North Carolina State University Extension
If you plan on deworming cattle, spring is the right time. Parasite burdens in pastures peak during the spring, drop over the summer, and rise again in the fall. Internal parasites cause subclinical effects that are then followed by clinical signs. Subclinical effects show up as productions losses. The animals don’t look sick but they experience reduced gain, decreased milk production, lowered conception rates, etc. Clinical effects can be seen and include rough coats, anemia, and edema. Subclinical effects have major economic impacts, so it’s important to deworm cattle before you see the physical signs.
How often you deworm depends on the type of cattle you have and how much of a parasite load they are exposed to. Mature cows usually only need to be dewormed once a year. They should be treated shortly before calving. Calving is a stressful time in a cow’s life and it can lead to suppressed immune function which makes her more susceptible to parasites. Treating twice a year may be needed if you have a large parasite load. Bulls are naturally more susceptible to parasites so they should be treated in the spring and fall. Calves require more frequent deworming. Treatment should start at 3 to 4 months of age and be given again at weaning. Depending on your farm’s parasite levels, deworming every 3 to 4 months until they reach 1 year may be necessary. Yearlings can be dewormed in the spring and fall until they reach maturity. Heifers aren’t technically considered mature until they are pregnant with their second calf.
The two main types of classes of dewormers are benzimidazoles and avermectins/milbemycins. Please note that these are the active ingredients and not brand names. The avermectin/milbemycin group also offers external parasite control and works well in calves. When choosing a dewormer, keep in mind its spectrum of control, withdrawal time, cost effectiveness, product efficacy, and method of application. It’s important to follow the product dosing guide. There is always a chance that parasites can develop resistance and continual under-dosing could contribute to resistance. If you can’t weigh each animal and dose based on that, calculate your dose based on the heaviest animal in the group, not the group’s average weight. It is also important to not overgraze pastures. This causes cattle to graze closer to the ground and they will ingest more worm larvae. Internal parasite control should be a combination of pasture management and treatment with dewormers.