RESPONSE OF WHITE CLOVER GENOTYPES TO INTERGENOTYPIC AND INTERSPECIFIC INTERFERENCE

Six white clover genotypes that were easily distinguishable from each other on the basis of leaf lamina marks and morphology were grown at L odi, Italy, during 1990 and 1991 in dense swards, under field conditio ns and a mowing regime, as (i) pure stands, (ii) a complex mixture of all genotypes, (iii) binary mixtures of each genotype with each of two ryegrass varieties, and (iv) complex mixtures of all clover genotypes in binary association with each grass variety. The grass components w ere of known, different vigour. The study assessed both intergenotypic and interspecific interference and related dry matter yield responses to morpho-physiological traits of the clovers, and also determined wh ether a high level of morpho-physiological heterogeneity conferred a y ield advantage on clover populations. Greater heterogeneity (i.e. a co mplex mixture of clover genotypes) did not produce higher clover yield s either in the presence or absence of interspecific interference from grass; thus, the use of blends of varieties or the development of var ieties with a fairly high degree of heterogeneity was not recommended for short-term meadows in environments with relatively low spatial and temporal variability. Interactions for yield occurred between clover genotypes and the presence or absence of intergenotypic interference ( P < 0.001), and between clover genotypes and the presence or absence o f interspecific interference from the grass variety characterized by g reater vigour and aggressiveness (P < 0.01). The variance of the forme r interaction tended to be consistently larger than that of the latter interaction, indicating that competitive effects were greater between clovers than between the clover and grass components. A lower Spring: Summer yield ratio and taller canopy tended to confer a competitive ad vantage under intergenotypic interference. Relatively better performan ce under interspecific interference was related to higher stolen densi ty, suggesting that selection for this trait may increase the general ecological compatibility of large-leaved white clover types grown with vigorous grass companions.