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Persistence on pasture of the infective larvae of nematodes parasitizing Maine dairy cattle.

Yearling calves infected with Ostertagia ostertagi, Cooperia oncophora, and Nematodirus helvetianus were allowed to graze on a 0.8 ha pasture plot contaminated during June and August 1977. In June 1978, the plot was subdivided into 10 subplots of equal size. Grazing in 5 of these plots was allowed, each for 1 month, during June, July, August, September, and October by groups of 2 parasite-free 16-week-old calves. The procedure was repeated in 1979 with the remaining 5 subplots. Each group of calves was allowed to graze for 2 weeks, then put into stalls for 2 weeks and killed and examined for worms. There was a good survival of the 3 worm genera over the winter period of 1977-78. In June, calves averaged 250 Ostertagia, 5,345 Cooperia, and 6,955 Nematodirus. Ostertagia and Cooperia were recovered from calves grazing in September and October. The 3rd genus, Nematodirus, also became less plentiful over the summer period, but persisted in calves grazing during September and October. This genus survived a 2nd winter on pasture and worms were recovered until July, 1979.