Abstract
Faecal examination for Trichostrongyle eggs of ewes of known ages sired by two different rams were made weekly over a period of two months from September to November 1937. All data on the egg counts were quantitative. Though the numbers of ewes studied were somewhat limited, the results were consistent and indicate that at least three different factors influence the degree of stomach-worm infestation. The season of the year is the first variable. Age of the ewe is the second, although, in these data, age beyond 2 years 8 months ceased to be important. The third and most important variable influencing stomach-worm infestation is genetic constitution for resistance and susceptibility. Judging from these results, genetic selection should effectively change the degree of resistance or susceptibility in populations of sheep.
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