The distribution of trichostrongyle infective larvae on pasture and grazing behaviour in calves.

The distribution of trichostrongyle infective larvae was investigated at three different times during the grazing season on a calf pasture in a district in the west of France. The grass was collected around the dated pats in several successive rings measured every 10 cm, and the larvae were extracted. The larval population was higher on August 20 than on June 10 or October 7. The main species were Ostertagia in June and October, Cooperia in August. Migrations were correlated with pat ageing for the two species; Ostertagia larvae migrated further than Cooperia ones. The distance of the calves' grazing location from the nearest refusal was observed at the same time. In August and October, one third of the observed grazing locations were upon the refusals, where larval density was maximum. Pat density and stocking rate increased during the grazing season, grass availability decreased, refusal area decreased after the August maximum. The observations give a better understanding of animal infection and are of great help for the interpretation of numbers of larvae near to and away from the pats.