DEVELOPMENT AND DISTRIBUTION OF BOVINE TRICHOSTRONGYLE INFECTIVE LARVAE ON A PASTURE IRRIGATED BY FLOODING OR BY SPRAYING

To compare the ecological effects of irrigation of a pasture, by flood ing or by spraying, on the gastro-intestinal parasites of calves, two contiguous plots, one flooded and the other sprayed, were,orated for 1 0 days by naturally infected animals. Faeces, grass and soil under pat s were sampled for 17-20 weeks for larval counts. The experiment was p erformed once in the spring and once in the summer. In the spring the maximal development rates in sprayed and flooded plots were 7.8 and 12 .6 stage 3 larvae (L3)/100 eggs, respectively, for Cooperia and 3.5 an d 3.2 L3/100 eggs, respectively for Ostertagia. During the summer, dev elopment rates were less than 1%. The main differences were found in t he vertical distribution of the larvae: spraying favoured migration on grass and flooding favoured migration in soil, this finding being sim ilar for both genera. The spatial distribution, studied by dividing ea ch plot into 18 subplots, was a little more regular on grass in the sp rayed plot. We conclude that the infection risk for calves is lower in flooded pasture than in sprayed pasture.