Short grass is commonly thought of as causing wormy cattle by the cattle grazing down where the worms are, but this is a misunderstanding. Cows look “wormy” when actually they are in poor body condition, and rough hair coat conditions are related to undernutrition. When pasture availability is limited due to dry conditions and overgrazing, the worms are not down in the soil. Actually, the larvae have died from the dryness and from the heat in the summer.
When stomach worm eggs are passed in cattle manure, the eggs hatch in the manure. The larvae must be washed from the manure by rains. The larvae can only crawl (swim) up the grass blades while the grass is wet from rain or dew. As the grass dries, the larvae go back down to find moisture. As cattle graze dry grass, they don’t ingest larvae. After weeks of dry weather, larvae die, and without grass mats for cover, all the larvae die, and the pastures become free of contamination.
While pastures are contaminated with larvae, it may be cost effective to deworm if the cattle have become exposed to enough larvae during three to six weeks of continuous rains with soil temperatures of 55° to 85°F when the larvae are active. If these conditions occur in the spring, the time to deworm the cattle is three to six weeks following continuous rain. If the rains come on contaminated pastures, that would suggest deworming cattle in May. If rains don’t come, the cost effectiveness of working cattle in the spring is questioned. When working conditions are cost effective, target deworming nursing calves and young cattle and probably not mature cows and bulls.
Adult stomach worms live for about two months and die of old age, and their larvae are transmitted during moderate temperatures and wet conditions. Because cattle do not have stomach worms throughout the year, determining the proper timing for deworming is critical.
