Short grass is commonly thought of causing wormy cattle by the cattle grazing down where the worms are. But that thought is a misunderstanding. Producers assume that this is the right time to deworm since the cattle look wormy, when actually the poor body and hair coat conditions are related to malnutrition from low intake of roughages on short grass pastures. When grass is short from dry conditions and overgrazing, the worms are not in the soil; the larvae have died from the dryness and sunlight, and also from the heat in the summer.
Don't look at the cattle, look at the ground...any wet grass over a period of weeks?
To be ingested by cattle, the larvae swim up the grass blades while the grass is wet from rain or dew and the soil temperatures are above 55º F. As the grass dries, the larvae go back down with the evaporating moisture. When cattle graze dry grass, they do not ingest larvae, because the larvae cannot crawl up dry grass. While the grass is dry without dew or rain, the cattle do not get worms. After weeks of dry weather, larvae die, and without grass mats for cover, the pastures become free of contamination. Therefore, pastures during drought conditions and under good management practices are not contaminated.
The key is to time the deworming during recent optimal transmission when larvae are developing in the spring or fall or during inhibition of larvae in the summer before the larvae emerge in early fall. While pastures are contaminated with larvae, it may be cost‑effective to worm if the cattle have become exposed to enough larvae during 3‑6 weeks of continuous rains when the larvae are active in soil temperatures of 55‑85º F. If these conditions occur in the spring, summer or fall on contaminated pastures, the timing to deworm the cattle is following 3‑6 weeks of continuous rain. When the timing is right for deworming, the cost-effectiveness is expected in nursing calves and young cattle since they are more susceptible but is not always expected in adults since they are more resistant.
If rains do not come, cost‑effectiveness of deworming cattle is questioned. When it is dry in the spring, do not deworm; wait until the cattle get the worms. Spotty rains this year may be an indication that it is coming for some, but that has not been for many places.
The farms that did not receive rainfall during last fall did not receive pasture contamination going into the winter. Contamination on any farm was lost during the winter due to pasture larvae killed by desiccation if the farm has been under drought conditions. Cattle on drought pastures since last fall do not have worms and do not need deworming at this time. Ones must wait for the right time on their farm. It is understood by science that deworming drought-pastured cattle would not result in economic benefits.
Floron C. Faries, Jr., DVM, MS
Professor and Extension Program Leader for Veterinary Medicine
Texas AgriLife Extension Service
Texas A&M System
College Station, TX 77843-2487