A common question veterinarians receive is, “When is the best time of year to deworm the cowherd, if I’m only going to deworm one time?” To answer this question requires a basic understanding of parasite population dynamics.

Parasites have two basic functions: one is to feed off the animal they infest and the other is to produce a large number of eggs and contaminate the environment so their species will survive. However, in order for parasite “off spring” to survive, eggs production needs to coincide with environmental conditions that are favorable for survival. Such conditions typically occur in the spring of the year and again in the fall. Therefore parasite activity and number of parasite “off - spring” contaminating the pasture follows a pattern similar to the one seen in figure 1.

The basis of strategic deworming is to deworm cattle with the intent of not only eliminating the parasites in the animal but also to reduce parasite contamination in the pasture. Therefore, the optimal time to deworm theoretically would occur right before the adults begin to lay their eggs.

According to Dr. Jeremy Powell, an extension veterinarian and parasitologist for the University of Arkansas, producers in northern Arkansas and southern Missouri should consider a deworming date of around March 15th as the optimal time to deworm. If a second deworming is desired he recommends it occur around September 1st. As for central and northern Missouri, conditions will differ slightly and moving the target dates back by two weeks may be indicated.

In the end, remember that only ten percent of the parasite problem is in the animal. The other ninety percent of the parasite problem is in the pasture. By strategically deworming, pasture contamination is kept to a minimum which reduces the likelihood of re-infestation and keeps parasite populations under control.

Parasite population dynamics

Footnote: There are several different species of parasites that can impact cattle performance but many parasitologists still consider ostertagia ostertagi (brown stomach worm) to be the most signifi cant. Therefore, the above recommendations are based upon population dynamics of ostertagia.

Source: Craig Payne, Beef Veterinarian