A primary objective in plant breeding programs is to develop high yielding varieties that have consistent performance. Selection for pure line varieties has been responsible for much of the genetic improvement in self-fertilized crop plants. Pure line varieties are usually very productive and are highly uniform in maturity, disease resistance, and market quality. Recently, multiline populations or mixtures have been suggested by several plant breeders to stabilize and perhaps increase yield (1, 9, 12, 13, 23). Mixtures might offer greater protection against diseases and insects and provide a broader adaptation to environmental conditions, thereby minimizing unfavorable genotype x environment interactions (12, 23).
Conclusive evidence has been presented which indicates that an interaction exists among genotypes. For example, \u27Vaughn\u27 barley (Hordeum vulgare L.) was found to be higher yielding than \u27Atlas\u27 in pure stands but lower yielding in mixtures (14-). Hartmann and Allard (11) found that this competition effect was present at intermediate levels of fertility and moisture but not at higher moisture levels. Competition effects for light and nutrients in Phalaris tuberosa L. and Lolium perenne L. were observed by Donald (5). Other work suggested a cooperative or synergistic effect in a few varieties of rice (Oryza sativa L.) (20), certain genotypes of barley (9), and between flax and linseed (Linum usitatissimum L.) varieties (10). A decrease in yield of mixtures compared to the mean of the component varieties grown in pure stands has been observed in lima beans (Phaseolus lunatus L.) (1) and hard red winter wheat (Triticum aestivum L. em. Thell.) (21). Results from experiments with various crop plants showed no difference between mixtures and pure stands for yield (4, 6, 7, 17, 22).
Suneson and Wiebe (24) indicated that the performance of a variety mixture cannot always be accurately predicted by its performance in pure stands. The Montgomery effect (15) further implies that a particular genotype may be inferior in yield in pure stands but superior in competition. Since varieties differ in their response to the environment and their ability to compete for nutrients and other growth factors, mixtures may be of value for more efficient production.
To augment various findings related to the performance of mixtures, three barley varieties were studied in pure stands and mixtures. The objectives of this study were:
1. To determine if yields of mixtures of barley varieties differ from the mean yield of the varieties grown in pure stands;
2. To determine if mixtures provide a stabilizing effect by decreasing genotype x environment interactions;
3. To measure competition effects of yield components and other agronomic characters of variety mixtures by deviations from the mean of their components grown in pure stands; and
4. To determine if the performance of the varieties in mixtures is predictable from their performance in pure stands
