Heifers should be dewormed at about three weeks after arriving on contaminated pasture. Some overwintered larvae will still be alive on the pasture, and infection will re-occur with adult worms developing over the second three weeks. Few larvae will have survived beyond six weeks, so heifers can continue to graze for the rest of the summer with only limited exposure to worms. Heifers managed according to this plan outgained control heifers by 87 pounds during the period they were on pasture. Another acceptable control scheme involves the use of safe pastures.
Pastures previously grazed by adult cows are relatively safe for heifers.
Pastures grazed by horses or sheep are safe, as the species of worms that infect other domestic or wild animals do not infect cattle. Pastures or hay fields that have not been grazed by heifers for a period of six week or longer in late spring or summer are also safe pastures. When moving heifers to safe pastures, it may be advisable to treat them to avoid contaminating the new pasture. Coumaphos is given in feed at low doses for several days. It is less effective than the other wormers and has a lower safety index.