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February 2, 2004

UK Pasture Monitoring Report
Comparison of 2002 and 2003 Data, December 10, 2003

Wayne Long, Jimmy C. Henning, and Michael Collins
Department of Agronomy
University of Kentucky


The University of Kentucky Mare Reproductive Loss Syndrome pasture monitoring program was established at the beginning of 2002 to identify and measure several important pasture parameters on a bi-weekly basis during spring and early summer of the 2002 and 2003 seasons. This intensive monitoring program has provided us with critical background information on season and year-to-year variation in several characteristics of Central Kentucky horse pastures with potential implications for animal health. This report provides a summary of these two seasons of activity and the data generated from this effort. A comparison of 2002 and 2003 values for some pasture parameters and a brief interpretation follows.

Pasture samples were collected from those fields occupied by mares that are part of the MRLS monitoring program. Because of this sampling structure, the pastures monitored during 2003 would not necessarily be the same as those monitored during 2002. Therefore, these comparisons do not allow a perfect comparison of 2002 and 2003 on a field-by-field basis. However, we believe that they do provide useful information about seasonal levels of nitrate, cyanogenic compounds, K/Ca ratio and tall fescue ergot-type alkaloids.

 

 

Summary of Emergency Farm Activities

2002: Other farms in addition to those monitored routinely were visited when veterinarians or farm managers identified MRLS type symptoms and referred them to us. These included nine farm visits of which 6 had MRLS cases totaling 27 early fetal losses and 3 late fetal losses. Other farms where additional samples were collected included a control farm (a farm with black cherry trees (BCT) (Prunus serotina) present but showing no losses) and 2 farms with concerns because of losses in 2001. A total of 22 additional fields were sampled resulting in 204 samples.

2003: Visits were made to five farms located in Jessamine, Woodford, Fayette and Jefferson counties. An average of 3 fields per farm was sampled resulting in 73 additional samples. Four of the five farms experienced MRLS-type loses in 2002 and were sampled again in 2003 as a precautionary measure. Three of the five farms experienced losses in 2003. Only one of these losses was suspected to be MRLS while the others were thought to be related to tall fescue. Percent tall fescue (as a proportion of the total pasture) was generally high in fields where abortions occurred, and ergovaline levels in some fields were at or above the 0.30 ppm threshold level above which toxicosis concerns are increased.

Summary of Monitored Parameters

Nitrate in Composite Pasture Samples
The results presented are minimum, maximum and average concentrations of nitrate - nitrogen (NO3-N) in the overall composite pasture sample, expressed in parts per million (ppm) on a dry matter basis. Nitrate can cause toxicity (including asphyxiation and abortion) after being converted to nitrite by the normal microorganisms present in the gastrointestinal tract of livestock. In the horse, this would occur in the cecum. Because most nitrate is absorbed before digesta reach the hind gut, horses are much less sensitive to nitrate concentrations of pasture than are ruminant animals such as cattle and sheep. Nitrate-N levels below 1200 ppm are considered generally safe for cattle and other ruminants.

Mean and maximum values of nitrate-N in 2003 have been generally higher than 2002 until late April. Values over 2000 ppm have been reported in both years (Cycle 5, 2002 and Cycles 3 and 4, 2003) without problems in pregnant mares on monitored farms.

Potassium/Calcium Ratio (K/Ca)
A hypothesis put forth in 2001 as a possible 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 have been suggested to lead to mineral imbalances in pregnant mares. However, further study of K/Ca ratios from past years revealed that even 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. The K/Ca ratio in 2003 has generally been greater than 2002. High K/Ca ratios were not correlated to any mare health problems on monitored farms.

Cyanide Potential of White Clover
White clover is common in many Central Kentucky pastures. White clover samples from each pasture were assayed for their potential to release cyanide (HCN) 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 seasonal patterns within 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 cyanide as HCN.

Levels of cyanide potential in white clover from sampled pastures have been consistently higher in 2003 than 2002. Maximum values in 2003 exceeded values reported for varieties of white clover known to have high HCN. However, 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 during this monitoring program.

Ergovaline Alkaloid Concentrations of Tall Fescue
Alkaloids in some genotypes of tall fescue (lolines and ergopeptides) are produced in association with the presence of the endophytic fungus (Neotyphodium coenophialum) (Long et al., 2003). Tall fescue plants were selected randomly across each pasture to determine the potential toxicity to pregnant mares. The values reported are ppm of ergovaline. Ergovaline is an alkaloid produced in tall fescue 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 0.500 ppm or greater in the total diet 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. Ergovaline especially accumulates in the seeds of infected tall fescue. Some samples taken during and after late April contained some tall fescue seedheads in both years, which would account for some of the increased ergovaline. Seed of infected tall fescue may contain very high ergovaline concentrations.

Infected tall fescue may vary widely in toxicity level 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. Interpretation of these values is not precise since they are of selected plants in a given pasture and probably do 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.

Tall Fescue Content of Monitored Pastures
The percent tall fescue present was noted visually in monitored pastures in both years. Values ranged from a low of 1% to a maximum of 65% (Cycle 3 of 2003, Barren/Maiden fields). The fields monitored in 2003 generally had larger amounts of tall fescue compared to 2002. There were no reports of fescue toxicosis symptoms in any mare group.

 

References:
Long, W., J. C. Henning, B. Coleman, L. Lawrence, C. Peterson, and A. Reinowski. 2003. Overview of the Mare Reproductive Loss Syndrome Monitoring Program for 2002. Proceedings of the First Workshop on Mare Reproductive Loss Syndrome, 2003; 102-112.


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