White clover population effects on the productivity and yield stability of mixtures with perennial ryegrass and chicory

Due to complementary resource use of the components, grass-clover mixtures are usually higher yielding compared to unfertilized pure grass stands. The number and identity of species in a mixture are known as important factors for the mixture's productivity. How intraspecific variability of component species affects the productivity of mixtures has received less attention. We established an experiment to investigate how and to what extent different populations of white clover (Trifolium repens L.) affect mixture performance. Eight novel populations of white clover and one variety of perennial ryegrass (Lolium perenne L.) and chicory (Cichorium intybus L.) were grown as monocultures and in two- and three-species mixtures at two sites differing in soil fertility. Aboveground herbage was cut twice in the establishing year and four times in each of the three following years. The accumulated dry matter yield was calculated as the sum of the four annual dry matter yields. There was no significant interaction between white clover population and crop stand (i.e., white clover monoculture or mixtures with partner species) on dry matter yield accumulated over four years; the white clover population that performed well in monoculture also did so in mixtures. At both sites, the binary mixtures of white clover and chicory produced significantly higher dry matter yields than the white clover monocultures and other mixtures. Inclusion of chicory in mixtures significantly increased the stability of yield production as well. Site conditions strongly affected the performance of the forage species mixtures. We found that the benefit of mixtures over monocultures was more related to the identity of species in mixture than to the white clover population.