Genetic progress in yield of United States maize (Zea mays L.)

Maize grain yields in the U.S. started to rise in the late 1930s, concurrent with introduction of hybrids and improved cultural methods. On-farm yield gains aver- aged 115 kg ha-1 yr-1 during the years 1934-2004. Genetic improvements have been responsible for ca. 50-60% of the on-farm gains, and changes in cultural practices (manage- ment) are responsible for the remainder. But because breeding and management interact with each other, nei- ther factor could have raised yields without concurrent and complementary changes in the other. Numerous esti- mates of genetic yield gain of maize hybrids have shown, without exception, that genetic yield gains during the past 70 years have been positive and linear. Estimates of the average annual gain vary but tend to fall in the range of 65-75 kg ha-1 yr-1. Over the years, hybrids have changed significantly in phenotype, in tolerance to biotic and abiot- ic stresses, and in genotype as measured by pedigree and molecular markers. Phenotypic changes may indicate im- provements in efficiency of grain production (e.g., smaller tassels may release more energy for gain production). Stress tolerance is greatly improved; newer hybrids out- yield the older ones not only in high yield environments but also when trials are subjected to abiotic stress (e.g., heat and drought) or to biotic stress (e.g., insect pests). Absolute heterosis for grain yield has increased over the years to a small extent, in particular when trials are sub- jected to abiotic stress, but the increase is considerably less than total genetic gain. Yield potential per plant has not increased over the years. Newer hybrids yield more than older hybrids because of continuing improvement in ability of the hybrids to withstand the stress of higher plant density, which in turn is owed to their greater toler- ance to locally important abiotic and biotic stresses.