The impact of internal parasites on the productivity of young cattle organically reared on semi-natural pastures in Sweden

A grazing experiment with young cattle was conducted over two consecutive ( 1997, 1998) grazing seasons on semi-natural pasturelands in central-eastern Sweden. Comparisons were made between groups of animals that were either u ntreated and set-stocked, ivermectin bolus treated and set-stocked or untre ated but moved in mid-summer (mid-July) to ungrazed pasture. The whole expe rimental area had remained virtually free of cattle during the previous two seasons and the cattle had been raised indoors since birth. To introduce l ow-levels of parasite infection into the experimental system, each animal r eceived a 'priming dose' of approximately 10,000 infective trichostrongylid larvae at the time of turnout for both years. Results of the first year st udy showed that the level of parasitism was so low that it failed to induce any productivity losses in both groups of untreated cattle, which grew as well as those given boluses at turnout. In contrast, in 1998 both groups of untreated cattle suffered varying degrees of sub-clinical and clinical par asitism to result in an average of 30 kg liveweight depression, compared wi th the bolus treated cattle, at the end of the season. The only major depar ture between the two years was that in the latter, the cattle in the untrea ted groups were exposed to infective larval pickup, which had overwintered on pasture. Cattle in the move treatment grazed in the same sequence on pas tures used by similar classes of animals during the previous year. That is, their pastures at turnout had not been grazed since mid-summer of the prev ious year. Clearly this early season (1997) grazing by young cattle resulte d in sufficient overwintered larvae at the start of the following year (199 8) to cause productivity losses of the same magnitude as those recorded for young cattle grazing on pastures contaminated for the entire grazing seaso n of the previous year. This was confirmed by tracer tests that were carrie d out on all treatments, at the time of turnout and the mid-summer move in 1999. These results have major significance to organic cattle producers in Sweden who have a much higher tendency to practice a variety of grazing man agement techniques aimed at controlling nematode parasite infections in you ng cattle, than their conventional farming colleagues. It has been identifi ed that one of these strategies is to simply use summer/autumn saved pastur es for young stack at turnout, which if grazed by young stack prior to this , could prove to be counter-productive. (C) 2000 Elsevier Science B.V. All rights reserved.