By Dayna Locitzer, DVM
Pasture season is the time of year we all look forward to in the organic farming community, but it is not without challenges. One challenge important to consider is parasites cows face when on pasture. A parasite is an organism that relies on another organism (the host) for its resources at the host’s expense. The parasite can drain the resources of its host, putting the host at severe risk. On pasture there are external parasites, like flies and ticks, and internal parasites, namely strongyles. This article is going to address internal parasites, specifically strongyles. It will discuss the risk factors, how to diagnose, and how to treat.
Strongyles are a type of roundworm (or nematode) that have part of their lifecycle in the gastrointestinal (GI) tract of the host animal. The strongyle species we are mainly concerned about are Ostertagia, Teladorsagia, Haemonchus, Trichostrongylus, and Cooperia. They attach themselves to the wall of the GI tract and cause damage to the tissue. Additionally, some species feed on the blood of the host. This results in ill thrift, anemia (decrease in red blood cells), and hypoproteinemia (decrease in blood proteins). While all pastured cattle are at risk of a strongyle infection, they develop immunity over time. This leaves youngstock at the highest risk.
To detect whether your animals are infected with strongyles, it is best to do routine screening. In the spring time when the days start to become longer and the animals have been on pasture for a few weeks, take individual samples of manure from your youngstock and consult with your veterinarian about getting them tested. Another good time to check parasite loads is in the fall before they head into their winter housing. Screening provides multiple opportunities. For one, it shows you who in your herd is shedding large amounts of parasites. It is often the case that 20% of the herd will shed 80% of the parasites. This can factor into culling decisions. Secondly, screening allows you to identify who has increased parasite loads before they become symptomatic and losses are incurred on the farm. If you don’t use the screening method, you should take samples from youngstock that are poor doers. They might have a rough coat or have a low body condition score. In more extreme cases you might see chronic diarrhea or even bottle jaw. Poor doers are often more susceptible to parasites. Once you get your results, you are then going to decide who and how to treat.
Animals that have a count above 200 eggs per gram are considered to be heavily infected. In organic dairy, we are allowed to use the conventional treatments for strongyles when the strongyle egg counts are at levels that warrant treatment. These treatments include products with the active ingredient of either moxidectin or fenbendazole. The products should be used according to the label dose and you should check with your certifier about milk withhold times. There is a minimum milk withhold of 90 days, making it hard to use in lactating animals and more amenable to use in youngstock. These treatments can only be used in animals that are in need of deworming and it is important to note that any animal treated with one of these above dewromers can NEVER be marketed as organic beef. After deworming, check a sample from the treated animal again in 10-14 day to ensure that the dewormer used was effective. It is best to see a 95% reduction in parasite eggs to make sure we aren’t creating resistant populations of strongyles.
There are a number of herbs and plants that have been proven to be effective against strongyles. Some pasture grasses with high tannins like birdsfoot trefoil and chicory have been shown to have anti-strongyle properties. Herbs like wormwood, black walnut, and garlic are known to have qualities that can reduce strongyle loads. With any product, even herbal ones, please follow the label directions. While there are many choices of organic approved herbal treatments and tannin rich pasture grasses, it is important to note they will not be effective against a severe infection. When you have an animal that is symptomatic from a large internal parasite load, I recommend using a chemical dewormer for effective treatment.
Since there are no great options for treatment, prevention is key! Focus on your grazing heifers because they are at highest risk. Understanding the parasite cycle is a good place to start. When a cow first poops out strongyle eggs, the larvae hatch and climb up the base of grass blades. They become infectious as early as 3-5 days after the eggs are laid. They can survive a mild winter and their ideal conditions are warm and wet. With this in mind, graze your animals so that they leave a four inch residual and rotate paddocks every 3-5 days to avoid the infectious larvae ingestion. Try to leave a 4-6 week rest period for each paddock in order to give the larvae time to desiccate in the sun. Get a sense of which pastures have high loads and avoid putting at risk animals there. Other tactics include harvesting hay before returning the animals to graze, giving more time for the eggs to desiccate or be destroyed in the haying process. You can also follow the grazing cattle with another species of ruminant like sheep. Sheep are dead end hosts for some of the species of strongyles.
This fall make sure you are on top of your parasite load because they can build up over the course of the pasture season. The best time to start thinking about parasites is now! Strongyles can cause significant losses in your herd and getting fecal counts of eggs can give you a good sense of what you are dealing with on your farm. This is a problem that can change over time. Maybe you have introduced new animals to the herd, maybe you had low counts in previous years but weather patterns didn’t kill off the parasites like normal. Regularly screening for parasites is valuable in catching a problem before it becomes hard to manage. When you have a serious infection it is vital that you treat the animals with chemical dewormers and monitor their effectiveness so as to not encourage resistant populations. Remember that your young stock are at highest risk and good grazing practices with some choice pasture plants can go a long way in prevention.
Dr. Dayna Locitzer has over 10 years of experience working with pasture-based dairies in the Northeast. She is a member of the staff at Columbia Veterinary Services, Hudson, NY.
Do you have a questions for Dr. Locitzer or an area you'd like her to focus on in future issues? Please send thme to the NODPA News Editor, Nora Owens at noraowens@comcast.net and she will share them with Dr. Locitzer.
Posted: to Organic Production on Fri, Sep 5, 2025
Updated: Fri, Sep 5, 2025
