Competitive effects in monocultures and mixtures of spring barley (Hordeum vulgare).

The presence of significant levels of intergenotypic competition amongst barley (Hordeum vulgare) genotypes has profound consequences for barley breeding programmes. Breeding programmes based on the pedigree system attempt to identify genotypes in genetically heterogeneous populations but the elite genotypes are grown in monoculture. Thus, to attain varietal status genotypes produced by this breeding strategy must perform well in mixtures as well as in pure stands. The effectiveness of early generation selection may be hampered by intergenotypic competition. To examine this problem in spring barley, a modified substitution experiment (Mather and Caligari 1981, 1983) was used and included genotypes sampled from a random set of inbred lines generated without conscious selection. This approach to the investigation of competitive effects in barley indicated the presence of significant levels of intergenotypic competition for a range of agronomic characters. The analyses allowed a distinction to be made between aggression (a) and response (r) with the component r displaying greater variation than a. The lack of correlation in the distribution of a and r suggested that they were under separate genetic control and hence adjustable by selection. The implications of these results for barley improvement, the use of varietal mixtures and mixed cropping are discussed.