The seasonal incidence of helminth infestation in sheep on newly established irrigated pastures was determined by the slaughter of pairs of tracer lambs exposed for periods of approximately 33 days from October 1968 - July 1970 and of sets of 3 lambs from March 1971-May 1973.
It took several months for the infestation to become established on the pasture, but thereafter, of the various species present, Haemonchus contortus was most prevalent. In general, peak burdens of this species were recovered from January-May or June, while marked inhibition of larval development was evident from April - August. Ostertagia circumcincta usually occurred in increased numbers from April-October, and larval development was inhibited during July and August. Although the sizes of the burdens varied considerably from year to year, Trichostrongylus spp. Were present mainly from April-August.