Genetic analysis to improve drought and low nitrogen tolerance of corn in monoculture and in a kura clover intercropping system

Direct selection for grain yield under stress conditions is often inefficient because the heritability for grain yield is greatly reduced under stress. The objectives of the first study described in this thesis were to determine the efficiency of indirect selection for corn (Zea mays L.) grain yield under drought and low N conditions using secondary traits or molecular markers. Testcrosses of 238 intermated B73 x Mo17 recombinant inbreds were evaluated under drought and low N stress conditions. Results indicated that direct selection for grain yield in the targeted stress environment is more efficient than using secondary traits under both drought stress and low N stress. Using significant markers only was not more efficient than direct phenotypic selection for grain yield. The relative efficiency of genomewide selection was significantly greater than 1.0 for grain yield under drought stress but not for grain yield under low N stress. The results suggest that selection based on molecular markers is more efficient than phenotypic selection alone for the improvement of grain yield under drought stress, while for grain yield under low N stress, selection based on markers alone can only be more efficient if gains per unit time and cost are considered. In addition to the genetic improvement of corn for stress tolerance, the use of drought tolerant corn in a kura clover (Trifolium ambiguum M. Bieb.) intercropping system can reduce the competition for moisture between the grain crop and the cover crop. The objectives of the second study were to determine if drought tolerant corn can minimize the yield losses incurred when corn is intercropped with kura clover. Results indicate that drought-tolerant corn can maintain high yields and allow sufficient regrowth of kura clover and therefore significantly reduce the risk associated with intercropping corn with living mulch.