Although a drench may remove a worm burden, the pasture will remain ‘wormy’ (carry worm larvae) for some time, and unless the cattle are moved, they are likely to pick up more worms. Extra drenching can often be avoided by planned pasture management.
It will not always be practicable to keep cattle off particular pastures for the time needed to become ‘sterile’ of worm larvae, but even partly-prepared pastures will usually be suitable for cattle of less worm-susceptible classes.
Figure 1. High larval contamination of pastures can be managed with rotational grazing and drenching of affected animals. Image courtesy of Jenny Cotter
However, spelling usually involves the absence of cattle for some weeks or months. An ideal management tactic to avoid production loss is to rotate with sheep, as cattle and sheep share few worm species. (Unfortunately, goats and alpaca can carry the worms of both sheep and cattle, and so are not suitable for between-livestock pasture rotations.)
Pasture spelling is based on the length of time required for most worm larvae to die off, it depends on the climatic zone, the season, pasture type (open or upright plants) and pasture length. For example, in winter rainfall regions, some L3 can survive on the pasture for 4 or 5 months during cool winter periods. Survival periods in this environment are much shorter under hot summer conditions, and on dry pastures most larvae die off within a few weeks at most.
Some worm types have larvae that are better at surviving in the environment than others, for example, some larvae of the small brown stomach worm (Ostertagia ostertagi) can survive through dry summer periods in dung pats. This means that pastures for grazing in autumn should be chosen on the basis of their likely contamination with worm eggs in late spring.