Best-practice first summer drenching strategies

Dr David Rendell, former veterinarian and founder with Livestock Logic, Hamilton, Victoria.

How do sheep producers progress from the somewhat blunt ‘No Christmas dinner until all mobs are drenched’ to the more nuanced and, in my experience, clearly achievable mob-by-mob first summer drenching (SD1) strategy?

This involves worm egg count (WEC) testing every mature-age mob at hay cutting time. For most of Victoria, that is November. Why then? This is when sheep faeces start to harden. Worm larvae survival over summer in faecal pellets is around 20 times higher in harder pellets than in soft, unformed faeces.

If the WEC is high, you should promptly drench every sheep in the mob. If the mob will not be moved to a low-worm-risk paddock after drenching, consider scheduling a second drench in six weeks. For high-worm-risk mobs, no Christmas dinner until a second drench is given!

If the WEC is moderate, you can safely delay the SD1 drench until December. The later the SD1 is given, the less likely the mob will require a second summer drench, which reduces the risk of selection of worms with drench resistance.

In mobs with a low WEC, it is advisable to repeat the WEC in December. If two WECs a month apart are both low, you can with some confidence, safely forgo SD1 in that mob and enjoy your Christmas dinner with sanctimonious satisfaction.

I hear the screams ‘All this monitoring would cost a fortune’. Well, what are the costs? Table 1 provides a breakdown.

Mob size WEC cost (cents/head)
WEC cost (cents/head)
2 WECs
Savings/mob for 2 WECs and no SD1 ($)
Drench A
Savings/mob for 2 WECs and no SD1 ($)
Drench B
50 0.80 1.60 -45.00 -10.00
100 0.40 0.40 -10.00 60.00
200 0.20 0.40 60.00 200.00
300 0.13 0.27 130.00 340.00
400 0.10 0.20 200.00 480.00
500 0.08 0.16 270.00 820.00
600 0.07 0.13 340.00 760.00


  • WEC laboratory cost: $30.60; labour cost of collection — $9.40 per mob
  • Drench A: $90/L; drench B: $200/L; both at a dose of  one mL/10 kg and ewes 65 kg maximum weight; labour cost: 10 cents/head

Table 1 shows the costs of doing one or two WECs (first two columns). The cost per head decreases as mob size increases. The last two columns show the savings per mob that would be made if two WECs were conducted and no drench is required. The savings are higher if you would have had to drench with an expensive option (drench B). Drench A is valued at $90 per litre and drench B at $200 per litre. For small mobs (120 or less), the cost of doing two WECs to determine if it’s safe to forgo SD1 is higher than the cost to drench. However, if drench resistance is an issue (as it is for many farms) and you need to use a more expensive drench type (drench B), there are cost savings from not having to drench even for small mobs (60 or more). Importantly, reducing unnecessary drenching is critical for managing drench resistance.

Even in mobs of 100 that are still given an SD1, the maximum extra cost of this nuanced SD1 strategy is relatively low, at 80c per head, which diminishes to less than 20c in mobs of more than 400.

A typical property may end up WEC testing two-thirds of mobs twice and forgoing SD1 on at least one-third of mobs. In this scenario, you’ll save money if using the dearest drench where mobs are larger than 130 ewes, and if using the cheapest drench where mobs are larger than 300 ewes.

Quite apart from the modest cost and potential substantial savings of a mob-by-mob strategy is the satisfaction of managing your sheep well, while having a real impact on reducing your property’s level of drench resistance.

When drenching mature-age mobs with moderate WECs, consider leaving five to ten per cent of the fattest individuals in the mob undrenched. In my experience, on numerous farms over many seasons, this did not cause any discernible worm control problems, and research indicates that it can have a big impact on reducing drench resistance.

Although there are sound reasons for conducting a WEC before any drench between now and Christmas, there are some exceptions. For example, it may well be prudent to just promptly drench mobs of 2020 drop lambs grazing paddocks where ewes have lambed down that have not been drenched for six weeks. Similarly, if the two or three lowest-worm-risk mobs on a property are WEC tested and the results are all very high, you might consider promptly drenching all mobs. On the other hand, if the two or three highest-worm-risk mobs are WEC tested and the results are all very low, consider delaying WEC of all mobs until December.

I have quite deliberately avoided specifying WEC levels that would trigger drenching, because they vary enormously throughout Victoria. These levels are best developed for each farm by an experienced vet who is familiar with your farm results over a few seasons and has knowledge of the likely worm species present on your farm.

Guidelines for collecting samples for worm egg counts:

  1. Submit samples only if, when first picked up, they are warm when squeezed; all those that are not should be discarded.
  2. Collect at least 20 and preferably 30 samples per mob, regardless of mob size.
  3. Where samples are mixed before submission, collect similar volumes of each sample.
  4. Ensure that lamb and ewe samples are placed in separate labelled containers because, even if they are running together, they are likely to have quite different WEC results.
  5. Immediately on collection, either refrigerate samples or place them in an air-tight container.
  6. Before leaving the paddock, label the sample legibly.
  7. Submit the samples to a laboratory that participates in regular interlaboratory quality assurance.

Refer to the article on WormBoss — this tool provides critical levels for your region, where to get WECs done and other valuable worm control information.

Page last updated: 21 Jul 2023