Parasitic gastroenteritis (PGE), largely attributable to the abomasal parasite Ostertagia ostertagi is a cause of major economic loss in the cattle industry. It has been suggested from studies on infected housed animals, together with pair fed controls, that the characteristic loss of weight occurring as a consequence of PGE is due mainly to loss of appetite, (Dargie, 1980). Thus in the present study the intention was to measure the effect of parasitism on intake under natural grazing conditions.

2 groups of 20 Freisian steer calves (175 kg at turnout), were grazed from May to September 1984 on adjacent 5 acre paddocks which had been contaminated the previous autumn by cattle Infected with 0. ostertagi. The allocation of calves to each group was such that the mean weight and range of weights within each group were similar. At turnout each calf in group A received a (Morantel slow release bolus), group B being untreated controls. The calves were weighed and sampled regularly for faecal egg counts.