Maize (Zea mays L.) yields have risen continually wherever hybrid maize has been adopted, starting in the U.S. corn belt in the early 1930s. Plant breeding and improved management practices have produced this gain jointly. On average, about 50% of the increase is due to management and 50% to breeding. The two tools interact so closely that neither of them could have produced such progress alone. However, genetic gains may have to bear a larger share of the load in future years. Hybrid traits have changed over the years. Trait changes that increase resistance to a wide variety of biotic and abiotic stresses (e.g., drought tolerance) are the most numerous, but morphological and physiological changes that promote efficiency in growth, development, and partitioning (e.g., smaller tassels) are also recorded. Some traits have not changed over the years because breeders have intended to hold them constant (e.g., grain maturity date in U.S. corn belt). In other instances, they have not changed, despite breeders' intention to change them (e.g., harvest index). Although breeders have always selected for high yield, the need to select simultaneously for overall dependability has been a driving force in the selection of hybrids with increasingly greater stress tolerance over the years. Newer hybrids yield more than their predecessors in unfavorable as well as favorable growing conditions. Improvement in the ability of the maize plant to overcome both large and small stress bottlenecks, rather than improvement in primary productivity, has been the primary driving force of higher yielding ability of newer hybrid.

The Contribution of Breeding to Yield Advances in maize (Zea mays L.)

Food security is an urgent challenge It is a global problem that is set to worsen with current trends of population, consumption, climate change and resource scarcity The last 50 years have seen remarkable growth in global agricultural production, but the impact on the environment has been nsustainable The benefi ts of this green revolution have also been distributed unevenly; growth in Asia and America has not been matched in Africa Science can potentially continue to provide dramatic improvements to crop production, but it must do so sustainably Science and technology must therefore be understood in their broader social, economic and environmental contexts The sustainable intensifi cation of crop production requires a clear defi nition of agricultural sustainability Improvements to food crop production should aim to reduce rather than exacerbate global inequalities if they are to contribute to economic development This report follows other recent analyses, all arguing that major improvements are needed to the way that scientific research is funded and used

Reaping the Benefits: Science and the sustainable intensification of global agriculture

Yield potential, yield stability and stress tolerance in maize

https://www.cell.com/trends/genetics/pdf/S0168-9525(06)00409-4.pdf

Heterosis: revisiting the magic

Heterosis, or hybrid vigor, refers to the phenomenon that progeny of diverse inbred varieties exhibit greater biomass, speed of development, and fertility than the better of the two parents ([Figure 1][1]). This phenomenon has been exploited extensively in crop production and has been a powerful

/pdf/in-search-of-the-molecular-basis-of-heterosis-13qor96oav.pdf

In Search of the Molecular Basis of Heterosis

Hybrid vigor, or heterosis, usually refers to the increase in size or rate of growth of offspring over parents; for example, hybrid vigor in crop plants can be observed as in increase in yield of grain, or reduction in number of days to flower. Heterosis in plants has been utilized on large scale for the past 75 years, as carefully selected and reproduced hybrid cultivars. Field crops such as maize, sorghum and sunflower are produced as hybrids in all of the industrialized world; they also are grown as hybrids in increasing amounts in the developing world. Hybrid rice is grown extensively in China, and increasingly in India (Virmani, 1994). Many commercial vegetable and flower crops are grown almost entirely as hybrids. Heterosis is credited for large increases in production per unit area, thus sparing large amounts of land for other uses such as environmentally benign nature preserves. Examination of the historical record suggests that the major gift of heterosis was its stimulation of interest in the entire system of breeding and use of hybrid crops, rather than a simple exploitation of hybrid vigor per se. Development and use of hybrid seeds can enhance crop yields and performance in ways that are different from and not necessarily dependent on heterosis by itself. This essay will examine the historical record on utilization of hybrids to exploit heterosis in some of the major field crops, discuss the present use of heterosis and hybrids, and attempt to predict how heterosis and hybrids may be used in the future. The purpose of this analysis will be to show how hybrids and heterosis can help to supply food for burgeoning populations and also help to improve environmental health of the global food production system.

D. N. Duvick

Papers

Heterosis: Feeding People and Protecting Natural Resources