Sentinel Farm Summary; 2002 Pasture Monitoring Program

The following information was produced by the University of Kentucky College of Agriculture from Wayne Long ( and Jimmy Henning ( of the Department of Agronomy (859/257-3144). This article also can be seen on the university's web site at

Data collected by the monitoring program clearly eliminated some of the potential causative agents for mare reproductive loss syndrome (MRLS), such as cyanide content of white clover, mineral balances in forages, and the presence of certain poisonous plants. The monitoring program also strongly confirmed the link between MRLS and the presence of Eastern tent caterpillars (ETC), while finding a possible involvement of endophyte infected tall fescue with late fetal losses on certain farms.

The 2002 MRLS monitoring program began Feb. 21 and ended June 28. During that time, data and samples from 12 horse farms (Thoroughbred and Standardbred) and one hay production farm (alfalfa and timothy for horses) were collected and analyzed. As a result, we were able to be in close contact with farms, farm managers, and practicing veterinarians and assist in consultation to moderate or prevent MRLS.

Correlations of Pasture Data to MRLS
Statistical analysis of monitoring data found significant correlations between cases of MRLS and the presence of black cherry trees in proximity to pasture. There was also an indication that tall fescue alkaloids could account for some late term abortions on farms without clear exposure to Eastern tent caterpillars or black cherry trees.
The 2002 monitoring data found that other pasture and environmental parameters were not correlated to MRLS. These included presence of poisonous plants, cyanide content of white clover, fungal mycotoxins, soil microbiology, weather, and mineral content of forages.

Summary of Sentinel Farm Activities
• An average of nine visits per farm, 13 farm visits every two weeks.
• Six of the monitored farms collected blood (378) and urine (205) samples from horses for future analysis.
• An average of 175 samples were taken per farm, which does not include blood and urine samples from horses.
• Over 3,000 total samples were collected, including blood and urine and emergency farm samples.
• A total of 83 fields were sampled from the 13 sentinel farms, averaging 6.4 fields/farm.
• Six of the 12 sentinel horse farms experienced losses--29 early fetal losses and nine late fetal losses--between April 25 and June 13 of 2002.
• UK personnel spent over 1,700 person-hours of actual time on monitoring activies.

Summary of Emergency Farm Activities
Other farms were visited when veterinarians or farm managers found MRLS-type symptoms and referred them to UK. These included:
• Nine farm visits, of which six had MRLS cases totaling 27 early fetal losses and three late fetal losses. Other farms included one control farm with black cherry trees (BCT) and no losses, and two with concerns because of losses in 2001.
• In this group, 22 fields were sampled, resulting in 204 additional samples.

Summary of Pasture Parameters
The following contain the average, minimum, and maximum for most of the parameters measured by the 2002 UK Pasture Monitoring Program. A few notes follow to aid in interpretation of each pasture characteristic or graph. These pasture parameters were not related to cases of MRLS and caused no toxicity in grazing horses.

Nitrate from Composite Pasture (Table 1 and Figure 1)

The results presented are the concentration of nitrate - nitrogen (NO3-N) in the overall composite pasture sample, expressed in parts per million (ppm). Nitrate can cause toxicity (including asphyxiation and abortion) by being converted to nitrite by the microorganisms in the gastrointestinal tract of livestock. In the horse, this would occur in the cecum. Because most nitrate is absorbed before the hind gut, the horse is much less sensitive to nitrate content of pasture than ruminants. The levels reported for cattle (ruminants) considered to be generally safe are anything less than 1200 ppm. Horses would have a much greater tolerance than cattle. Nitrate levels in all pastures were low.

Cyanide Potential of White Clover (Table 2 and Figure 2)

White clover samples from each pasture were assayed for their potential to release cyanide from cyano-sugars that naturally occur in many strains of clover. To release cyanide, the cellular structure of the leaf must be disrupted and cell contents mixed. As with nitrates, horses are much less sensitive to cyanide poisoning than ruminants. The level of cyano-sugars and therefore the cyanide potential is genetically determined and can be raised or lowered by environmental stress and season of the year. Cyanide potential in white clover will vary according to the variety, with some agronomically important lines having HCN content as high as 1000 ppm. In general the varieties used in Kentucky are low in cyanide. For example, ëRegal’ ladino has been found to contain 50 to 200 ppm HCN.

Tall Fescue Total Alkaloids (Table 3 and Figure 3)

Tall fescue plants were selected randomly from across each pasture to determine the potential ergot alkaloid intake if it accounted for 100% of the diet. The values reported are ppm of ergovaline plus ergovalinine and are termed “total ergot alkaloids.” Ergovaline and ergovalinine are produced in tall fescue, which is infected with the endophyte. While not all tall fescue in Kentucky is infected with this endophyte, the samples from all monitoring farms and fields had measurable ergot alkaloids. Therefore, it is reasonable to assume that at least some of the tall fescue plants in these fields were infected. These alkaloids are responsible for tall fescue toxicosis in pregnant mares, whose symptoms include prolonged gestation, agalactia (lack of milk production), thickened placenta, and dystocia. Total alkaloid levels of 0.600 ppm or greater are considered toxic to pregnant mares in late gestation. The level of ergot alkaloid in tall fescue is very low to non-detectable in early spring and increases as the plant increases its growth rate in April. Some tall fescue plants had reached or exceeded the toxicity threshold (0.600 ppm) in early May, and these plants remained high in ergot alkaloids even into late June. However, no typical fescue toxicity symptoms were observed. Mares were either not in late gestation at this time or tall fescue formed a minor part of the diet.

Adjusted Total Ergot Alkaloids (Table 4 and Figure 4)

A composite sample of the general pasture was collected at each date and analyzed for total ergot alkaloids (ergovaline plus ergovalinine). This assay was an attempt to determine what the alkaloid intake might be if the pasture were eaten without discrimination by species. It is also an indirect measure of the relative toxicity due to fescue present in the pasture. Low adjusted total alkaloid concentrations could mean that there is little tall fescue in the pasture or that the fescue might not be infected with the endophyte that causes alkaloid production. No field exceeded 0.600 ppm for total ergot alkaloids.

Yeast and Mold Comparison (Table 5 and Figure 5)

Yeast and mold counts were determined in the soil from monitored pastures. These are reported in colony forming units per gram (CFU/g). The numeric format for CFU/g used on the graphs is a shortened version of scientific notation. For example, 6.00E+05 is equivalent to 6.00 X 105 or 600,000 CFU/g. All counts were between 200,000 and 2,000,000 CFU/g. High levels of yeasts and especially molds in soil might be a good predictor of a field’s potential to produce fungal mycotoxins if environmental conditions were suitable.

Mycotoxins (Tables 6, 7, 8, and Figures 6, 7, 8)

Samples from each field were analyzed for the presence and level of several fungal mycotoxins (listed below with their detectable limit) and all data is reported in ppb (parts per billion). Most fields had no detectable mycotoxins. The three principal compounds that did appear in a few pastures were DON (deoxynivalenol), T-2, and Zearalenone (abbreviated ZEA on graph). A “zero” value on the graph means that concentrations were below the detectable limit. Zearalenone below 500 ppb in the absence of measurable levels of other mycotoxins has not been documented to cause any ill effects. Aflatoxin B1, B2, G1, G2 (Detection limit 1ppb), Fusarenon-X (Detection Limit 1.0 ppm), 3-acetyl DON (DL 0.2 ppm), 15-acetyl DON (DL 0.2 ppm), DON (DL 0.2 ppm), Nivalenol (DL 1 ppm), Neosolaniol (DL 1 ppm), diacetoxyscirpenol (DAS) (DL 0.750 ppm), HT-2 Toxin (DL 0.2 ppm), T-2 Toxin (DL 0.2 ppm), Zearalenone (DL 0.2 ppm), Fumonisin B1, B2, B3 (DL 0.1ppm), Ochratoxin A (DL 1ppb).

Potassium/Calcium Ratio (K/Ca) (Table 9 and Figure 9)

One early theory for the cause of MRLS was that pastures in 2001 had excessively high ratios of potassium to calcium. Values for K:Ca greater than 5:1 were suggested to lead to mineral imbalances in the pregnant mare. K:Ca values for most sampling dates were less than five for all pastures and never exceeded seven. For perspective, the average K:Ca ration for May 2001 across several farms was 6.76:1. However, the K:Ca for 1996 for the same farms in May was 7.58:1.

Seasonal Distribution of Early Fetal Loss/Late Term Abortion (EFL/LTA) Losses (Tables 10, 11 and Figures 10, 11)

The distribution of losses was more spread out compared to 2001, indicating that caterpillars hatched over a longer period of time and that cool weather slowed development.


Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More