After being exposed to infestation for 4 or 6 weeks, pairs of tracer calves were slaughtered to determine the seasonal incidence of helminth infestation in cattle on natural pasture. \
Haemonchus placei was recovered from March-July and from November-February, and peak burdens were recorded during December and January. There was marked inhibition in larval development from April-July.
Trichostrongylus spp. were recovered from March-June and from November-February, the greatest number of worms being recorded during December.
Cooperia spp. infestation occurred from March- August and November-February with peak burdens during April-June and December. From February-August more than 50% of the worms recovered were in the 4th larval stage of development.
The greatest number of Oesophagostomum radiatum were recovered from June-January. Longistrongylus sabie and lmpalaia tuberculata, parasites normally found in impala, were recovered from many of the survey animals, this being the first record of their occurrence in cattle.