Changes in drought tolerance in maize associated with fifty years of breeding for yield in the US Corn Belt [Zea mays L.]

Understanding the changes underlying past breeding progress may help to focus research efforts and accelerate future genetic gains. The major abiotic stress affecting maize production on a worldwide basis is drought. We addressed the improvements in drought tol- erance over a 50-year period of hybrid breeding by evalu- ating, under targeted stress conditions, a set of 18 Pio- neer-brand hybrids that had been released during the 1953-2001 period. Stress treatments were designed as overlapping windows of water deficit covering the pre- flowering to late grain filling development stages. Data were collected on grain yield, yield components and an- thesis-silking interval (ASI), and were analyzed using a linear mixed model approach. Genetic gain was mea- sured as the slope of the regression of the trait on the year of hybrid release. Significant, positive genetic gains of varying magnitude were observed for grain yield in all windows of stress evaluated. The largest genetic gains for grain yield were observed under conditions of full irriga- tion and severe flowering stress. ASI and barrenness, es- pecially under stress at flowering, were significantly re- duced by selection. Though flowering remains the most susceptible stage to drought in maize, selection has re- duced its negative effects, and susceptibility during early grain filling is now of similar importance in many modern hybrids. Yield under drought at flowering has more than kept pace with the increase in yield potential because of the emphasis breeders have placed on improved floral synchrony.