Mare Reproductive Loss Survey Confirms Some Beliefs; Belies Others

Results from the survey of 133 farms in Central Kentucky regarding the early fetal loss and late-term abortions that occurred this spring substantiated the preliminary ideas held by researchers and veterinarians this spring. The survey also put to rest some fears of causes put forth by individuals in the industry. It is perhaps this second result that makes the survey good news for horse owners.

Four factors were associated with increased chances of mare reproductive loss syndrome (MRLS):

1. Breeding date in February 2001;
2. Moderate to high concentration of caterpillars in mare areas;
3. Presence of cherry trees around pastures; and
4. Having more than 50 mares on the farm.

The survey noted that a larger farm size, and hence more mares, can be explained as a risk factor by the likely presence of more barren and maiden mares on these farms.

Having those types of mares would mean a larger percentage of mares probably being bred during February. On the other hand, an association between some factors and MRLS was not detected in this survey for the following:

Pasture composition (fescue, clover, orchard grass, bluegrass, or other grass types);

Mowing before, during, or after the frosts of April 17 and April 18, 2001;

Fall 2000 or spring 2001 fertilization with ammonium nitrate, urea, or any other fertilizer;

Treatment with lime in 1999, 2000, or 2001;

Manure spread on pastures;

Presence of surface water or source of drinking water;

Chain harrowing of fields;

Feeding grain to mares, type of grain fed to mares, source of grain (bags, bulk source, other), grain/hay contamination by domestic or wild animals;

Bedding type used;

Evidence in the mares' environment of mice, rats, raccoons, skunks, coyotes, fox, deer, or opossums;

Mares' contact with cattle, sheep, goats, poultry, pigs, deer/elk, dogs, or cats; and

Use of various dewormers.

The survey did note that one factor warrants further investigation and consideration. While nearly all farms observed water fowl at least sometimes, a higher percentage of farms with early foal loss reported observing water fowl "often."

The survey said it is strongly emphasized that this is in no way an indication to eliminate water fowl from farms. Rather, this might indicate that some environmental factor that is associated with water fowl presence is also associated with the syndrome.

The frequent observation of water fowl on farms with losses deserves further investigation to fully understand what part, if any, they might play in the syndrome.

Those involved in the equine industry should remember that the weather seems to be a key factor in the onset of foal and fetal loss. The survey noted that in 1980, 1981, and 2001--three years in which there were reports of some above-normal foal/fetal losses--there were below normal temperatures in March.

This resulted in grasses not growing as quickly or having as long of an early growing season. This was followed in all three years by above normal temperatures in April, leading to "explosive biological activity and rapid growing season for plant materials."

Also, all three years experienced frost/freeze dates the third week of April, followed by warm temperatures within a few days. Review of historical weather data dating back to the mid-1970s show that observed weather patterns of 1980, 1981, and 2001 appear to be unique.

The survey was done in cooperation with 133 equine farms in Central Kentucky and conducted by the University of Kentucky Gluck Equine Research Center and the U.S. Department of Agriculture. The two-part survey examined farm and pasture management issues as well as individual mare reproductive concerns.

The survey was conducted to analyze risk factors associated with MRLS, which resulted in the loss of nearly 500 late-term pregnancies and approximately 2,000 early fetal losses during the spring of 2001. Early fetal losses in equines occurred previously in the area (1981 foaling season and to a lesser extent in 1980) with an undefined causal agent.

However, reported losses during those years do not approach the intensity or severity of MRLS in 2001.