FUNGAL DISTRIBUTION AND LIVESTOCK DEFOLIATION PATTERNS IN PASTURE ECOSYSTEMS, AND THE DEVELOPMENT AND CONTROL OF DIETARY-DEPENDENT DISORDERS

Abstract

The fungal saprophytes, Pithomyces chartorum and Fusorium culmorum, have similar distribution patterns in grazed-pasture ecosystems. Spore loads (and toxin levels) of each are highest at the base of pastures and rapidly decline up the profile. But variation is considerable from site to site within a pasture, with the highest levels associated with high N (viz: urine-patch) sites. In vegetative tillers of ryegrass the fungal endophyte, Acremonium lolii, has a similar vertical distribution pattern to the above-mentioned saprophytes, the greatest concentration being in the leaf sheath component in the pasture base. It also develops better within ryegrass at high N sites. Grazed-pastures are not uniformly defoliated by livestock during summer and autumn - urine-patch sites are grazed more frequently and intensively and dung-patch sites less frequently and intensively than the remainder of a pasture, especially when set-stocked. The close grazing which occurs at urine-patch sites, especially in grass-dominant pastures can, therefore, contribute disproportionately to the acquisition of fungal toxins by livestock. Observations of field outbreaks and results of grazing experiments show that the risk of outbreaks of ryegrass staggers in sheep is greater under set-stocking and during the latter stages of defoliation of a pasture in a rotational system. When necessary defoliation, especially at urinepatch sites, and hence acquisition of fungal contaminants (and toxins), can be controlled adequately by a rapid rotation in which stock are moved daily. Moving stock less frequently does not control defoliation or the development of dietary-dependent disorders. Keywords: Pasture fungi, saprophytes, ryegrass endophyte, distribution patterns, feeding behaviour, toxin acquisition, dietary-dependent disorders, facial eczema, ryegrass staggers, ill-thrift, infertility, grazing management and control.