Strategies for the avoidance of faeces by grazing sheep.

Experiments were conducted to investigate which environmental cues were used by sheep when discriminating against patches of pasture contaminated with faeces. The influence of the spatial distribution of contaminated patches and the parasite infection status of sheep on avoidance of contaminated patches and ingestion of parasite larvae was also investigated. In experiment 1, sheep infected with the parasite Ostertagia circumcincta were given the opportunity to graze in uncontaminated or aggregated contaminated patches. Patch contamination comprised of either faeces from sheep infected with O. circumcincta larvae, faeces from uninfected sheep, or O. circumcincta larvae only. Infected sheep discriminated against faeces from parasite-infected animals and faeces from uninfected animals equally. Sheep did not discriminate against patches contaminated with parasite larvae only. In experiment 2, sheep infected with O. circumcincta and uninfected sheep grazed experimental plots with differing spatial patterns of faecal-contaminated patches, allowing animals the opportunity to forage in contaminated or uncontaminated patches of herbage. Plots were also grazed by infected and uninfected animals that were fistulated at the oesophagus to enable the collection of ingested herbage. Sheep spent a greater proportion of their time foraging in uncontaminated patches than in contaminated patches. Where patches were highly aggregated, infected animals spent a greater proportion of total grazing time in uncontaminated patches than did uninfected animals, and grazed uncontaminated patches for longer on each sampling occasion. On grazing plots where all patches were contaminated, the difference between the numbers of larvae isolated from pasture herbage and ingested herbage was greatest for infected animals. In this situation, infected animals avoided parasites most. On grazing plots consisting of both contaminated and uncontaminated patches, the difference between the numbers of larvae isolated from pasture herbage and ingested herbage was greatest for uninfected animals. In this situation, uninfected animals were most effective at parasite avoidance as they consumed fewer parasite larvae relative to what was available on pasture.