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Developing maize-breeding methods and cultivars to meet the challenge of climate change Marcelo J. Carena, North Dakota State University, USA 1 Introduction 2 Early developments in breeding 3 Hybrid breeding: heterosis 4 Inbred–hybrid breeding 5 Limitations in current breeding techniques 6 Exploiting genetic diversity 7 Breeding for marginal environments 8 High-throughput phenotyping 9 Case study: use of exotic germplasm 10 Case study: short-season quality maize hybrids 11 Case study: cold and drought-resistant varieties 12 Summary and future trends 13 References

1 Introduction The United Nations, through its Food and Agriculture Organization, has stated numerous times that people have the right to access adequate food. The agricultural sector has the potential to produce more food for a growing population, but the challenge is to produce it in a sustainable way. Many public and private institutions promote the agricultural sector, including breeders and farmers, to ‘feed the world’. But there is still a long way to go, and developing the next generation of sustainable maize-breeding methods and cultivars is part of the solution. Breeders can develop improved cultivars for both extensive and intensive production systems in a sustainable manner that can reach all people, including farmers in the world’s marginal environments growing maize to sustain their families. © Burleigh Dodds Science Publishing Limited, 2016. All rights reserved.


Developing maize-breeding methods and cultivars

Sustainable breeding is a viable scientific alternative to maintain enough food supply under the environmental challenges facing our planet. It includes responsible breeding with the most efficient use of resources, genetic diversity, maximizing conservation and minimizing waste. In order to achieve a more sustainable maize production, a commitment from independent scientists is needed for new ideas to solve breeding paradigms, improving the quality of life for all human beings, not just a few. The problems posed by climate change warrant innovative solutions to tackle the challenges that lie ahead. This chapter addresses ideas to develop not only unique cultivars but also breeding methodologies to assist breeders selecting most desirable genes in genetically complex traits in an efficient and sustainable way. Climate change calls for increasing efforts in the integration of germplasm improvement with cultivar development, adapting germplasm carrying unique genetic properties and diversity not available in sequenced genomes or current cultivars available in expensive research projects (e.g. NAM, G2P, among others). It is also an opportunity to move breeding stations from traditional production centres to strategic marginal locations. Scientists are encouraged to be creative in their search for needed research to address current and future challenges. World maize breeders, with the support of their national leaderships and international centres, have the task of developing the next generation of sustainable cultivars and cost-effective breeding methodologies that will facilitate successful and sustainable results for the common good of all world citizens and their environments. Sustainable maize production will be possible only with the development of the next generation of maize products carrying genetic diversity. Sustainable maize breeders face current and future climate challenges that can be addressed only by germplasm diversity. Only those maize-breeding programmes utilizing large samples of genetically broad-based germplasm have the potential to contribute useful and unique alleles to face climate challenges. These programmes need to integrate germplasm adaptation and improvement with cultivar development adapting and improving exotic and unique germplasm. Developing new products from these genetically broad-based germplasm sources should supplement the typical elite-by-elite line pedigree selection cultivar development process often used in breeding programmes focused on genetically narrow-based germplasm. In order to obtain elite germplasm to develop new sustainable maize cultivars, breeders need to adapt and improve diverse genotypes to their target environments. However, the choice of germplasm determines the success of a breeding programme. No breeding methodology or technology can provide successful cultivars with a poor germplasm choice. However, once elite germplasm is chosen, breeders should be able to develop not only new cultivars but also new breeding methodologies for efficient selection of quantitative traits. There is a need for breeders to develop not only products but also methodologies that can save time and cost in measuring economically important traits that are genetically complex, difficult to screen and largely influenced by the environment. Breeders will need to develop not only new products but also new breeding methodologies to increase the high-throughput screening efficiency of quantitative traits of economic importance. Rate of dry down (Yang et al. 2010), cold tolerance (Carena 2013b) and drought tolerance (Sharma and Carena 2016) are examples of new and successful inventions on breeding methodologies that have facilitated the screening of thousands of genotypes for traits that are difficult to measure and significantly influenced by the environment. These are breeding techniques that avoid the destruction of plots and save millions of dollars in maize-breeding and production costs. Ideas to overcome the evaluation of genetically complex traits will continue to be a priority for developing the next generation of sustainable maize products. Molecular techniques were initially targeted at economically Š Burleigh Dodds Science Publishing Limited, 2016. All rights reserved.

Developing maize-breeding methods and cultivars3

important traits, which could be the reason why this technology is now utilized in genetically simple traits. Technology, if used wisely, however, should target traits according to their genetic complexity. Flowering time is of the more simple traits for breeders to move maize to higher latitudes at a very low cost. On the other hand, drought tolerance and nondestructive root-screening systems (Sharma and Carena 2016) and the rate of dry down (Yang et al. 2010) are successful examples of genetically complex traits targeted with new breeding methodologies that can facilitate a high-throughput screening of large samples. Breeders should focus on new breeding methodologies for genetically complex traits, including the use of unmanned aircraft systems (UAS).

2  Early developments in breeding Maize (Zea mays L.) is considered one of the important natural resources of the Western Hemisphere. It appeared as a wild species about 7000 to 10 000 years ago. N. I. Vavilov proposed that maize was derived from teosinte and that its centre of origin was in Central America (Fig. 1 and 2). After domestication, maize was widely grown by Native Americans in the United States before the arrival of Columbus in 1492. These first maize breeders are a successful example of breeder-directed evolution. As cultivation of maize became

Figure 1 ‘Nikolai Vavilov NYWTS’ by World Telegram staff photographer – Library of Congress. New York World-Telegram & Sun Collection. Licensed under Public Domain via Commons – jpg#/media/File:Nikolai_Vavilov_NYWTS.jpg. © Burleigh Dodds Science Publishing Limited, 2016. All rights reserved.


Developing maize-breeding methods and cultivars

Figure 2 ‘Maize diversity in Vavilov’s office (3421259242)’ by Luigi Guarino from Suva, Fiji – Maize diversity in Vavilov’s office. Licensed under CC BY 2.0 via Commons – https://commons.wikimedia. org/wiki/File:Maize_diversity_in_Vavilovs_office_(3421259242).jpg#/media/File:Maize_diversity_in_ Vavilovs_office_(3421259242).jpg.

more extensive, selected open-pollinated strains were developed. By the end of the 1800s, U.S. farmers had 800–1000 unique landrace options to grow (Sturtevant 1899), some of them showing yields of over 6 t ha1 when properly cultivated (Leaming 1883). During this time, seed cost was barely a concern for farmers who save their own seed for sowing the following year. Still this is a very low-cost and sustainable system for maize production worldwide. In short-season dryland North Dakota (ND) environments, unique varieties improved by recurrent selection methods have often provided grain yield values of 8–10 t ha1 per se in experiment trials evaluated across years. Improved populations are also used for the development of genetically broad-based inbred lines and hybrids with unique grain quality and stress-tolerant properties not available in commercial hybrids. They should be freely available for farmers all over the world to take advantage of this system if backed up by active public breeding programmes that improve varieties. It is desirable that improved populations be stored at the U.S. National Plant Genetic System (NPGS) in Fort Collins, CO for future generations as local university cold storage conditions are not often appropriate for not only the long term but also, in many cases, for the short- and mid-terms. In addition to the significantly low seed costs, the following year’s crop of improved populations is expected to deliver, genetically, a similar crop. On the other hand, if farmers plant the seed harvested from a single-cross hybrid (e.g. where all plants are genetically identical) typically grown on extensive farms today, the following year’s crop is expected to deliver, genetically, a poor crop. The system, therefore, makes farmers buy seed annually.

3  Hybrid breeding: heterosis The use of landraces, and later on improved varieties, for production per se was discouraged with the discovery of heterosis. During the early 1900s, extension and research maize © Burleigh Dodds Science Publishing Limited, 2016. All rights reserved.

Developing maize-breeding methods and cultivars to meet the challenge of climate change  

Sustainable maize production will be possible only with the development of the next generation of maize products carrying genetic diversity....

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