“…Varieties or crosses that are genetically low yielding, but which have a measure of resistance to pests and diseases, as might occur in wild, weedy or primitive types, may perform better in stress situations than varieties or crosses of high yield potential but no resistance. This effect is similar to that of frost in determining yield and maturity in soya bean crosses (KALTON, 1948, WEISS et al, 1947. Therefore to select crosses or genotype of high yield potential, it is important to analyse and separate possible confounding factors which may cause yields to vary.…”