Effects of mixed grazing of first- and second-year calves on trichostrongylid infections in Lithuania

The objective of this study was to examine whether susceptible calves grazi ng together with second-year resistant heifers are less exposed to trichost rongylid infection than are calves grazing on their own. Two groups of anim als representing each age category were turned out onto pasture on 24 May 1 997 and grazed at comparable stocking rates. The grazing of calves and heif ers together was compared to groups of each age category grazing separately . The results indicated that herbage larval counts were significantly reduc ed in the second part of the grazing season on the plot grazed by the mixed group compared to the plot grazed by the first-season calves only. The mix ed grazing strategy protected the young calves and no clinical signs were o bserved in this group, while most of the calves that grazed alone exhibited clinical signs. The availability of herbage was reduced towards the end of the season, with subsequent competition for the grass forcing all the anim als to graze the tufts around the faecal pats, where the quality of the gra ss is poor and the numbers of infective larvae are high. The effect of this was visible in the form of increased parasite burdens in the calves that w ere grazed together with the heifers, confirmed by increased blood serum pe psinogen concentrations and reduced daily weight gains in the second part o f the grazing season. The lower numbers of infective larvae on the pasture were probably achieved through the heifers ingesting many of the larvae but subsequently depositing relatively few eggs, since they had acquired some degree of resistance against trichostrongylid infections during their first grazing season. Thus they did not suffer any parasitological ill-effects d uring mixed grazing with first-season calves.