Managing larvae on pasture

By Rochelle Smith

Wouldn’t it be great if we could see parasite larvae on the pasture?! 

Do you ever start to feel like your lambs keep on looking wormy in autumn, despite a drench? 

This could be the start of drench resistance or it may be a symptom of management decisions made much earlier in the season.  Either way, it warrants further investigation.

The immune response begins as soon as the animal eats infective larvae on pasture and continues through the development of the larvae into an adult and can continue as long as the parasite resides in the gut. The immune response requires energy and this energy will be diverted from growth into the response resulting in reduced performance. This results in reduced appetite, the gut becomes leaky (nutrient loss) and less absorptive (less nutrient in) reducing feed conversion and lowering weight gains.  This is all before you see diarrhoea, weight loss or high FEC.

Reducing larval intakes is key so how do we keep these down?

  1. Reduce larva on pasture

Drench – Maintaining 28 day drench intervals throughout the lamb season is a tool to reduce faecal egg output.  If drench intervals are stretched, more eggs may be released, increasing the amount of infective larvae developing in autumn.

Stock management – Vacuum pastures with older stock and other stock species.  Consider reducing stocking rate also.

Pasture management – cropping can create ‘clean’ new pastures.  Paddocks last cut for hay will also have lower contamination.  Certain pasture types e.g. Lucerne, chicory are less suitable for larval development and should pose less risk.

  1. Reduce intake of larva on pasture

Pasture management – high pasture residuals

The exception: Occasionally in drought like conditions larval development and survival will be slowed and the need to drench may be reduced.  You could consider trigger drenching based calculated need.  To help with this consider:

  • FEC – estimate of adult worm burden, and an indicator for future paddock larval contamination
  • Live-weight (or changes in weight gains)
  • Pasture length – lower grazing leads to higher larval intakes. Also if pasture is short nutritional stress could reduce the animals’ ability to combat parasite infection.
  • Previous paddock use – to help determine likely degree of larval ingestion

When lambs still appear wormy after a drench it could be the result of drench resistance.  Or it could be that the larval challenge we have set up in autumn is simply too great.  Have the discussion with your vet about testing for drench resistance and for advice on managing parasite burdens.