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Regeneration guidelines for maize

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The information on this page was extracted from:
Taba S. and Twumasi-Afriyie S. 2008. Regeneration guidelines: maize. In: Dulloo M.E., Thormann I., Jorge M.A. and Hanson J., editors. Crop specific regeneration guidelines [CD-ROM]. CGIAR System-wide Genetic Resource Programme, Rome, Italy. 10 pp.

Before reading the regeneration details for this crop, read the general introduction that gives general guidelines to follow by clicking here.

Introduction

Maize (Zea mays L. subsp. mays) is an outcrossing, monoecious annual crop that evolved in southern Mexico, possibly from the close relative teosinte. The seed bearing ears are borne laterally at the mid-nodes of the plan, while the male flowers (tassel) are on top of the plant. There are more than 250 races and local cultivars of maize in Latin America. Some of the mid-altitude races in Latin America have growing seasons of more than 10 months while some early maturing races take less than 3 months from planting to harvest. Some races are 4–5 metres tall, making artificial pollination difficult. Traditionally, maize breeders classified maize ecotypes by their adaptation to growing environments: tropical (<1200 m), mid-altitude (1200–1900 m) and highland (1900–2600 m) for those growing between 26° north and 26° south; and temperate for those cultivars growing in latitudes above 26° north and below 26° south.

Maize (Zea mays L. subsp. mays)

The diverse phenotypes and widely differing adaptations of the races of maize and local cultivars are often constraints to regeneration. The germplasm accessions are either genetically heterozygous (panmictic populations) or homozygous (inbreds). The recommended practices and procedures for maize germplasm regeneration are compiled from experience and consultation of theoretical studies on sample size and mating system.

Choice of environment and planting season

Climatic conditions

Preparation for regeneration

When to regenerate

Pre-treatments

Precautions

Method of regeneration

Regenerate maize using controlled pollination.

Artificial pollination

This method is the most commonly used for germplasm accession regeneration and multiplication. It can be done either by plant-to-plant or chain crossing. Chain crossing is recommended for regenerating large numbers of accessions.

Natural or open pollination

Prevention of GMO contamination

Floral induction

Planting layout, density and distance

This photograph shows the first stage of hand pollination. One ear is selected on each plant and the silk is covered with a glassine bag to prevent pollination from any foreign pollen. On a second trip through the field, pollen from the male tassel are collected and applied directly to the silk of the next plant’s ear. This “chain crossing” is necessary to ensure that the highest level of genetic diversity is maintained in each accession (Photo: CIMMYT)

Crop management

Maize is generally grown under rainfed conditions but can also be grown under irrigation.

Irrigation

Fertilization

Common pests and diseases

Contact plant health experts to identify the symptoms of the likely pests and diseases and the appropriate control measures. The following are common pests and diseases for maize:

Pest and disease control

Consult a plant health expert for guidance.

Roguing off-types

Others

Harvesting

1. Before harvest, record all relevant agronomic traits (see ‘documentation’ below).

2. Immediately before harvest, record the number of plants lodged and number of plants pollinated.

3. At harvest, the black layer of the seed is formed and most leaves, especially husk leaves, have dried. Remove the pollinated ears from the plant and place the ear either under the plant or in front of the row for inspection (photos 5 a, b).

Harvested ears for inspection in the field (photos: CIMMYT)

4. Further inspect the ears individually and remove diseased, contaminated or abnormal kernels on the cob before and after shelling.

5. Include clean ears with good grain quality to represent the regeneration cycle and record the number of ears forming the seed accession in the regeneration field book.

6. Treat the harvested ears with insecticide to protect them from insect damage during seed processing.

Number of seeds harvested per pollinated ear

Post-harvest management

1. Pre-dry harvested ears in a chamber with heated air (not more than 35°C) blown through the piles of ears to reduce the seed moisture to about 13–15%. If the maize is quite damp at harvest, keep drying temperatures below 30°C. Where special drying facilities are not available, dry ears in the shade with good air circulation.

2. Shell the ears to the individual seed envelope and balance seed samples prepared from all ears to represent the regeneration cycle, normally by taking the same number of kernels per ear. Further cool-dry the seed bulks of the accessions. It is ideal to make several regeneration packets of two seeds each from the individual ears (for long-term preservation) for subsequent regeneration cycles (Crossa 1989).

3. Perform secondary seed drying by placing the seed in cloth or paper bags and putting these in a cool dry room at low temperature and humidity (10–15°C and 15–20% relative humidity) for at least 4 weeks, until seed moisture reaches 6–8% in equilibrium. This is normally done using special driers which combine cooling and dehumidifying functions. If such equipment is not available, dry seeds to a moisture content of 7–8% with silica gel or another appropriate desiccant.

4. Prepare several sets of the balanced bulks for preservation in active, base and safety duplicate collections. Send a sample of each accession to a seed health laboratory for quarantine requirements.

5. Register seed test weight (1000 seed weight) and germination percentage before storage.

6. Record other regeneration data (see documentation below) into the genebank management system. Check the original passport data to see if the seed characteristics are the same as described in the original records in order to replace, if necessary (see 8 below) the old seeds with the regenerated seeds.

7. Store seed samples at the respective storage locations according to genebank norms (active, base and safety backup collections).

8. Replace old seeds in the active and/or base collections with new regenerated seeds to facilitate management and save space. A small sample of original seed may need to be kept as reference material.

Monitoring accession identity

Documentation of information during regeneration

A field book of the regeneration nursery is recommended to document identification, characterization, seed origin, number of plants pollinated and harvested and agronomic traits of the accession and introduction. The field book can contain the following detailed information:

References and further reading

Bioversity International, CIMMYT. 2009. Key access and utilization descriptors for maize genetic resources. Bioversity International, Rome, Italy; International Maize and Wheat Improvement Center, Mexico. Available here.

CIMMYT Maize Program. 2004. Maize diseases: A guide for field identification. 4th edition. CIMMYT, Mexico City, Mexico.

Crossa J. 1989. Methodologies for estimating the sample size required for genetic conservation of outbreeding crops. Theoretical and Applied Genetics 77:153–161.

Crossa J, Taba S, Eberhart SA, Bretting P, Vencovsky R. 1994. Prac tical considerations for maintaining germplasm in maize. Theoretical and Applied Genetics 89:89–95.

FAO/IPGRI. 1994. Genebank standards. Food and Agriculture Organization of the United Nations, Rome and International Plant Genetic Resources Institute, Rome. Available in English, Spanish, French and Arabic.

Franco J, Crossa J, Taba S, Shands H. 2005. A sampling strategy for conserving genetic diversity when forming core subsets. Crop Science 45:1035–1044.

Hartkamp AD, White JW, Rodriguez Aguilar A, Banzinger M, Srinivasan G, Granados G, Crossa J. 2000. Maize production environments revisited: A GIS-based approach. CIMMY T, Mexico City, Mexico.

ISTA. 2008. International Rules for Seed Testing. International Seed Testing Association. ISTA Secretariat, Switzerland.

Lafitte HR. 1994. Identifying production problems in tropical maize: A field guide. CIMMYT, Mexico City, Mexico.

Mezzalama M, Gilchrist L, McNab A. 2005. Seed Health: Rules and regulations for the safe movement of germplasm. Mexico. D.F., CIMMYT. Available from: URL: http://libcatalog.cimmyt.org/download/cim/93586.pdf. Date accessed: 3 September 2010.

Ortega AC. 1987. Insect pests of maize. A guide for field identification. CIMMYT, Mexico City, Mexico.

Pardey PG, Koo B, Van Dusen E, Skovemand B, Taba S, Wright BD. 2004. CIMMY T genebank. In: Saving Seeds: The economics of conserving crop genetic resources ex situ in the Future Harvest centers of the CGIAR. CABI Publishing, UK . pp. 21–47.

Salhuana W. 1995. Conservation, evaluation and use of maize genetic resources. In: Engels JMM, Rao RR, editors. Regeneration of Seed Crops and Their Wild Relatives. ICRISAT, India.

Vencovsky R, Crossa J. 1999. Variance effective population size under mixed self and random mating with applications to genetic conservation of species. Crop Science 39:1282–1294.

Wang J, Crossa J, van Ginkel M, Taba S. 2004. Statistical genetics and simulation models in genetic resource conservation and regeneration. Crop Science 44:2246–2253.

Acknowledgement

These guidelines have been peer reviewed by Jose Crossa, International Maize and Wheat Improvement Center (CIMMYT), Mexico; Major Goodmann, USA; and Zachary K. Muthamia, National Genebank of Kenya (NGK ), Kenya.

The Genebanks

The 11 CGIAR genebanks currently conserve 730,000 of cereals and grain legumes, forage crops, tree species, root and tuber crops, bananas and crop wild relatives.

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