Yield and yield components ofF 1 hybrids were studied in three experiments at 30 Γ 30 cm spacings and in one experiment at 15 Γ 15 cm spacings. In the 30 Γ 30 cm experiments, l0 of the 41 hybrids tested significantly outyielded their high parents. However, only 2 hybrids significantly outyielded the best cultivar: one hybrid yielded 23 ~ and the other 16 ~o more than their respective check cultivars. The four hybrids in the 15 Γ 15 cm experiment yielded only 59 to 92~ as much as their high parents.
In areas where rice is transplanted at relatively wide spacings, the observed levels of F~ heterosis in selected hybrids may be sufficient to warrant production of hybrid rice, if enough hybrid seed can be produced. For direct-seeding at the high rates normal in the USA, the relatively small levels of heterosis and the difficulties of hybrid seed production preclude use of F1 hybrid rice cultivars at present.
None of 19 bulk F 2 and F3 hybrids in two experiments yielded significantly more than its high parent. Similarly, none of the 12 mixtures included in one experiment yielded significantly more than its high parent. On the basis of yield alone, using bulk F 2 or F 3 or simple mixture populations is not merited.