The role of parasite epidemiology in the management of grazing cattle

Epidemiology of ruminant helminths is the foundation on which strategic par asite control programmes are designed. Without this information one is not able to use anthelmintics to provide the optimal benefits for controlling b oth the adult worm and the pasture larval populations. The absence of strat egic programmes generally results in using anthelmintics at the convenience of the producer, which may have little if any impact on parasite populatio ns. The design of a strategic parasite control programme requires a knowled ge of the dynamics of egg shedding from the host and the resulting pasture larval populations. It is important to know if larvae are available when an imals are turned out onto pasture, when larval populations reach their maxi mal numbers and when they are induced to become hypobiotic. The goal is to keep pasture larval populations as low as possible. The use of pasture rota tion adds another dimension to control programmes. The longer a pasture is allowed to remain fallow, the lower the pasture larval burden will be when it is grazed next. However, when we use intensive rotational grazing, anima ls may return to the pasture about 28 days later, when the larvae resulting from the eggs shed in the previous grazing are infective. This practice fo rces cattle to eat all of the forage available, including the grass closest to the faecal pat, where most of the infective larvae are available. If we treated cattle before turning them onto a clean rotationally grazed pastur e, we should be able to control parasitism. Using a long-acting anthelminti c should enhance helminth control in rotationally grazed pastures and actua lly help to clean the pastures. Another grazing management practice is to a lternately graze different species. This programme with the strategic use o f anthelmintics should reduce parasitism in both host species. (C) 1998 Aus tralian Society for Parasitology. Published by Elsevier Science Ltd. All ri ghts reserved.