Worm control for the grazing season

Worm control for the grazing season

Nematodirus is usually the first of the gastrointestinal (GI) worms to affect lambs at the beginning of the grazing season.

The eggs deposited by lambs the previous grazing season usually overwinter, needing a period of cold weather, followed by warming conditions in the spring before the eggs will hatch. This can lead to a "mass hatch" where the majority of nematodirus eggs hatch in a short space of time.

Timing of the Nematodirus hatch is weather dependant, usually between the end of April and mid-June, and can be largely avoided by not grazing young lambs in the same field in consecutive years. Where this is not possible, strategic treatments are usually required to prevent the lambs from suffering extensive gut damage because of the resulting sudden severe nematodirus challenge.

The Nematodirus forecast on the SCOPS website takes weather station data from around the country to give daily updates on local nematodirus risk, to allow accurate timing for nematodirus treatments. With very low levels of resistance, Group 1 (BZ) white wormers are the active of choice for nematodirus.

Check the forecast now to see what the nematodirus risk in your area is today:

The next GI worms to trouble the lambs during the grazing season will be Teladorsagia and Trichostrongylus. These are the most pathogenic worms in what is usually a mixed infection that causes a drop in growth rates, scouring and weight loss as the worm burden increases. Like Nematodirus, when they appear on pasture and the level of challenge faced by the lambs depends entirely on previous grazing management and weather conditions. Unlike Nematodirus, these worms will usually complete several lifecycles during a grazing season, with pasture worm burdens increasing rapidly when lambs continually graze the same pastures. Also, unlike nematodirus, these worms are likely to have some level of resistance to one or more of the three older wormer groups1.

Recent prevalence survey report findings of anthelmintic resistance from across the UK

The impact of wormer resistance on the farm will not be immediately obvious. As resistance develops, there is a gradual increase in the number of worms surviving treatment. The worms left behind will hold the lambs back, decreasing growth rates by up to 50% before there are any visible signs that the wormer treatment has not been fully effective2.

As mentioned above, the level of challenge, and therefore the need to treat for these worms during the summer and autumn, is weather dependant. To reduce the selection pressure that drives the development of worm resistance, lambs should only be treated when their worm burden reaches the point where it starts to decrease their growth rate, the same wormer group should not be used repeatedly or for consecutive treatments, and the newer Group 4-AD (Orange) and Group 5-SI (Purple) wormers should be used in every flock3.

As one of the newer groups, Zolvix™, the Group 4-AD wormer, should be used as a late season break dose for lambs as shown above, replacing one dose of one of the older 3 groups, and as part of routine quarantine treatment for all incoming and returning animals.

As the impact of resistant worms remains invisible until resistance levels reach the tipping point, it is important to include newer actives in worm control plans now. Taking action before there are obvious problems on the farm, will help maintain the activity of wormers for the future, as well as allowing lambs to reach their growth potential today (see www.scops.org.uk for full details on effective quarantine treatments).

1 - https://www.scops.org.uk/workspace/pdfs/prevalence-of-ar-.pdf
2 – www.scops.org.uk
3 - https://www.scops.org.uk/workspace/pdfs/using-group-4-ad-and-5-si-anthelmintics.pdf

For more information on worm control, please speak to your local in-store Registered Animal Medicines Advisor (RAMA).