On the persistence of infective Ostertagia ostertagi, Cooperia oncophora and Nematodirus helvetianus on pastures.

By grazing parasite-free susceptible calves on naturally infected paddocks for eighteen day periods, Nematodirus helvetianus was shown to survive over two winters on marshland pastures under Maritime climatic conditions in three successive years although the numbers surviving the second winter in two of the three years were low. Climatic conditions of below normal temperatures and little snowfall would appear to have a detrimental effect on the survival of Nematodirus over a second winter although large numbers did survive one winter of such adverse weather. However, under certain conditions, large numbers of Nematodirus can overwinter two years on pastures since over 12,000 worms were picked up by individual calves grazing paddocks in 1969 which had been vacant since 1967. The findings indicated that few, if any, Ostertagia ostertagi and Cooperia oncophora survive over a second winter under Maritime conditions. The sharp decline in numbers of infective Ostertagia and Cooperia on pastures during the first grazing season following pasture contamination observed in this and previous investigations indicate that few worms of these species survive longer than a year under Maritime conditions.