When should lamb drenching start?

In most New Zealand farming systems, young lambs will require some level of drench input to remove internal parasites. There is variation between farms, districts and forage systems as to the ‘ideal’ time to start drenching lambs.

For many farms this is a ‘calendar date’ and is related to other management activities that are happening at the time, such as docking, a first draft of milk lambs, or weaning. Sometimes it’s a useful exercise to stand back and check that your current policy is still ticking the boxes with regard to productivity, profitability and sustainability.

Here are some things to think about when assessing your lamb drenching options:

Docking drench:

While this practice is fairly uncommon nowadays it is still routine on some farms. It is unlikely to have a positive cost-benefit for the following reasons:

  • A transient scour is often seen in very young lambs, from early infection by a tiny thread-like worm called Strongyloides. This worm penetrates through lambs’ skin and mucous membranes and can be transferred in the ewes’ milk. It is not very pathogenic and lambs very quickly expel the parasite on their own. Thus, an apparent ‘response’ to a docking drench in otherwise healthy lambs is often a self-limiting Strongyloides infection that was about to come right on its’ own!
  • The growth and health of very young lambs is not affected by worm larval intake as much as you might imagine. The immune system of lambs in their first few weeks of life does not recognise incoming worm larvae as foreign invaders and makes no effort to expel them. The scouring, dagginess and reduced weight gain we associate with worms in lambs happens as the lambs’ immune system starts to try to repel the worms; a process which starts sometime after docking. Removing a few worms that have established by docking is unlikely to be very beneficial from a production viewpoint.

Pre-weaning drench:

In some parts of the country it is normal practice to drench lambs after docking, but before weaning; say in November.

Where the cost-benefit of this practice has been studied in terms of lamb growth, the results are variable. Even lamb mobs with high faecal egg counts (FECs) do not necessarily show a liveweight gain response to a drench. There are probably a number of farm factors that come into play here.

Infective worm larvae are concentrated in the bottom 2cm of the pasture sward. If this is where lambs are grazing, then they are more likely to be picking up worm burdens that will limit their production as the weeks go by. Their mums will also be under pressure for feed and won’t be producing as much milk as a well-fed ewe. An early drench may be well worthwhile in this situation.

Though early growth rates of lambs may be unaffected by high FECs, but they will certainly be creating pasture contamination, and the hatchability/survival of eggs and larvae from lambs at this time is high.

Thus, drenching lambs prior to weaning may act to reduce summer pasture contamination on the lambing platform. The benefit from this depends on the farm system, but where lambs will be weaned back onto lambing country, a pre-weaning drench may be a helpful tool.

There are other management reasons for drenching pre-weaning on many farms, (such as making lambs less attactive to fly) but if faecal egg contamination of pasture from lambs is a concern, faecal egg counting is an option – one thing we do know is that egg counts in suckling lambs vary greatly between farms and between years. So, the timing for that first drench can vary too and can be later than we expect.

The addition of a pre-weaning drench to an existing programme needs to be evaluated from a resistance perspective, especially if you don’t plan to change anything else. Have a chat to one of our sheep enthusiasts to discuss your own situation.


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