Drenching with an eye for the future

Sheep farmers are being encouraged to consider the future when undertaking drenching practices.

According to Dan Salmon, senior district veterinarian with the Riverina Livestock Health and Pest Authority, as temperatures increase worm larvae start to die off, providing a good opportunity to control internal parasites.

"During the cooler months almost every egg that any worm lays will hatch and survive on the pasture so that the vast majority of worms are on the pasture rather than in the sheep. The only effective way of killing worms is with a drench which will only get the worms inside the sheep, leaving all of the worms on the pasture free to reinfect sheep at their leisure," he said.

Although it would make sense to drench during the hot months when worms can only survive in the sheep and drenching would kill almost every worm on the property, Dr Salmon advises that this is not necessarily the best thing to do.

"If there are no worms on the pasture the only worms which will be available next season are the ones which have survived the drench because they are resistant to it."

Dr Salmon said that there are two ways to slow resistance down, but unfortunately both of them require sheep farmers to learn to live with a few worms.

"One approach is to drench the sheep before the weather gets properly hot and dry so that there are a few larvae surviving on the pasture making sure that not all of next year’s worms are the offspring of those which survived the drench.

"The other approach is not to try to kill every worm. This will work, particularly if the sheep selected for no drench are the healthiest ones so that they don’t actually die of worms through the summer."

Another option is not to drench any sheep, which will also slow resistance.

"If a faecal egg count shows that there are not a huge number of worms, why not let them be? Unless we have another wet summer the worms won’t build up and the drench can be saved until it’s really needed," said Dr Salmon.

The key to successful worm control without encouraging resistance too much is measuring the worm burden with faecal egg counts. If there are a lot of worms it’s recommended to drench the sheep before it gets too hot and dry but if there are not a lot of worms, leave them be as they won’t get much worse over the summer.

The same principles apply for the second summer drench - only give it if necessary and if so give it a bit later in the autumn so that the next generation of worms are not just the ones which have survived a drench.

For more information about drenching, please contact Riverina LHPA or your private vet.

Media contact: Vanessa Delaney 02 6391 3787, 0428 465 378