There are numerous schemes in conventional farming that combine grazing management with anthelmintic therapy. In general, these methods fall into three types: (i) dilution; (ii) evasive; and (iii) suppressive (Taylor and Andrews, 1992).
The dilution strategy is the best method for organic farming, and consists of grazing paddocks in relays by groups of calves followed by groups of older cattle. The basis of the technique is that the greater consumption of infective larvae by the immune or partially immune adults will delay and reduce the build-up of infective larvae on the paddocks. Moving calves at monthly intervals to clean pasture has also been shown to be an effective method for the control of parasitic gastroenteritis (Eysker et al., 1998a; Poot et al., 1997). No anthelmintic treatment should be necessary using this method, although alternative wormers may help to prevent the disease (Duval, 1994). The lower stocking rates applied in organic farming may also contribute to a lower larval challenge in this system (CEC, 1804/1999; Mitchell, 1996; Shaw et al., 1998; Vaarst et al., 1994).
Research on alternate grazing with sheep has shown that this method is not very successful (Bairden et al., 1995; Coop et al., 1985). Alternate grazing also carries the risk of nematodirus infection (Coop et al., 1988; Coop et al., 1991; Mitchell et al., 1985).
The evasive strategy uses available knowledge of the epidemiology of parasitic gastroenteritis. Calves are moved from contaminated pasture to aftermath just before the summer increase in infective larval numbers (Nansen et al., 1987). In conventional farming, the calves receive anthelmintic treatment at the time of movement to prevent contamination of the clean pasture (Eysker et al., 1998b). As anthelmintic prophylaxis is not permitted under organic standards (UKROFS, 2000), timing is very important in this method. However, alternative wormers may be useful (Duval, 1994). This method is inferior to the dilution method, as it puts the calves more at risk from the disease.
The suppressive strategy is not suitable under organic farming, as it relies almost entirely on anthelmintic treatment.
Breeding for resistance
There is little doubt that faecal egg counts are heritable, and genetic selection can reduce egg production and pasture contamination. Stear and Bishop (1999) state that the procedure is relatively simple. One or more faecal samples are required from calves that have been grazing the same or similar pastures. The concentration of nematode eggs is then measured and standard procedures are applied to identify genetically superior individuals for breeding.
An effect of herbage species on the population dynamics and vertical migration of Trichostrongylus colubriformis and T. circumcincta larvae has been found (Niezen et al., 1998). This finding is especially relevant to organic farming, as ruminant production is based on roughage feeding and on a more diverse use of herbage species (Niezen et al., 1996).
Use of nematophagus fungi
The potential of the nematophagus fungus Duddingtonia flagrans in the control of nematode parasites has been investigated and shows some promise (Fernandez et al., 1999a; Fernandez et al., 1999b; Gronvold et al., 1999). However, more research is required, especially in the area of application.