The genetic basis of variation among wheat F sub(3) lines in tolerance to competition by ryegrass ( Lolium rigidum)

The heritability of tolerance of wheat F sub(3) lines to competition from annual ryegrass ( Lolium rigidum), and its relationship to morphological traits, were determined using crosses between wheat cultivars with good and poor competitive ability. Forty F sub(2)-derived F sub(3) lines from a cross between two late flowering varieties (Machete x Spear) and 40 from a cross between early flowering varieties (Wilgoyne x Gutha) were grown in the field with and without annual ryegrass. There was significant genetic variation between lines within each of the two populations in a number of aspects of plant growth and development, including yield in monoculture. The estimates of heritability for % yield loss due to competition were 0.25 and 0.57 in the two crosses respectively, indicating that selection for high tolerance to competition in the F sub(3) generation or later should be effective. Fairly strong relationships between height and % yield loss and between leaf length and % yield loss suggest that these may be useful auxiliary traits when selecting for low % yield loss. However, differences between crosses in the magnitude and sign of genetic and phenotypic correlation between traits indicate that competitive ability is a complex character influenced by many factors.