Agent P: Parasites Pasture Grass Parasite Management


Lush, spring pastures are pleasing to all horse owners. It can provide your horse with nutritious forages, exercise and enrichment. However, pasture grasses can contain an antagonistic visitor: internal equine parasites.

Parasite management is an important step in pasture grass management and with successful techniques, the total amount of parasite eggs and larvae a horse may consume can be reduced. Without proper management, these parasites can cause detrimental effects to your horses and cost owners hundreds in treatment.

Where it Begins
Manure is the primary source of parasite transmission. Parasite eggs from an infected horse are passed into the fecal balls. The eggs develop into larvae that live on pasture grasses, using moisture on the grass to crawl up the short blades of grass (under 3 inches tall) so they are more likely to be consumed. Contrary to belief, parasite eggs and larvae can survive freezing weather and heavy rains act as an effective method for dispersing larvae through pasture grasses due to a protective sheath. Once a horse eats the contaminated grass, the larvae develop within the horse’s digestive tract, primarily in the stomach or intestines. Signs of parasites can vary but the most common symptoms include:

  • Weight loss
  • Dull, rough looking coat hair
  • “Potbelly”
  • Diarrhea
  • Colic
  • Tail rubbing
  • Lethargy

Parasite Pasture Prevention
Parasite prevention all starts with proper pasture management. Two to three acres should be allotted per horse, and having horses on one acre or less limits exercise, nutritional availability from pasture grasses and increases the chances of parasite exposure due to higher amounts of manure in the area. Before turning horses on pasture, wait until pasture grass is about minimum six to eight inches (Eight to ten inch height is more ideal) before grazing. Once the grass is grazed down to four to five inches remove horses and mow grasses to three to four inches.. Never mow below three to four inches, as this increases recovery time and damages tall grasses. This type of management can be achieved through rotational grazing practices. 

Extreme heat (over 85 degrees Fahrenheit) can also kill parasite eggs but it takes more than just temperatures to kill the eggs. Dragging or harrowing pastures to break up manure piles can expose parasite eggs in manure at those extreme temperatures. Do NOT drag pastures that are being actively grazed on and allow at least two weeks of rest between dragging and turning horses back onto pasture.

Anthelmintic Management and Treatments
Horses typically develop immunity to most internal parasites by two to three years old. However, that does not mean owners should not treat their horses for all possible internal parasite infection. For internal parasite testing, it requires a fecal egg count test (FEC). FECs are quick and affordable, but owners must submit fresh manure samples to their veterinarian’s office. Once submitted, your veterinarian’s office will conduct the test to determine what types of parasite(s) are present in your horse (if any) and how many eggs per gram of manure. Horses are classified based on their shedding level: 
   Low shedder: <200 eggs
   Moderate: 200-500 eggs
   High: >500 eggs 

Your veterinarian will guide you to the best course of treatment action with anthelmintic (dewormers). DO NOT randomly give anthelmintics to your horses without knowing what types of parasites are present since some anthelmintics are tailored to certain parasites. This type of deworming can also lead to possible anthelmintic resistance. 

Deworming can be done anytime of the year but the American Association of Equine Practitioners (AAEP) recommends deworming in the spring and fall. When deworming, it is important to give the appropriate amount of medication. Read the dewormer label to determine how much to give your horse based on its weight. It is always better to SLIGHTLY overestimate your horse’s weight as too little dewormer will NOT kill all the parasites who can build up resistance. As with any medication, make sure to record your horse’s deworming in their health records. 

For further information about types of parasites and which anthelmintic(s) to give, please visit the following UTK info page: 

Any further questions about pasture grass parasite management, please contact your county’s Extension office. UT-TSU Shelby County Extension can be reached at 901-752-1207 during normal business hours Monday through Friday 8am-4:30pm CST. 

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