|College of Agriculture|
December 23, 2002
Sentinel Farm Summary
2002 Pasture Monitoring Program
University of Kentucky College of Agriculture
Report from Wayne Long, Jimmy Henning
Department of Agronomy
Data collected by the monitoring program clearly eliminated some of the potential causative agents for 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 1 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.
Summary of Sentinel Farm Activities
Other farms were visited when veterinarians or farm managers found MRLS type symptoms and referred them to UK. These included:
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 caterpillar 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 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.
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.
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,
‘Regal’ ladino has been found to contain 50 to 200 ppm HCN.
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, 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.
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.
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
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).
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 5 for all pastures and
never exceeded 7. For perspective, the average K/Ca ration for May 2001
across several farms was 6.76 to 1. However, the K/Ca for 1996 for the
same farms in May was 7.58 to 1.
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.
Contact: Wayne Long, firstname.lastname@example.org; or Jimmy Henning, email@example.com, 859 257-3144.