Relationship of Different Species of Forage to the Survival and Infectivity of Haemonchus contortus in Lambs

Abstract

Third-stage Haemonchus contortus larvae that had been placed on outdoor forage plots in December survived overwinter and were ineffective for lambs the following May. The plots were 8 ft square and each consisted of one of the following species of forage: perennial ryegrass, velvetgrass, red clover, alsike clover, subterranean clover, and white clover. Four replicates of each forage species were used in the first experiment and six in the second. One helminth-free lamb grazed each plot for 18 consecutive days. All lambs contained adult Haemonchus at necropsy except the controls which had grazed on noninfested plots. Enough parasites were present in some animals to produce a significant decrease in their packed cell volume. Highest numbers (means) of adult worms were recovered from lambs which grazed white clover and subterranean clover.. The grass groups contained the lowest numbers of parasites. These results suggest that greater numbers of third-stage H. contortus larvae will overwinter and retain their infectivity if they are on forage species which have heavy dense growth characteristics rather than on forages which have an open, sparse type of growth. Larval behavior on different plant species is discussed. A comprehensive review on the survival of free-living stages of Haemonchus contortus by Kates (1950) indicates that freezing temperatures and desiccation are primary factors in the survival of free-living stages of this parasite. Only a few studies have indicated the influence different species of forage might have on survival of preparasitic nematode larvae. Furman (1944) found that free-living Ostertagia circumcincta larvae could survive the entire summer on irrigated pasture in northern California. He also reported that in the summer months, irrigated ladino clover was more favorable to nematode survival than irrigated alfalfa or ryegrass. A report by Pullar (1953) indicated that development of free-living larvae may be aided by pasture plants with broad horizontal leaves which conserve surface