Worms getting the upper hand

01-09-2010 | Marie Taylor

Drench resistance is getting worse in Wairoa sheep, vet Joe Bennett says.

At the annual VetEnt seminar recently he reported on the Wairoa Vet Club's drench resistance project this year.

Bennett says increased drench resistance threatens the sustainability of sheep and beef farming. A recent study demonstrated a 14% difference in carcase value between animals drenched with a 100% effective drench and those drenched with a drench not 100% effective.

This translates to a difference of nearly $12 a head.

Drench tests were done on 14 sheep and beef farms with the help of Vet Club subsidies. There were six treatments, including a control, to look at how well the three main drench families worked against parasites.

The 2006 national drench survey on 97 New Zealand farms set a benchmark by classifying resistance as being present whenever there was a less than 95% reduction in parasite numbers.

On the East Coast, 22 farms were again surveyed in 2008 and showed resistance was continuing to increase.

The new tests again show the problem is real, Bennett says. "The percentage of resistance in Wairoa for any of the test groups is much higher than 2008 or 2006."

Bennett says that by the time drench resistance tests show a less than 95% reduction in egg count following a drench, 40% of the worm population on that farm carries one or both genes for drench resistance.

"By the time we identify it in our reduction test it is already a problem and we are on a slippery slope.

"A combination of our environment, the lambs' poor immunity, and our farming practices which have led us to this point."

For farms with resistance the next move is to address the risk factors: the main one of which is using drenches.

"Every time we drench every animal we are one step closer to progression of drench resistance," Bennett says.

"Natural selection dictates that resistant animals survive, susceptible ones die. The more you drench the worse it will be."

Other risk factors are:

• having a high number of lamb days on the farm;

• using long-acting drenches - long-acting injections, capsules and the oral moxidectin drench;

• lamb-finishing blocks.

Lamb days on the farm is a function of the number of lambs and the number of days they are on the farm.

"Lambs are great multipliers of worms; their immune system is relatively undeveloped compared with a ewe," Bennett says. That means lambs need drenching, and in late summer and autumn lambs are exposed to Barber's Pole worms. Long-acting drenches must be used to control Barber's Pole worms because the day after you give a short-acting drench, the Barber's Pole larvae can come in and have a negative effect on stock, he says.

He suggests three ways to reduce the number of days lambs are on-farm. The first is to sell lambs store earlier, particularly those that will take longer to finish.

It may be better to quit the scrawny little lambs, that will take a hell of a lot of input and grass, so they are someone else's problem, Bennett says.

Then there might be something else more profitable to grow or more grass can be fed to the ewes to get them up to condition at scanning.

The second way is to grow them fast and get better lamb weaning weights so more can be sold off their mothers.

"The objective should always be to grow the lambs as fast as we can and get them off the place. Treating some ewes that need a hand pre-lambing may be a way to help achieve that."

The third way is to select ewe lamb replacements early at weaning based on bodyweights.

The old attitude of waiting for twin lambs to catch up for selection in April is not well-founded, Bennett says. That's because the heritability of twinning in sheep is low, about 0.1.

This means 90% of the reasons a ewe has twins versus single lambs is nothing to do with genetics. Body size, body condition and feeding level play much greater roles.

Selecting the biggest ewe lamb replacements early means the surplus lambs can be sold earlier and you can feed the replacements better.

Ewe lamb replacements should be grown out to 40 to 42kg by May 1 to ensure they are set up for good performance for their whole lifetime.

"You have to balance profitability now with long-term profitability as well.

"Part of our job as responsible sellers is to ensure the right advice is given when clients want to purchase long-acting treatments. We try to advise people against blanket treatments of ewes."

There does need to be more information about managing the use of long-acting drenches and more ethical use of products, Bennett says.

Meanwhile farmers can get a $200 subsidy from the vet club towards drench testing this year. "We need to keep monitoring drench resistance on our farms and in the district as a whole."

Bennett says drench resistance is also present in cattle parasites but they haven't yet looked at it a lot in the Wairoa district.

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