A major source of gastrointestinal worm infestation of grazing heifers is larvae that have survived the winter on pasture grass. When these larvae are swallowed and mature inside the heifer, they produce eggs that are shed in the feces, resulting in a higher pasture contamination later in the grazing season. To break this recycling of pasture worm infestations, multiple deworming is needed. This adds labor and other costs, since the heifers are on pasture and may not be easily accessible.

The authors thank D.J. Walstrom and W. S. Swafford, Pfizer Inc., for technical assistance and research funds, and the J. Ruch and A. Van Groningen Dairies of Visalia for providing animals and facilities; and they wish to acknowledge the use of data presented at the 81st Annual American Dairy Science Association Meeting, 1986.