Variation in the harvest index of tropical maize: evaluation of recent evidence from Mexico and Malawi

In temperate zones, the potential grain yield of wheat has increased during the twentieth century owing to progressive increases in the harvest index of new varieties, which are principally associated with reduction in plant stature. Crop biomass has not increased substantially. In contrast, the pot ential grain yield of maize in the USA has increased owing to progressive i ncreases in biomass, principally associated with selection for grain yield at higher population density. Harvest index was already around 0.5 for reco mmended varieties in 1930, and has not increased significantly since. Howev er, for both crops, the harvest index of a given variety has proved to be a highly-heritable character, except under severe stress. Less is known about the physiology of tropical maize. This paper reviews ev idence from Mexico and Malawi that tropical maize can respond to selection for reduced stature following the same pattern as temperate wheat, but, und er other circumstances, the magnitude of harvest index is not highly herita ble, varying inconsistently with season, management and environment. It is proposed that these differences arise out of the unique vulnerability of th e grain-setting process around flowering. The plasticity of harvest index u nder long, favourable conditions, however, remains to be explained, althoug h it is probably also related to the events around grain setting. Neverthel ess, to the subsistence farmer, higher harvest index may not be a high prio rity in crop improvement, because of the need for large quantities of high- quality stover.