Internal parasites are a part of our farm systems. Reducing worm larval challenge and the number of worm larvae consumed each day improves lamb growth rates and increases milk production.¹ So, how do you reduce something you cannot see?

Egg hatch and larval development require rainfall and speeds up as temperatures rise, much like grass growth. Eggs can develop to larvae in one week under ideal conditions but are more likely to be two to four weeks in a New Zealand summer. Pasture larval challenge is a relative concept, considered low, moderate or high. It is difficult to measure in a useful manner and can rise quickly after rainfall when temperatures allow.

Considering what has been grazing the paddock in the previous three to four months is useful to gauge potential worm challenge. In general, cattle and deer worms do not survive and breed in sheep and vice versa, although nothing is absolute. Adult sheep use their immune system to reduce the number of worms completing development to adulthood and lower the survival of their eggs. This, however, comes at an energy cost to the sheep.

Worm larvae on pasture are susceptible to ultraviolet light and high temperatures. Larval survival is much shorter in the summer than in the winter, so it takes three to four times longer to get the same reduction in larval population in the winter.

It is good practice to take every step possible to reduce the larval challenge to improve stock performance and profitability. Worm control, without solely relying on a drench, is easier to achieve in mixed species farm systems, using specialist forages for lambs post-weaning, or where a large proportion of young stock are sold at weaning. In general, you want to begin with fewer larvae by preventing or reducing egg deposition and preventing the larvae on pasture from completing their lifecycle.

To lower the worm challenge in lambing paddocks, reduce the number of lambs grazing in these areas in the autumn and replace them with adults or another species. Effective drenching prevents egg deposition for about 21 days but this effect is greatly reduced when worms can survive. When considering weaned lambs, paddocks where ewes have lambed and grazed over lactation will be wormier than paddocks where cattle have grazed over the same period.

Due to the longevity of larval survival over the winter as they tolerate frosts well, leaving pasture ungrazed may not result in a reduction over this period. These larvae may need to be harvested (eaten) by an adult sheep or another species. Silage and hay paddocks have a lower larval population after spelling and cutting. Specialist forages grown for lambs can provide low worm challenge, improving growth rates and reducing the need to re-drench.

Discuss strategies for reducing worm challenge with your local PGG Wrightson Technical Field Representative.

¹ Beef + Lamb New Zealand. (July 2019). Wormwise National Worm Management Strategy. Retrieved 5 April 2024 from https://beeflambnz.com/knowledge-hub/PDF/wormwiseresource-book.pdf.