Grazing Management for Worm Control

A major problem is the emergence of resistant strains of parasites. Resistance to all the major classes of anthelmintics has been reported. Where strains of parasites resistant to more than one drug exist, the options for treatment are seriously restricted. Hot countries where parasite turnover is high and anthelmintics are used often are, generally, worst affected.

A 50/50 System Timing is also Important References and Further Reading   See also: Alternative Dewormers

Attempts have been made to develop alternative control strategies such as the use of genetic resistance. The red Masai sheep, for example, is noted for its resistance to worms (Mugambi et al., 1997; Wanyangu et al. 1996).

Other possibilities for control include improved grazing management to avoid infected pastures. This option is, however, difficult to implement where animals cannot be confined. However, where animals can be confined by use of fencing, grazing management provides a way of limiting the spread of infections.

Nutrition also plays a major role in how well animals are able to overcome the detrimental effects of internal parasites. In practice, the signs of parasitism can often be used as a symptom of some other problem, usually poor nutrition.

A new grazing system, (referred to as the 50/50 system, is now being promoted in South Africa (see Van Wyk et al. 1998). In this system, all the animals on a given farm are concentrated on half the pasture, while the other half is rested, with these treatments being rotated annually. In this way practically worm-free pastures are available annually and it should be possible to obtain high levels removal of worms, on the condition that special precautions are taken to prevent exposure to resistant larvae surviving in places of common access on the farm, e.g. the surroundings of handling pens, roadways and tracks for common access and for herding sheep between paddocks, etc.

This system of grazing management depends on the environmental conditions and the abilities of the parasites to remain in the soil. It is essential that grazing is not resumed in an area until enough time has passed so that the parasite eggs and larvae no longer survive - and can therefore not reinfect the livestock. An understanding of the life cycles of the different parasites within the soil-plant-animal system will help show the interrelationships between these three components.

The major part of the parasite life cycle is outside of the animal. This fact helps the livestock keeper to choose management strategies that reduce parasite levels on his or her farm and to decrease the need for use of chemical dewormers (Wells, 1999).

Animals that continuously graze a pasture eat the grass close the ground, while contaminating the soil with large numbers of parasites. Nothing apart from regular deworming with chemicals will control them. Alternatively, by using controlled grazing methods that allow pastures to rest and soil life to function well, parasite contamination can be reduced. This reduction occurs because soil organisms, including earthworms, dung beetles, and nematophagous fungi will destroy or keep a lot of the parasite eggs and larvae from developing. In addition, environmental conditions such as extended dry periods also help to kill some parasite eggs and larvae. Keeping the grass in a more vegetative stage, and tall enough to provide the animal with adequate forage, will provide better nutrition to keep the animal healthier, strengthening the immune system to prevent the adult worms from producing eggs. In general, most parasites do not cause as much harm to a healthy, well nourished animal. The parasites that are present will not deplete the host's resources as much as in an animal that is malnourished. Parasite loads affecting wildlife generally do not cause the death of the host, because the parasites need the host to survive. The same principle applies to livestock.

Pasture contamination can be reduced through management. Livestock will avoid manure piles and the grass surrounding them. This behaviour also helps them avoid eating larvae. The height of the pasture sward can affect parasites. The majority of worm larvae crawl only a few cm from the ground up grass stems and other plants. Not allowing animals to graze below that point will cut down on a lot of infestation. This is one reason sheep tend to have more problems with internal parasites. They eat much lower to the ground than cattle do, picking up higher numbers of larvae. Therefore, it is important to monitor grazing sheep closely so they don’t graze too low. Larvae migrate only a short distance from the manure pile. If livestock are not forced to eat close to their own manure, they will eat fewer larvae.

With sheep and goats, the most important time to control pasture contamination is during the periparturient rise, which is the sudden release of infective larvae and eggs within the ewe’s intestinal tract. This occurs right after lambing, and is due to the ewe or doe’s immune system becoming temporarily less effective. By treating animals at this time, the exposure to newborn and young lambs (those most susceptible to parasites) is minimized.