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Mass Selection in a Composite of Intercrosses of Mexican Races of Maize¹

Mass selection was used to incorporate desirable traits from 25 Mexican races of maize (Zea mays L.) into a single population with maturity and plant type that would be useful to temperate zone maize breeders. Ten generations (cycles) of selection have been completed. The original population ranged from early to very late at Blacksburg, Va, where it was grown in isolation with 600 or more ears selected each cycle. Selection was practiced primarily for erect, disease-resistant plants with mature grain, without particular regard to plant and ear type. Earlier maturity was gradually achieved by a combination of natural selection against very late plants and artificial selection of ears from several of the earliest maturing plants each cycle. Seed increases were made in 1972 from C1, C4, and C7 remnants. These, together with C10 produced in 1972, were grown in performance trials in 1973 and 1974. Over the 10 cycles, yield increased 171%, days to mid-silk decreased 11 days, and moisture at harvest decreased 7.7 percentage points. The ratio of plant-to-ear height decreased; in C10, ear height averaged 115 cm — 50% of plant height. Average spread between pollen shed and silk emergence decreased from 9.1 to 7.0 days. Incidence of corn smut [Ustilago maydis (DC.) Cda.] decreased from 43.6 to 19.3 smut galls/100 plants. Selection had little effect on root lodging, but stalk lodging increased. C10 is highly variable but has a sufficient frequency of plants with satisfactory maturity and morphology to provide a readily usable source of tropical germplasm in major temperate zone maize production areas.

Key Words: Population improvement • Genetic variability • Genetic vulnerability • Exotic germplasm • Zea mays L.

² Professor of agronomy, Virginia Polytechnic Institute and State Univ., Blacksburg.

Received for publication January 17, 1976.