Drench Longevity Study

July 2011

A new approach to target selective treatments for improved livestock parasite control

Lecturer and researcher Andrew Greer is applying the Scottish development of targeted selective treatments (TST) for parasite control, reducing anthelmintic drench usage by only treating those lambs and calves which need parasite control, not the whole flock or herd. This approach is only possible with radio-frequency identification (RFID) tags which mean that animals can be automatically drafted at weighing according to a pre-determined treatment threshold. Greer believes TST will prolong the useful life of current drench chemical families, and any new active ingredients, by slowing down the development of drench resistance in the internal parasites.

Ashley Dene demonstration farm, located about 11 km west of Lincoln University and 3 km from Burnham, is well known through extension work, field days, agricultural science research literature, and the personal experience of generations of students and staff members who have studied there and worked on aspects of its operation. The 355 ha farm is devoted to sheep research programmes managed by the Agriculture and Life Sciences Division. The average rainfall is 620 mm, and with over 900 mm potential evapotranspiration/year and soils that are mainly shallow and stony, the farm is an ideal site for dryland research and teaching.

The farm runs the Lincoln University registered Coopworth ewe flock.

Andrew Greer has also been working with young dairy cattle on four dairy farms near Lincoln.

Sheep and cattle farmers need to use anthelmintic drugs to control gastrointestinal nematode infections in their livestock, commonly called worms. But the intensive use of such drenches since mid-20th century has seen parasites develop resistance to the common treatments. For instance 80% of NZ farms now have worms resistant to benzimidazoles (white drenches) and some are also developing resistance to levamisoles (clear drenches) and the ivermectins. Two new actives with different chemistry have been developed and released on the market recently, but they are much more expensive than the three older families.

As the older drench families become ineffective, researchers are looking for ways to prolong their effectiveness and to slow down the development of resistance to the new products.

Potential savings are huge: at present internal parasites cost sheep farmers about $700 million a year in lost production and drench; while about $60 million is spent on cattle drenches.

If animals which really need drenches are treated, and not every animal in the flock or herd, then the development of anthelmintic resistance is slowed through providing a parasite population that is not exposed to the drug, effectively diluting the frequency of anthelmintic-resistant genes within a parasite population. The simple proposition put forward by Andrew Greer is that treatment decisions can be based on performance, using the assumption that the animals which continue to perform well (utilise their feed and gain weight) are not suffering from parasitism and therefore unlikely to need a drench with anthelmintics.

However there are other animal and environmental factors which must be included.

When he was doing post-doctoral research work at Moredun Research Institute in Scotland some five years ago, he was exposed to the TST concepts and contributed the term “happy factor”, which predicts a growth rate for sheep or cattle which would be considered acceptable on that farm in those climatic conditions. “In effect it is how happy those animals are, which is a catchy and memorable term,” he said. Therefore each animal will have a current performance base expressed as a proportion of the theoretical optimum, known as its happy factor efficiency value. It is a measure of the animal’s ill-thrift.

With the maximum happy factor expressed as 1, the farmer would decide what threshold below 1 he or she would accept. In other words what level of productive loss from the effects of parasitism is acceptable, which is a trade-off between the cost of treatments and the targeted growth rates, returns for finished stock, animal welfare considerations, anthelmintic resistance in the parasite population, and the opportunity for breeding worm resistance or resilience in the livestock.

The Prattley electronic weighing and drafting system installed at Ashley Dene can be programmed with a pre-determined sub-optimal happy factor, sidelining sheep which fail to reach that threshold so they can be treated.

RFID enables a record of when and how often each animal was drenched. This information can be utilised as part of the selection of flock or herd replacements with the aim of selecting animals that are less reliant on chemical intervention.

Greer said farmers are understandably reluctant to invest in RFID technology incorporated with an automated weighing and drafting system. But the technology offers the potential to revolutionise on-farm management of animals from a flock or herd level to an individual level.

“Now we have the tools to develop production systems that are both sustainable and ethical while promoting the responsible use of chemicals in food-producing animals. However, while we can show some savings on anthelmintic costs and provide long-term benefits of slowing the anthelmintic resistance, regimes such as the happy factor will not justify investment in RFID alone. But TST regimes are just part of the picture and the overall benefits need to be considered.”

Trial work so far with young dairy cattle which should be gaining 700g/day has shown that it is possible with TST to have a 90% reduction in drench use with only a 5% trade-off in reduced live weight gain.

One Wairarapa farmer with RFID cattle has begun using TST and happy factor and has reduced drenching down to 20% of his 300 cattle.