THREE DOSES OR LESS EACH YEAR
AS a rule of thumb, flockmasters should try and reduce the number of worm doses down to three or less a year.
Dr John Vipond from the Scottish Agricultural College says those producers practising clean grazing can drop to two doses for ewes and avoid drenching lambs.
"But it is a high risk policy because if there is a breakdown, the result is worse than a set challenge. The flock should therefore be allowed to develop its own natural immunity," he says.
There are two major sources of worm infestation. Over-wintered infection is passed on in the faeces of sheep other than dosed ewes grazing the grass in the previous summer and autumn. "The levels and effects of intestinal worms will increase if ewes and lambs are grazed on the same grass year after year," Dr Vipond explains.
Also, the output of worm eggs from the ewe increases rapidly at lambing. These eggs develop into larvae on the herbage and become a source of infection for the lambs later in the season.
Dr Vipond suggests three simple rules for minimising infection:
• Graze ewes and lambs on "clean" pasture which has been free from lambs and hoggs during the previous 12 months. This breaks the life cycle of the most important stomach worms.
• Dose ewes and any lambs over four weeks old after lambing, but immediately before they enter the clean grazing. This virtually eliminates worm egg output and safeguards lambs against subsequent infection
• During the autumn do not allow store lambs or hoggs access to next years grazing, particularly if they have been on dirty ground. It is safe and often necessary to use mature ewes instead because they do not contaminate pasture in autumn to such an extent.
"The additional output that can be achieved makes the system worthwhile to most farmers and it has wide application in a variety of situations because clean grazing is a principle and not a rigid system," says Dr Vipond.