cows in field

Pasture, Calves and Internal Parasites

By Hue Karreman, VMD

At this time of year, well into the pasture season, it is usually good to discuss internal parasites in calves. I really think that parasitism, whether internal stomach worms and coccidia or external flies and mange are truly a weak link in the chain of organic livestock health. I say this coming from being on the “front lines” for many years now. I see too many calves on pasture that don’t look so great late in the summer – whether “natural raised”, certified organic or conventional.

It must be remembered that if pasturing animals in the same areas year after year, there will be parasites waiting for each group as they arrive. Pastures look really nice early on but those stomach worm larva that are invisible to our eye are out there rapidly multiplying and loading the animals that are out there eating the pasture plants. That’s because the stomach worm larva hatch from the eggs in the freshly deposited manure and crawl onto the grass blades to be taken in again by the animal to start their life cycle all over again - to feed from and reproduce themselves within the animal’s digestive system.

This is why I am in favor of clipping pastures or at least dragging pastures with a set of chains: it smears out the manure paddy and those larva will dry out in the sun and wind and not live to climb onto the grass blades to be eaten and taken in again. You should, however, wait about 3 days before clipping or dragging pastures so the dung beetles can do the majority of their work in helping to break down the manure. Additionally, the horn flies will have laid their eggs within the first three days of the fresh manure being deposited, so dragging after that will help reduce their offspring (maggots) from developing into more horn flies that torment animals.

Classic Signs of Parasite Infestation

What do your calves on pasture look like right now? Are they sleek and in good body condition just like when you weaned them or set them out to pasture? They certainly can be if fed well – which usually means making sure that there is sufficient energy (carbon/carbohydrate) in their diet to balance out all the protein (nitrogen) from green pasture. Or do they look a bit more ragged now - perhaps a bit pot-bellied, their hair somewhat dry looking and reddish black (not shiny black as it could be), with thin back leg muscles and some dried diarrhea up high on their legs and tail? If so, these are classic signs of internal stomach worm infestation.

It would be wise to catch a few up and look in their eye sockets to see how pink or pale white the sockets are. In sheep and goats, it is common to use the FAMACHA test which basically looks at their eye sockets and, depending on how white (indicating anemia), this will indicate when to treat them with a conventional wormer. While the FAMACHA test is technically not valid for calves, looking at their eye sockets will still reveal the degree of blood loss as well. Calves just hide it until later in the disease than do sheep and goats.

In organic agriculture, with the requirement of animals 6 months and older to get a minimum of 30% dry matter from pasture over the grazing season, it is only a matter of time before the young stock, which are not immunologically mature against stomach worms, will become challenged (and possibly infested) if feeding and pasture management is not top notch. A big part of it all is proper feeding to ensure excellent energy intake while on pasture. This can be from high energy forages or giving some grain. The immune system depends heavily on proper daily energy intake. It should be noted that adult animals do NOT need to be wormed as they can live in balance with a stomach worm challenge in their environment – unfortunately young stock cannot because they haven’t experienced worms previously. Note: lung worms can, and do, infect adult cattle especially in wet years.

I think a good goal is to raise calves that do have some challenge with stomach worm larva in the pasture, yet are managed and fed well enough that instead of becoming infested, they instead build immunity due to a low level exposure. This is a kind of a natural vaccine effect. Unfortunately not many farms seem to be able to achieve this. The result is somewhat stunted calves that likely will freshen a month or two later since they won’t reach breeding size as quickly. However, calves that do make it through this tough period of life – usually between 4-11 months of age – start looking really nice again by a year old and go on to do fine. Even if they did look crummy due to a significant stomach worm infestation, after about a year old they will be really strong against pasture stomach worm challenges for the rest of their lives.

Certified Organic Internal Parasite Infestation Treatment

So how do we treat internal parasite infestations on certified organic farms? Well, as of this writing, ivermectin, moxidectin and fenbendazole are allowed to be used – but only for an emergency need when methods acceptable to organic have not succeeded in restoring an animal to health. If a farmer uses these in lactating cows, a 90-day milk withhold is required. Typically in the past I have recommended a wormer as a one-time single treatment – essentially to re-set the individual animals which are infested – and then get the management in place to keep things in prevention mode rather than reaction (treatment) mode. Note that the NOSB is voting to sunset (remove) ivermectin and moxidectin at their next meeting in Vermont at the end of October. If you have strong feelings about this topic please write to them. However, when I was on the NOSB and we voted to allow fenbendazole (Safeguard® crumbles) it was with the intention of removing ivermectin and moxidectin someday. The reason is that both are technically antibiotics as well as ivermectin being lethal to dung beetles and moxidectin being harmful to fish if it gets into the water.

Fortunately, there are many plant based medicines being used around the world against internal parasites. In a chapter I wrote on plant based medicine for the book Veterinary Herbal Medicine by Wynn and Fougere (Mosby, 2007), I reported on a study that showed birdsfoot trefoil and chickory interplanted into pasture decreased stomach worm larva burden significantly compared to straight white clover and rye pasture. This is because of tannins contained in the birdsfoot trefoil and chickory which helps the digestive tract of the animal to repel the stomach worms.

However, we are later in the grazing season, so what should we treat with right now if our young stock looks crummy? One treatment is to give 10cc orally of the high iron and mineral supplement “Ferro” once daily for 5 days in a row – this is highly effective. Another would be to try Agri-Dynamics Vermi Tox as both have some positive benefits as shown by clinical trials at Chico State University. Weaned cattle are dosed at 1 oz. /300-400 lbs. for 3 days in a row. Remember, at this time you can use ivermectin, moxidectin or fenbendazole if your animals are in really bad shape – and you probably should at that point.

Remember, now is the time to really check your young stock on pasture for signs of internal worm infestation. If they are infested and nothing is done about it, the first batch of damp cold weather will likely bring on pneumonia – and that is not at all desirable. Remember, animals always tell us the truth by the way they look. So be mindful: stop and observe your animals and take action now if needed, not later.

Hubert Karreman is a veterinarian who has been working with organic dairy cattle since 1988. After a few years of consulting in the organic sector, he is happy being back in practice and working hands-on with organic livestock in the Lancaster PA area. He has given invited talks about organic livestock health care in many countries and is owner of CowMaster, LLC, the supplier of Phyto-Mast multi-purpose tubes. He can be contacted at 717-405-8137 or