He suspects the vast majority of farmers are not testing for drench resistance.
Lambs have most of the worm species present between January and May, so this is the best time to do the FECRTs. They are worthwhile only where faecal egg counts have identified worm burdens of above 300 eggs a gram (epg) of faeces in sheep or goats, and 150 epg in cattle.
To carry out the FECRTs, animals should be sorted into groups of 10-15 to ensure faecal samples of 3-4g (one tablespoon) are collected from at least 10 of them.
Mckenna explains that depending on the drench history of the property, including previously identified drench resistance, each group of animals may be tested with a different type of drench. For example, one group may be tested with Levamasole while another tested with Invermectin.
Once drenched and identified the animals need to go back into either their original paddock or a clean paddock to avoid recontamination.
After seven to 10 days the animals are re-sampled.
Failure to reduce egg counts by 95% indicates drench resistance and further testing can identify the resistant nematode.
Ben Hodgson, a sheep and beef vet from VetENT Te Kuiti, says that when worms are resistant to a certain type of drench, lamb growth rate is reduced from the average expected 100g/day weight gain.
A five-month trial compared lamb growth rate in lambs drenched with effective drench versus worm-resistant drench.
“Those treated with an effective drench were 4.7kg heavier on the hook than those treated with a drench that worms were resistant to.
“At $4/kg carcaseweight, this would be worth $18.80 per lamb.
“You need to know you are using a drench that works, and you need to know what drenches don’t work on your farm.”
The cost of testing ranges between $1200 and $1500, but Hodgson says the impact of using an ineffective drench goes beyond this year’s lambs.
Thin multiple-bearing ewes around lambing may benefit from the use of long-acting persistent activity treatments such 100-day drench capsules. If the drench capsule used is made up of a drench to which worms on the farm are resistant, the ewes will not get the expected benefits and resistant worms will survive to dominate the worm egg output that contaminates the pasture over lambing.
The farm ends up with more resistant worms in the pasture, which challenge the grazing lambs during the following summer/autumn period. Lamb weight gain is compromised again if an ineffective drench is still being used during the season.
