Abstract
In grazing systems, heterogeneous distributions of forage resources and faeces result in
localised accumulations of nutrients and parasites (both macroparasites and
microparasites), creating trade-offs between the costs of exposure to infestation or
infection and the benefits of nutrient intake. Each contact between livestock and faeces
in the environment is a potential parasite/pathogen transmission event. Thus,
herbivores must make foraging decisions in complex environments which will affect
their intake of both nutrients and parasites. However, the pattern of forage and faecal
resources in agricultural environments will also be affected by the grazing management
system in place. The aim of this study was to investigate the effect of grazing
management on the risk of infection/infestation to livestock. We used a spatially
explicit individual based stochastic foraging model to simulate livestock contact (both
grazing and investigative) with faeces in the environment. The model was
parameterised to simulate cattle grazing under three types of grazing management:
set stock (i.e. where sward growth and cattle intake are in equilibrium in a single
field); a two pasture rotation grazing system with increasing number of rotations; and
a rotational grazing system with two rotations and increasing subdivisions of the
pasture. Overall the amount of cattle contact with faecal-contaminated patches was
similar in both set stocking and rotational grazing scenarios, suggesting no difference
in the risk of infection or infestation between the different systems. However, the
timing and absolute amounts of peak contact varied greatly indicating that different
grazing management systems expose livestock to risks of different types of parasites at
different times of the grazing season. Intensive rotational systems with small pasture
blocks (especially the first grazing period) maximised livestock contact with fresh
faeces, and thus exposure to microparasites (e.g. bacterial pathogens). Livestock reentering
pasture blocks in rotational systems and set stocked livestock had the highest
contact with old faeces and thus have a greater risk of macroparasite transmission
(gastrointestinal nematodes). This study highlights how livestock management affects
the highly dynamic interaction between livestock and distributions of parasites in the
environment and thus the levels of livestock exposure to parasites and pathogens via
the faecal-oral route.
Year
2009
Category
Refereed journal