Prevention is better than cure. Rather than worming your animals regularly, it is better to reduce the risk of parasite infection by strategic grazing. This will reduce medicine costs and prevent the development of resistant parasites.
When thinking about your pasture and parasite management, we can consider the pasture as ‘low-risk’, (sometimes called safe pasture or clean pasture) or ‘high risk’ (dirty pasture). Low risk pasture is grassland with low number of infective parasites, and high risk is when animals grazing this field are at high risk of ingesting infective larva. Pasture grazed by lambs, lactating ewes or young cattle in the previous year is considered to be high risk. There are many strategies that can be used to reduce the risk of infection. Some of these are outlined below.
Sometimes called ‘cross-grazing’ this is when different species graze the same pasture either simultaneously or in rotation. Generally cattle, sheep and horses are infected by different species of parasites (this is not always the case but more often than not), so when they share pasture the cattle for example, will remove some of the parasites that would otherwise be infective to the sheep. This is known as the dilution effect.
Another form of mixed grazing is when you send in the older animals to graze the pasture prior to younger stock to ‘hoover’ up the infective parasites to reduce the risk of infection. With a few exceptions older animals will generally build up good immunity to most gut parasites, the exceptions are:
Haemonchus contortus (barbers pole worm) in sheep
Liver fluke in cattle and sheep
It is thought that most parasites cannot survive the ensiling process, or drying for hay, although this needs further research.
True ‘clean’ pasture is when fields are not grazed by livestock for more than 5 years. Often this is not practical, although could work well on mixed arable and livestock farms.
The types of drugs used to treat worms are called ‘anthelmintics’. Some plants are thought to have ‘anthelmintic’ properties, meaning that they are able to reduce worm burdens of animals. How they do this exactly is not fully understood, it could be to do with bioactive metabolites inside the plant that affect the parasite, or it could be the plants enhance the hosts immune system, or it could be something as simple as the shape of the plant is not suitable for the parasitic larvae to inhabit. Research that has been carried out looking at anthelmintic properties of alternative forages has mainly been conducted in sheep. Some of the anthelmintic forages identified have other properties, see the Diverse Forages page for more.
More information on gut parasite control can be found at SCOPS.