Survival of infective Ostertagia ostertagi larvae on pasture plots under different simulated grazing conditions

This study was carried out to examine the survival of infective Ostertagia ostertagi larvae (L-3) on pasture under different simulated conditions of g razing, i.e. mixed grazing of cattle and nose-ringed sows, or grazing by ca ttle alone. Standardised pats of cattle faeces containing O, ostertagi eggs were deposited on three types of herbage plots, which were divided into zo ne 1: faecal pat; zone 2: a circle extending 25 cm from the edge of the fae cal pat; zone 3: a circle extending 25 cm from the edge of zone 2. For "tal l herbage" (TH) plots, the herbage in zone 2 was allowed to grow naturally, while the herbage in zone 3 was cut down to 5-7 cm fortnightly, imitating a cattle-only pasture. For "short herbage" (SH) plots, the herbage in both zones 2 and 3 were cut down to 5-7 cm fortnightly, imitating mixed grazing of cattle and sows. The grass in the "short herbage and scattered faeces" ( SH/SF) plots were cut as for SH plots, and the faeces were broken down 3 we eks after deposition and scattered within zone 2, imitating the rooting beh aviour of co-grazing sows. Five faecal pats from each plot group were colle cted on monthly basis, along with the herbage from zones 2 and 3 cut down t o the ground. Infective larvae were then recovered from both faeces and her bage. The numbers of L-3 recovered from zone 1 were higher in the TH plots than in the other two groups and, furthermore, the larval counts from SH pl ots were always higher than from SH/SF plots. The three groups followed a s imilar pattern during the season regarding numbers of L-3 in zone 2, and no clear patterns between plot types were obtained. The presence of L-3 in zo ne 3 was almost negligible. Important differences were seen throughout the study from the biological point of view; more L-3 were able to survive in f aeces on the TH plots, presumably reflecting a better protection from heat and desiccation compared to those in the other plots. The overall results s upport the idea that mixed grazing of cattle and pigs favour the reduction of O. ostertagi larval levels in pasture. This reduction is mainly due to t he grazing behaviour of pigs, which by grazing up to the very edge of the c attle faeces, will either expose the larvae in faeces to adverse environmen tal summer conditions or ingest cattle parasite larvae, or both. (C) 2001 E lsevier Science B.V. All rights reserved.