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 flooding or by spraying, on the gastro-intestinal parasites of calves, two contiguous plots, one flooded and the other sprayed, were grazed for 10 days by naturally infected animals. Faeces, grass and soil under pats were sampled for 17-20 weeks for larval counts. The experiment was performed 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 and 3.2 L3/100 eggs, respectively for Ostertagia. During the summer, development rates were less than 1%. The main differences were found in the vertical distribution of the larvae: spraying favoured migration on grass and flooding favoured migration in soil, this finding being similar for both genera. The spatial distribution, studied by dividing each plot into 18 subplots, was a little more regular on grass in the sprayed plot. We conclude that the infection risk for calves is lower in flooded pasture than in sprayed pasture.