Planned weaning can cut parasites

"It's not just about what to drench and when to drench – it's got to be part of a plan," he says.

Lambs have little immunity to internal parasites and as they approach weaning or are weaned that is the time farmers must act, he told Rural News.

"To some extent they've been protected by the amount of milk they are getting from the ewe. It's not necessarily a large amount of milk, but any milk seems to have a protective effect."

Once weaned, without that milk effect they become much more vulnerable due to the combination of stress of weaning and change in diet to all pasture.

Cook warns worm burdens can accumulate rapidly, so managing the whole weaning process and the risks to lambs needs planning.

He believes a combination drench at weaning is essential, and it is pretty much common practice.

"The expectation after that depends entirely on where they are going to graze. A common practice is to put them back on the paddocks they came from. So the lambs are going back onto what we know are contaminated pastures so they will start accumulating a worm burden straight away."

A better approach, if possible, is to put weaned lambs on pastures that don't have a recent history of sheep grazing. This will slow down the accumulation of worms in a lamb compared with those that go back onto contaminated pasture.

As for a drenching programme, on North Island farms the rule of thumb is drench every four weeks. But he says it will vary from farm to farm depending on the level of worm challenge. Regular monitoring to keep a track of worm burdens is important for determining any drenching programme.

Cook says cooperia will tend to dominate in summer and trichostrongylus can appear at any time.

Looking at the present season, he doesn't see the parasite issue being any different from normal, though with more pasture available to lambs they may suffer less exposure. The caveat on that is the rain.

"If it carries on raining throughout the summer that will present a risk. The worm cycle just loves wet summers.

"When it's hot and moist they hatch and develop very quickly. The worst problems occur in wet summers."