It has been repeatedly demonstrated that one of the most important factors in maximising growth rates of young animals is minimising the effects of worms. It is now known that high worm burden pastures cause reduced intakes. This is partly due to the reduced appetite in animals with subclinical worm burdens. However, there are some other less well understood factors which suggest animals can detect the ‘wormy pasture’ and do not want to eat it. With this in mind it is obvious that if we reduce pasture contamination, lambs and calves will grow better.

Experimentation has shown that un-drenched lambs challenged with 1000 worm larvae per day will still grow considerably better than lambs challenged with 5000 larvae per day and drenched monthly. Lambs fed on clean pasture will out perform lambs on contaminated pasture even when they are drenched every 21 days.

Controlling Pasture Larval Challenge

Forage and Pasture Factors
• Infected larvae develop the best on Ryegrass, and the highest density of larvae can be found on high density pasture.
• Lambs perform best if drenched and moved onto crops.
• Lamb paddocks tend to become heavily contaminated. Mixed grazing of lambs over the summer, though logistically challenging, works very well to enhance lamb performance, reduce pasture contamination and improve pasture quality.
• Nutrition is very important in building and maintaining gut immunity to worms, especially in ewes around lambing time. Protein levels seem to be the key factor and most pasture does not meet the nutritional demands of twin baring ewes at this time.

Animal Factors
Mixed grazing is the optimal way to reduce pasture challenge. Grazing lambs with a few mixed aged (MA) cows or dairy heifers with some MA ewes, drops the larval challenge as the older animals effectively hoover up the infected larvae but are unaffected by them. Ideally, mixed grazing means, using animals of a different species and of lower risk. If this is not possible then the most different stock class available is best e.g. reducing the risk to lambs by grazing with terminal ewes.

15% of the mob are responsible for 50% of the challenge. This is because some animals are predisposed to worms and favour worm development; it is extremely difficult and not practical to identify these animals but does explain why targeted treatments work so well.

Drenching factors
Most worm control programs are centred on drenching, but a balance needs to be found between protecting productivity, maintaining refugia and minimising the development of resistance.

The priorities of farmers, vets and industry are not always the same, but there are clear guidelines to help with the sustainability of drenching programs, such as quarantine, knockout and exit drenching. Targeted specific drenching of affected animals and specific treatment of specific worms e.g. Genesis Ultra for Barber’s Pole are other important control measures. Increasing drenching intervals in the face of challenge is not advised as recent research has shown that extending drenching intervals from 28 to 35 days can result in a 3-fold increase in pasture challenge and extending to 42 days can result in a 6-fold increase. Of course increasing drenching intervals when there is no challenge is a good way to save time and money, but the only clear way to know the challenge level is to perform repeated FECs.