Growth of cattle on phalaris and lucerne pastures II. Helminth parasite populations and effects of anthelmintic treatment

Abstract In young beef cattle, the effects on helminth parasite populations of anthelmintic treatment, pasture type and stocking rate are reported from a four-year experiment at Canberra. The animal production results are given by Morley et al. (1978). Anthelmintic treatment was at three levels — none, at weaning in autumn, and frequent treatment intended to be suppressive. In the last two years of the experiment when helminth populations were moderately high, treatment in early autumn recouped roughly half the production loss from uncontrolled helminth infection. Most of this effect occurred in spring and not soon after treatment. Monthly anthelmintic treatment failed to suppress Ostertagia ostertagi infection and delayed the natural development of host resistance. Treatment at intervals of two weeks or less largely prevented O. ostertagi egg output and eventually reduced pasture infectivity and worm burdens to low levels. In three of the four years, worm burdens in untreated cattle were lower on lucerne than on phalaris pastures.