Strange Behavior Exhibited by Cattle Consuming Toxic Tall Fescue

Here in the South it is not uncommon to see cattle standing in farm ponds that have not been fenced off.  That behavior is common in the summer months when it is hot and humid, especially if there is limited shade to escape the scorching sun.  However, cattle may exhibit strange behavior when grazing pastures dominated by toxic endophyte-infected tall fescue even in “cool” weather. 

Early one morning I drove past a farm where cattle were standing neck-deep in a pond at 6:30 am when the temperature was cool and the sun was yet to rise.  In fact, it was barely dusk and unless paying close attention the phenomenon would not have been observed.  I pass this farm most days on the way to work, and several days later cattle were again observed standing in water when the temperature was 66oF and the relative humidity was 60%.  This picture shows two of those animals. 

Various members of the herd were observed in the water over the next 3 days.  Some of the herd were Angus, others Charolais, Limousine, or mixtures thereof.

I recorded the temperature and relative humidity each day to determine whether the livestock might be suffering from heat stress.  Heat stress is estimated based upon temperature and humidity conditions to which cattle are exposed.  The following table illustrates what levels of heat stress cattle suffer under different environmental conditions based upon a calculated Temperature Humidity Index (THI).  The level of stress is categorized into normal (THI<75) , alert (THI=75-78), danger (THI=79-83), and emergency (THI>84) categories. 

The environmental conditions and calculated THI for each day that the cattle were observed standing in water is in the following table.

  DayTemp (oF)Humidity (%)  THIHeat stress? (yes or no)

So why are the cattle standing belly deep in water when they are suffering from no heat stress?  Since they are grazing toxic endophyte-infected tall fescue it must be assumed that this behavior is somehow related to the toxins in the tall fescue.  The toxins are known to cause a condition called vasoconstriction – the narrowing of blood vessels resulting in reduced blood flow.  Exposure to toxin contributes to lameness and cattle being “tender footed”, and under very toxic conditions the vasoconstriction can be so severe that it causes gangrenous tissue to develop just above the hoof.  Sometimes the gangrenous tissue becomes so severe that the hoof may slough off.   Undoubtedly the tissues affected by the vasoconstriction are sensitive to touch, or in this case the weight of the animal standing on the hooves. 

One way cattle can alleviate some pain is to immerse themselves into water and take advantage of the buoyancy the water provides, thus reducing the weight and pressure on the sensitive tissues.  This hypothesis has not been tested scientifically, but it is the only plausible reason for this strange behavior.  If your pastures have a tall fescue base and the cattle stand in the pond, or show other signs of heat stress or chronic lameness it is highly likely that they are suffering from the effects of toxic endophyte-infected tall fescue.

~ Dr. Nick Hill, Professor Emeritus (University of Georgia) and Founding Partner Agrinostics, Ltd. Co.

The Alliance for Grassland Renewal is a national organization focused on enhancing the appropriate adoption of novel endophyte tall fescue technology through education, incentives, self-regulation and promotion.  For more resources or to learn more about the Alliance for Grassland Renewal, go to

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