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Worm control under irrigation in Tasmania

by Paul Nilon, Nilon Farm Health, Perth, Tasmania

November 2015

Irrigation is frequently used to provide high quality summer and autumn forage for finishing lambs. Irrigated brassicas are mostly worm free because of the cultivation and “spelling” during the growth phase. Irrigated legumes (lucerne and clover) are often lower worm risk for a number of reasons: winter grazing is limited by their low growth; hay making and seed harvesting reduce spring burdens and their high protein may counteract the effect of worm burdens. Also, most worm larvae are found near the base of plants, with relatively few moving more than 10 cm up the plant—those crop plants that are taller than grasses will tend to have fewer larvae available for grazing.

The biggest worm problems occur on irrigated grasses and grass-dominant perennial pastures. Short-rotation rye grasses become heavily contaminated in their second year, but as they are replaced after two years the cycle is broken. Irrigated perennial pastures have the potential to be worm-fests year round, almost endlessly. Moreover, people use grass-based pastures because of their winter growth and so these pastures are grazed year round. Thus, integrating conventional grazing management into the system is difficult at commercial stocking rates.

Irrigation in the summer and autumn extends the seasonal availability of our most pathogenic species: Trichostrongylus vitrinus (black scour worm). This is exactly what we see in wet summers, e.g. 2010 and 2011. Additionally, we are starting to see some cases of barber’s pole worm (Haemonchus contortus) in places where it was not previously a problem.

Unfortunately, there is a dearth of good trial work to know what works and what does not with regards worm control on irrigated pastures. So, the following suggestions are based on things being tried by clients, and arguments from first principles.

Source: The epidemiology and control of gastrointestinal parasites of sheep in Australia. Edited by A.D. Donald, W.H Southcott and J.K. Dineen, Division of Animal Health, CSIRO 1978.
Figure 1. The availability of infective larvae of the winter scour worms on pasture, showing a peak around June–August (depending upon the timing of the autumn break)


The “Stetson Hat” image of larval availability, above, shows how larvae peak in winter and decline over spring. Under irrigation (and during wet summers), the spring fall-off is less pronounced if vulnerable classes (lactating ewes and weaned lambs) keep recycling worms onto pastures that support rapid larval maturation. The trick should be to get the most out of the spring die-off of worm larvae on pasture by clever use of grazing management and drenching. In no particular order things to consider include:

  • Keep clean pastures clean. Sheep should be drenched onto new plantings almost regardless of their egg count. This is particularly important for new, short-rotation rye grass. However, drenching onto clean pastures can accelerate the development of drench resistance. Therefore, when the sheep are moved to another paddock treat them with an effective drench that is from a different group to the drench used when the sheep first went onto the paddock.
  • Monitor religiously. This is the only way to know what contamination is occurring, and what may already be there. For new pastures be prepared to drench at low triggers to preserve the good parasite status.
  • Enhance the natural spring larval decline:
    • Grazing with cattle is a robust way—as little as 8 weeks grazing in the late winter and spring will help. As these are likely to be finishing steers, a drench will serve them well and help lower the levels of Trichostrongylus axei (the only significant worm of both cattle and sheep) on the paddock.
    • Avoid lambing on irrigated pastures that will be grazed by lambs post-weaning.
    • If you must lamb on these paddocks, consider using a long-acting pre-lambing anthelmintic each second or third year. This will aid the paddock rather than the ewes, but they will also benefit. The limitation is that lambs graze early and may provide enough contamination to kick-start infection post-weaning. Capsules provide longer activity against black scour worm than injectable moxidectin. Importantly, monitor your LA drench throughout its payout period to see how it is going.  
      ‚ÄčEditor’s note: recent evidence shows that lambs suckling from ewes who received a pre-lambing long-acting moxidectin injection receive a low dose through the milk, which can speed up the development of drench resistance.
    • Consider a “smart grazing” system to get the contamination down on heavily contaminated paddocks. This will provide safer grazing in the autumn and winter.
    • Long-acting drenches can be used on part of the finishing lamb drop. The stand-by has been the white drench capsule, because of its shorter ESI. To date, it has worked well, but monitor to make sure it is working on your place. Capsuled lambs need to be well identified lest they are sold within the ESI.