Progress in Developing Drought Tolerant Corn

Drought in corn and soybean growing areas of the Midwestern U.S. in the summer of 2012 and again this year has heightened interest in the development of drought tolerant corn. Africa corn growers have had an ongoing intense interest because of recurring droughts. 国际农业生物技术应用获取服务 (Ishaaa) recently re-released Progress in Achieving and Delivering Drought Tolerance in Maize – An Update authored by Greg O. Edmeades, former leader of the maize drought program at the International Maize and Wheat Improvement Center (CIMMYT) in Mexico, that had been originally released in February of this year. He wrote a similar update in 2008.

据博士. Edmeades, “Tremendous progress has been made in the past four years in developing drought tolerant products – both in temperate and in tropical maize.” This year Midwestern U.S. farmers have hybrids available from at least three multinational seed companies. The Drought Tolerant Maize for Africa (DTMA) program, funded by the Bill and Melinda Gates Foundation for ten years starting in 2007 and involving CIMMYT, the International Institute of Tropical Agriculture (IITA) å’Œ 13 national programs in sub-Saharan Africa, uses conventional selection and marker assisted selection (MAS) to improve germplasm adapted to the drier sub-Saharan maize environments.

The Water Efficient Maize for Africa (WEMA) 项目, a private/public sector partnership is also funded by the Gates Foundation for ten years starting in 2009. It involves CIMMYT, Monsanto and five eastern and southern African countries and uses conventional, marker-assisted recurrent selection (MARS) and transgenic components. It hopes to release the first biotech drought tolerant maize as early as 2017 in sub-Saharan Africa. Similar work is being done in South East Asia.

The yield gap between crop potential yield and water-limited yield can be large. 博士. Edmeades says a rough rule of thumb is that 20-25% of this gap could be eliminated by genetic improvement in drought tolerance and another 20-25% by water-conserving agronomic practices, but the remaining 50-60% only by irrigation. The analysis focused on drought tolerance and to a lesser extent on heat tolerance. Although heat and drought often occur together, tolerance to each appears to be independent of the other.

Genetic and management strategies target three variables; 1) water captured by the plant, 2) water use efficiency, å’Œ 3) the harvest index or the proportion of biomass forming grain. Each of these variables can be altered. 据博士. Edmeades, “The numbers of kernels set and filled under drought stress accounts for most of the variation in maize grain yield under drought.” Events at flowering play a critically important part in yield stability because kernel number is largely determined at that time. Improvement in drought tolerance can be achieved without an impact on yields under optimal conditions.

博士. Edmeades notes that there is less optimism and more pragmatism on yields for drought tolerant corn in 2012 比在 2008. Selection solely for drought tolerance in conventional breeding can result in yield gains of 1.6 bushels per acre per year, but drought tolerance must be gained while maintaining all the other qualities corn producers demand. A more realistic goal may be 0.8 bushels per acre per year from conventional breeding. MAS may double those gains in the U.S., 但是一个 50 percent gain may be more reasonable for Africa. Assuming 48 每英亩蒲式耳 (3.0 metric tons per hectare) as a base yield, on a percentage basis that would be a 1.4 percent gain for conventional breeding plus a 0.6 percent gain for MAS. Transgenics may add another 0.7 percent assuming a new gene is added with a 5 percent gain every eight years, down from the 2008 assumption of a 15 percent gain every five years.

Multinational companies have the resources to substantially exceed those rates of yield increases for drought tolerant corn, but programs with fewer resources should aim for about 1-2 每年两个百分点, 0.5-1.0 bushels per acre per year. Corn hybrids generally require 7-15 years from the first cross to adoption by farmers. That requires continuous commitments to drought tolerant hybrids across several decades to consistently deliver improved hybrids – something that does not happen often in public sector programs in developing countries. China’s decade long $3.5 billion commitment to biotech crops is an exception. 博士. Edmeades concluded that developing countries need a well-trained staff that is allowed to remain in the field and not be distracted as so often is common.

Corn producers in the U.S. have access to the latest genetics, conventional or biotech, because private companies compete in a well developed market. There is no equivalent marketplace in much of Africa to deliver the final product to corn producers and the producers are not prepared to participate in such a market. 博士. Edmeades gives support to breeding programs taking what U.S. farmers would consider a step backward and marketing drought tolerant open pollinated varieties and conventional hybrids as a step forward from using saved seed. This would allow the development of seed industries that can delivery improved hybrid and biotech seeds in the future.

Regulations on biotech crop development in the U.S. have resulted in costs of up to $100 million per gene, costs that only the well-resourced companies can undertake. Sub-Saharan Africa has the additional challenge that the biotechnology regulatory framework is still evolving. 博士. Edmeades is concerned that the biosafety framework is leading to country-by- country regulations when a regional approach would be far more efficient in using resources. Present regulatory systems are modeled on risks that experience suggests are overestimated. He wrote, “Thus the precautionary principle on transgenic crop regulation in its present form is hurting resource-poor farm families — the very people it was designed to protect, and forcing the development of transgenics back into the hands of a few large well-resourced institutions.”

Further gains in conventional breeding and MAS depend on the availability of adequate genetic variation and faster rates of genetic progress. Germplasm collections are assuming greater importance because further drought tolerance in the near term will come largely from native genes. Transgenic drought tolerance has been more difficult to develop than expected, partly due the lack of genes with large effects.

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