Seasonal availability of nematode larvae on pastures grazed by cattle in New South Wales.

Trends in the numbers of infective menatode larvae on pasture plots contaminated by cattle at different seasons of the year were defined in 3 different climatic regions. The main nematodes were Ostertagia ostertagi, Trichostrongylus spp, Haemonchus spp and Cooperia spp. On the North Coast of New South Wales with a sub-tropical climate, the numbers of infective larvae of all 4 nematodes rose rapidly to peak levels soon after each seasonal period of contamination began, then fell quickly within a few months. On the Central Coast of New South Wales, the trends were similar to those on the North Coast, except that the larvae persisted on the pasture for a much longer time. On the Northern Tablelands of New South Wales, where temperatures were much colder than on the coast, larval development was slower and major peaks of larval availability did not occur until early spring. These different seasonal trends in each region were considered to be related to the climatic differences between the regions. On pastures which were contaminated continuously, larval numbers reached maximum levels in mid-winter on the Central Coast and in early spring on the Northern Tablelands. It was concluded that the majority of these larvae were derived from the contamination of pastures in autumn and winter. Subsequently in summer, a rapid dying out of larvae was observed in all the regions, probably due to the effect of hotter weather. The studies suggest that a reduction in the contamination of pasture with nematode eggs in autumn and winter could result in pastures carrying fewer larvae and thus form the basis of effective worm control programs for cattle.