We went back and looked at the potassium and calcium ratios in past years when MRLS was not an issue. A few years ago, we had potassium-calcium ratios that were much, much higher than we had in 2001, and no MRLS.”

A lot of farms measure mineral content of forages. In fact, pasture grass is recognized as having naturally low levels of calcium—which goes into the separate topic of equine nutrition.

“A plant is not necessarily going to take up calcium,” Henning said. “Just because you put the mineral on the dirt, that doesn’t mean the roots are going to take it up.”

Regarding nitrogen, he said, “Frankly, farm managers don’t put that much nitrogen on to begin with.” In spring 2001, one farm that had fetal losses had not added chemical nitrogen.

“Are there management changes that farmers need to be making based on what we have learned? No,” Henning said. “They still need to soil test, and they still need to be concerned about what mineral composition the soil is so we’re supplying the right nutrition for the plants.

“Farms should get on a plan where they test the soil, and they may choose to test the forage as well, because the forage is the ultimate measure of what the horse is eating. Farms can use their university extension agent or a private consultant and get on a program of moderate fertilization use that will wind up producing grass that works into the overall program of nutrition for the horse. Some farms are going to need to use a little more than others, some none at all, and some have different philosophies.”

Even though grass health and mineral content, and soil mineral content, are not widely considered to be factors associated with MRLS, the horse industry’s increased awareness of pasture grass and soil is ultimately good for the horse.

“It’s a good thing to be able to study the environment of the horse, and find out about it,” Henning said. “There were some things you would have assumed we would have known, that we hadn’t had a chance to study in this detail, that we’ve had a chance to do now. It has raised the passions of a lot of people, but ultimately, it’s a scientific endeavor.”


Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.