Dairy producers who use pasture-based systems want to see healthy animals with good growth from the start. One aspect of managing cattle on pasture is managing their internal parasites.
Penn State extension veterinarian Dr. Robert Van Saun says that although internal parasites haven’t been a problem for most dairy herds in the past, they’re becoming more of an issue. Unfortunately, there’s no one-size-fits-all when it comes to parasite management. However, if producers identify problem parasites, it’s easier to target the animals most likely to be harboring parasites.
Internal parasites harbored in the digestive system reduce protein digestion and the cow’s ability to absorb protein. “Parasite species that attach to the wall of the gut and consume blood, which is most of them, causes loss of blood protein and hemoglobin,” said Van Saun. “The animal has to make up for that, which costs energy, and that’s less energy she has to maintain her body or productivity.”
Most farmers are aware of the clinical signs of parasite infestation including diarrhea, sunken eyes, weight loss, pale mucous membranes and rough hair coat. Severe infestation can lead to bottle jaw. “Bottle jaw is edema in the tissue under the jaw,” said Van Saun, noting that the condition is associated with blood-sucking parasites in the gut. “Because the animal has lost so much blood protein, fluid is leaking out, and that’s one of the areas that fluid leaks into.”
But Van Saun wants producers to realize that clinical signs are the tip of the iceberg. “A bigger problem for the producer is subclinical parasitic infections,” he said. “Suboptimal performance, fewer calves born, loss of milk, extra days to finish, increased time to breeding and more disease.”
Van Saun explains the life cycle of parasites. “It’s a direct life cycle and only involves one host,” he said. “The cow is out grazing and ingests larvae. That’s an important control point. Once the larvae get into the cow, they mature, lay eggs and migrate; and that’s where damage is done. Eggs develop in manure and work their way into the ground and become the infective larvae. The last step, which becomes the contamination stage, is when larvae emerge from feces or soil and ascend vegetation following rain or heavy dew.”
The time frame between the dung pile being deposited and the larvae getting ready to enter and infect a new host is key to the cycle and eventual infection of the cow. “L3 is the infective stage of intestinal and abomasal worms,” said Van Saun. “It’s going to develop, depending on the species, in the abomasum or in the small intestine. It goes through several developmental stages, then to the adult. The prepatent period, which is between the time the animal becomes infected and when they start to shed eggs, is only two to three weeks.”
The focus of parasite management is on L3 larvae. Van Saun says that larvae don’t eat during stage three, and could potentially be starved out to reduce numbers. The L3 larvae are also resistant to their environment because they maintain an outer coating, or cuticle, from the previous larval stage. However, it is susceptible to drying out and UV light.
Van Saun suggests that producers set management goals to treat animals before they show clinical signs of disease. The parasite management program should be based on production goals, length of grazing season, available pasture, and whether pastures are used heavily or rested for long periods. Producers should also consider the cost of anthelmintic (deworming) product, time, labor and handling facilities.
“Withhold food for better contact of treatment with parasite,” said Van Saun. “Deliver the drug in the proper manner (injection, oral, pour-on). If using white wormers — any products with benzimidazole in the name (Fenbendazole, Albendazole, etc), we might want to treat twice — 12 hours apart. Be aware of milk withholding time and meat/slaughter withholding time.” Dose for the correct weight – worms exposed to a non-killing dose is selecting for resistance.
Although it’s tempting to treat every animal when the herd is being handled, Van Saun wants producers to understand the concept of refugia. He describes refugia as a sub-population of worms that is not exposed to anthelmintic. This could include eggs and larvae that are on pasture at the time of treatment or worms in untreated animals. “It’s important to maintain refugia to keep resistance at bay,” he said “Having a population that hasn’t been exposed dilutes the resistant genes in the rest of the population and allows us to use various dewormers for longer.”
Another important concept, which has been proven in numerous case studies, is the fact that a small percentage of the herd harbors most of the parasites. “We don’t want to be treating every animal, every time,” said Van Saun. “This is where the 20-80 rule comes in – not all animals are equally infected. About 15 to 20 percent of the animals harbor over 80 percent of the parasites. There’s a small population that is susceptible to parasites, and they are the Typhoid Marys of the herd.” The parasite control program should be targeted toward that segment of the population. Producers can also identify and eliminate the 20 percent of animals that are the major carriers, or select animals that show resistance to parasites.
Pasture management should include rotation to starve the larvae. Limit grazing so that animals are not forced to graze the lower portions of the forage stand. “Larvae cannot climb very high,” said Van Saun. “They don’t get much higher than 3 to 6 inches off the ground. Reduce the stocking density so there are fewer animals per acre. Graze alternate species that don’t share parasites.” Producers can also clip and drag pastures to break up pies, then use that field for a hay crop.
Van Saun noted although some plants that are high in condensed tannins seem to suppress parasites, there is no definitive research to determine how much of such plants an animal would have to consume for parasite management. He also says there are no proven studies showing that diatomaceous earth is an effective dewormer. Although there’s no simple answer for parasite management, the producer’s best options are to deworm the herd when worms are in hypbiosis (late fall), use clean or safe pastures (especially for young animals), co-graze with other species to ‘clean up’ parasites, rotate anthelmintic products yearly and select animals for resistance.