(Previously published on Wormboss, Tests and Tools, Sheep)
Managing the frequency and intensity with which livestock graze pasture reduces the number of infective larvae ingested from the pasture each day.
Effective grazing management will reduce the exposure of vulnerable sheep to larvae on pasture and the need for chemical (drench) intervention and at the same time provide nutrition to allow sheep to better deal with parasites.
Grazing for worm control entails any strategy to provide ‘low worm-risk’ pastures for the most susceptible sheep.
The aim of grazing management is to have vulnerable sheep exposed to fewer larvae on pasture.
Grazing management for worm control entails a range of strategies:
- Preparation of low worm-risk paddocks in summer rainfall areas
- Smart grazing in winter rainfall areas
- Rotational Grazing
- Alternating grazing of sheep with cattle
What are the key components and concepts of grazing management strategies?
There are a number of approaches to rotational grazing that differ in the time used for graze and rest periods.
The graze period
This is the time livestock spend grazing a paddock. Controlling the amount of time a paddock is grazed is just as important as leaving adequate rest periods between grazing.
The rest period
The time in a rotational grazing system when the stock are not in the paddock to ‘break’ the worm life cycle. Most sheep parasites complete their life cycle only in sheep with a few exceptions. Thus, grazing with cattle represents a period of rest from sheep grazing, during which the number of infective larvae on pasture declines because larvae continue to die.
The auto-infection period
This is the time between eggs being deposited on pasture (in the sheep’s dung) and when the larvae that have developed from those eggs appear on pasture ready to re-infect sheep. This is 4–10 days; the shorter period when temperature and moisture conditions are ideal for the particular worm species. Grazing animals on a paddock for a time shorter than the auto-infection period allows them to continue through further paddocks, picking up infection at a slower rate from larvae already on the pasture.
Larvae location on pastures
Knowing where the worm larvae are found on the pasture and what influences their distribution and survival on pasture is important in grazing management.
Effect of pasture species on worm larvae distribution and survival
Different pasture species can influence the sward microclimate or possess a sward structure that is unsuitable for the parasites and therefore affect the larval population and number of L3 larvae ingested by grazing livestock.
Research shows that the differences between herbage species alter larval density in various height strata. Scales et al. (1994) and Moss and Vlassof (1993) observed fewer larvae on chicory and lucerne than on various grasses. Niezen et al. (1993) reported lower faecal egg counts from lambs grazing Yorkshire fog or ryegrass compared to common bent or tall fescue.
It has been suggested that greater numbers of larvae can overwinter on forage species with heavy, dense growth characteristics compared to those with a more open type of growth.
In addition, pasture composition can also affect the movement of infective larvae and the use of alternative forages can offer further means of parasite control. For example, chicory has been reported to lower faecal egg counts in lambs and improve weight gains and legumes such as red clover, have been shown to affect larval migration, thereby lowering infection rates.