Ewes’ immunity to worms can ‘slip’ in late pregnancy and early lactation.
Where ewes have lambed in good condition on appropriate feed levels, the productive impact of a worm burden at this time is minimal. If our barometer is the appearance of dags, we may feel like ewes are under pressure, though performance in terms of lactation and lamb growth is often unaffected.
Good feed levels after docking allow ewes to gain weight lost in peak lactation and make it easier for them to get back on top of any worm burden they may have accumulated. Drenching whole ewe mobs at docking has been shown to be a risk factor for drench resistance. However, this does not preclude drenching individual light ewes within mobs, and for productive reasons, we sometimes advocate drenching lactating hoggets at docking.
Ewes or hoggets which were treated with a long-acting drench product should have faecal samples taken to check the efficacy of the product used. If eggs are present, larval cultures allow us to determine the species of worm present, and whether there is a potential issue with drench resistance. An exit drench is an important strategy if you have used a long-acting pre-lamb product; the choice of drench and its timing will depend to some extent on your situation.
Lambs become ruminants and begin to get a lot of their nutrition from grass at around five weeks of age. The period after docking is the time where lamb growth rates start to diverge the most. This is driven initially by feed quantity and later by feed quality. I have never seen a farm lose control of quality before docking! And rarely in the few weeks after docking… so feed levels per se are really important for much of the lambs’ early growth period.
Where covers are too low, ewes and lambs compete for feed, and mum is always going to do a better job of foraging in this situation. Infective worm larvae are concentrated in the bottom 2cm of the pasture sward. If this is where lambs are grazing they are more likely to be picking up worm burdens that will limit their production as the weeks go by.
Where lambs develop high nematode egg counts early in life, their early growth rates may be unaffected, but they will certainly be creating pasture contamination. The hatchability of eggs passed by lambs at this time is high (much higher than the hatchability of eggs passed by the ewe over lambing). Thus drenching lambs prior to weaning may act to reduce summer pasture contamination on the lambing platform. The impact of 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, but if contamination from lambs is a concern, doing some faecal egg counts could be quite enlightening – one thing we do know is that egg counts in suckling lambs seem to vary greatly between farms and between years.