Internal parasitism of steers grazing extensively at different stocking rates.

Eighty-four first-year grazing Holstein-Friesian steers were observed through 2 years on 40 ha of reclaimed unfertilized grassland with the objective of investigating the influence of stocking rate (SR) on internal parasitism under extensive grazing conditions. Three groups of steers were grazed each year at low, medium and high SR in separate paddocks (0.53, 1.05 and 1.55 steers/ha). The faecal egg counts of predominantly Cooperia and Ostertagia spp. were influenced by SR in the first year (p < 0.05) with maximum geometric means of 135, 182 and 217 epg in the low, medium and high SR groups, respectively, and part of the second year (p = 0.08) with maximum geometric means of 88, 134 and 195 epg. In the second year, all steers were treated with levamisole by 26 July due to diarrhoea, being particularly severe at high SR and 7 steers were withdrawn from this group at the same time. Serum pepsinogen levels indicated a higher rate of uptake of Ostertagia larvae in higher SR groups in the second season before treatment (p < 0.01). The mean weight gains (+/- s.d.) for the first year were 630 +/- 58, 341 +/- 97 and 300 +/- 151 g/day in the low, medium and high SR groups, respectively, and in the second year the corresponding figures were 565 +/- 109, 357 +/- 135 and 247 +/- 129 g/day. Although the SRs were much lower than commonly found on improved pastures, we conclude that feed on offer was a limiting factor in late season and secondly, that the levels of exposure and uptake of nematodes in steers were affected by SR. The levels of parasitism at the medium and high SR were high enough to have affected production.