The plus-hybrid system: A method to increase grain yield by combined cytoplasmic male sterility and xenia

Cytoplasmic male sterility (cms) is used increasingly in hybrid seed production because of its superiority in terms of cost efficiency. Non-restored cms-hybrids often yield more than their male-fertile counterparts, especially when they are pollinated by unrelated plants. The combined effects of cms and genetically dissimilar pollen sources (i.e. xenia) are referred to as the Plus-Hybrid effect. The Plus-Hybrid system consists of blending a non-restored cms-hybrid with an unrelated male-fertile hybrid as a pollinator. The objectives of this study were to: (1) determine the effect of male sterility and xenia on the grain yield of cms-hybrids as pure stands in small-plot experiments and (2) to confirm the results, obtained from small-plot experiments, with real Plus-Hybrids as blends in large-strip mixture trials. European and US Corn Belt germplasm was used. Small-plot experiments were conducted from 1998 to 2000: Two dent x flint cms-hybrids were crossed with three fertile hybrids in nine environments in Switzerland; two dent x dent (Corn Belt adapted) cms-hybrids were crossed with four fertile hybrids in 12 environments, and one cms-hybrid was crossed with five fertile hybrids in 8 environments in the USA. Large-strip mixture trials were conducted from 1998 to 2000: One cms-hybrid was crossed with four pollinators in three environments in Switzerland; one cms-hybrid was crossed with two pollinators in 32 environments and three pollinators in 16 environments in the USA. The Plus-Hybrids, on average, outyielded their male-fertile counterparts. In Switzerland, the best Plus-Hybrid yielded 1.7 t ha -1 higher in the small-plot and 1.1 t ha -1 higher in the large-strip mixture trials. In the USA, the Plus-Hybrid effects were somewhat less strong and less consistent. Our results indicate that, in particular in European dent x flint germplasm, Plus-Hybrids may be a valuable option to further increase grain yields of high yielding maize hybrids. In the USA, where the dent x dent pattern predominates, further research is required to identify genetically diverse germplasm that will contribute a sufficiently large and consistent Plus-Hybrid effect to be of value to producers.