University of Kentucky Pasture Monitoring Interim Report--Comparison of 2002 and 2003

Jimmy C. Henning, Assistant Director for Cooperative Extension, Agriculture and Natural Resources, Former Extension Forage Specialist;and Wayne Long, Agriculture Extension Associate, Department of Agronomy, University of Kentucky (UK)

The University of Kentucky mare reproductive loss syndrome (MRLS) pasture monitoring program has been evaluating certain pasture parameters weekly since March 1, 2003. A comparison of 2002 and 2003 values for some pasture parameters and a brief interpretation follows.

Pasture samples are taken in those fields occupied by mares that are part of the MRLS monitoring program. Pastures monitored in 2003 would not necessarily be the same as in 2002. Therefore, these comparisons do not allow a perfect comparison of 2002 and 2003 on a field by field basis. However, they do provide more information about seasonal levels of nitrate, cyano-sugars, K/Ca ratio and tall fescue alkaloids.

Nitrate from Composite Pasture

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.

Mean and maximum values of nitrate-N in 2003 have been generally higher than 2002 until week 8 (late April). After week 8, 2003 values have been lower than 2002. Values over 2000 ppm have been reported in both years (week 8, 2002 and weeks 4 and 5, 2003) without problems in pregnant mares on monitored farms.

Cyanide Potential of White Clover

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.

Levels of cyanide potential in white clover have been consistently higher in 2003 than 2002. However, even the maximum observed level of cyanide (1215 ppm, week 7) in 2003 is similar to that observed in some commercial varieties of white clover. Pregnant mares are known to be very tolerant of low levels of cyanide. No problems were observed in mares corresponding to fields with high cyanide clover.

Potassium/Calcium Ratio (K/Ca)

One early theory for the cause of MRLS was that pastures in 2001 had excessively high ratios of potassium (K) to calcium (Ca). Values for K/Ca greater than 5:1 were suggested to lead to mineral imbalances in the pregnant mare. However, further study of K/Ca ratios from past years revealed that values exceeding 10:1 were not associated with equine abortions. K/Ca values for most sampling dates in 2002 averaged less than 6:1 for all pastures and the maximum never exceeded 8:1. The K/Ca ratio in 2003 has generally been greater than 2002.

Tall Fescue Ergovaline Alkaloid

Tall fescue plants were selected randomly from across each pasture to determine the potential toxicity to pregnant mares if it accounted for 100% of the diet. The values reported are ppm of ergovaline. Ergovaline is produced in tall fescue which is infected with the fungal endophyte. While not all tall fescue in Kentucky is infected with this endophyte, the samples from all monitoring farms and fields had measurable ergovaline alkaloids. Therefore, it is reasonable to assume that at least some of the tall fescue plants in these fields were infected. When consumed in high enough quantity, these alkaloids are responsible for tall fescue toxicosis in pregnant mares, whose symptoms include prolonged gestation, agalactia, thickened placenta and dystocia. Ergovaline levels of 0.300 ppm or greater are considered toxic to pregnant mares in late gestation.

The level of ergovaline in tall fescue is very low to non-detectable in early spring and increases as the plant increases its growth rate in April. Some samples taken during weeks 7 and after in both years contained seedheads, which helps explain the rapid rise in ergovaline. Ergovaline accumulates in the seeds of infected tall fescue.

Infected tall fescue may be more or less toxic from year to year, depending on many factors that are not well defined. Ergovaline levels in 2003 have generally been lower than 2002. However, levels show the same seasonal pattern, rising in late April (weeks 7 +). Interpretation of these values is difficult since they are of selected plants in a given pasture and probably does not represent the true diet of the grazing mare. They can be used, however, as an indication of potential toxicity.

Pregnant mares should be removed from fields containing infected tall fescue during the last 60 to 90 days of gestation. Where possible, infected tall fescue should be eliminated or minimized in pastures used for pregnant mares.

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