Ben drenching lambWhen 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 AgResearch trial compared lamb growth rate in lambs drenched with a 100% effective drench versus a drench which was very ineffective due to the presence of resistant worms. "Those treated with an effective drench were 4.7kg heavier on the hook than those treated with a drench that worms were resistant to," says VetEnt National Sheep & Beef Team Leader, Joe Bennett.  "At $4/kg carcass weight, this would be worth $18.80 per lamb. While this study might be comparing the extreme ends of the spectrum, it does well to drive home the point that a drench which is not 100% effective, will be costing you in production".

"You need to know you are using a drench that works, and you need to know what drenches don’t work on your farm," said Joe. "The management of worms on a sheep and beef farm is a very complex issue. Knowing the resistance status of the different worm species on your property is one important piece of a big puzzle".

VetEnt recommends that the best way to identify drench resistance is to do a Faecal Egg Count Reduction Test (FECRT). Feb/March is the best time to do the testing, because this is the time of year when lambs have most of the worm species present.

The test involves individual dosing of small groups of lambs which have a reasonable worm burden present with the various types of drench families. The lambs are then re sampled 10 days later to determine how effective the dosing has been at reducing the worm burden.  Where significant egg counts are still found, further laboratory testing can help identify the resistant worm.

"The biggest benefit to farmers of such a test is that they determine the resistance status of the different worm species on the farm, and can then set about developing a worm management program with their vet," says Joe.

The cost of testing is from around $1200 but can be more depending on individual clients needs.  However 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 you use a long acting product made up of a drench to which worms on your farm are resistant to, the ewes will not get the expected benefits of the drench capsule, and resistant worms will survive to dominate the worm egg output that contaminates the pasture over the lambing period. 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.

Now that drench resistance is more common, potent drench chemical families and combination drenches are coming under pressure.  "With lamb prices down on the high of last season, every dollar counts," says Joe.

To organise a drench resistance test, contact your nearest VetEnt clinic:

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