Farming

Maize Hybridization

Maize hybrid seed provides farmers with varieties containing improved genetics, such as high yield potential and unique trait combinations to counter diseases and adverse growing conditions. However, the quality of hybrid seed depends greatly on-field production methods, both in adherence to quality assurance standards and implementation of appropriate agronomic management. While open-pollinated maize seed production is relatively straightforward, hybrid seed production requires additional field practices that are critical to success. Hybrid maize seed production involves deliberately crossing a female parent population with a male parent in isolated fields.

Thus, from the very start of hybrid seed production, the identity and arrangement of the two-parent populations determine the outcome. Each hybrid variety is composed of a specific combination of female (seed-bearing) and male (pollen-providing) parents. The field management of the two parents is also important and requires attention to the timing of planting, elimination of off-types, removal of tassels from the females before pollen shedding, separate harvesting of the female seed, and careful shelling and processing of the seed to maintain seed quality.

The sequentially dependent nature of the process means that any errors in earlier stages have a significant impact on the following stages and major errors or problems can result in complete failure or rejection of the crop.

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The plant that bears the seed is called the female or seed parent, while the plant that provides the pollen to fertilize the female is called the male or pollen parent. In other words, the female plant is crossed with the male plant to produce a hybrid seed. This seed bears a unique genetic make-up from the female and male parents and will produce a plant with particular characteristics. Plant breeders produce the female and male parents of each hybrid to generate progeny with particular characteristics, such as plant maturity, disease resistance, grain color, food processing quality, and so on. It is this unique hybrid seed that farmers will sow in their fields. When a farmer purchases a particular hybrid, he or she expects the seed to perform in the field as designated by the variety description.

With maize, there are a number of possible kinds of hybrids, such as single-cross, three-way, doublecross, and top-cross hybrids. These hybrids differ in their parental composition but, in all cases, the hybrid seed sold to farmers is a cross between two parents – a female and a male. Since maize has separate male and female plant parts, it is relatively easy to make a cross between two plants. In a hybrid seed production field, male and female parents are planted in sequential row patterns, usually with three-to-six times the number of female plants or rows to a single male plant or row. The male flower (tassel) of the female plant is removed (detasseled) before pollen is shed so that the only source of pollen for the female flower (the cob or ear) on the female plants is the tassels on the male plants. Detasseling of the female is necessary to prevent any pollen from the female from pollinating the female silks. If this occurs, known as “female-selfing,” the result is a significant loss of seed quality that will clearly be seen in a crop grown from the seed. Female-selfing is to be avoided at all costs.

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The height of the male tassel relative to the female silk and the size of the female plant. Furthermore the timing of the pollen shed of the male and silking of the female must coincide. If the male and female plants are known to flower at different times, an adjustment in the sowing dates of each component will be required to ensure the flowering synchrony of the two parents. The hybrid seed that is useful to farmers is harvested from female plants. Plants and seeds from the male rows are usually discarded before harvest to avoid the mixing of seeds from the parents. With this preliminary description of maize hybrid seed production, it is clear that numerous key factors determine the success and quality of hybrid seed production, including the following.

  • Female and male parent identity, purity and identity preservation.
  • Ratio of female to male rows in the seed field.
  • Timing of planting of the female and male plants.
  • Timely removal of the tassels from the female plants before they shed pollen and before silk emergence.
  • Timing of female silk emergence relative to male pollen shed.
  • Avoidance of contamination of female silks with unwanted pollen, particularly from females, off type males and foreign pollen.
  • Avoidance of seed mixtures between and within the male and female plants.

Each of these key issues will be discussed in greater detail, along with important agronomic management factors.

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Hybrid composition

The basic building blocks of maize hybrids are inbred lines. Inbred lines are the result of repeated self-pollination of particular maize populations to produce a plant that essentially has a fixed and uniform genetic composition. Consequently, all the plants of a particular inbred line are identical, but each inbred line will differ in its genetic composition from other inbred lines. Because maize is normally cross-pollinated, inbred lines are usually smaller, less vigorous, and lower-yielding than open-pollinated maize plants due to a phenomenon called “inbreeding depression.” But, when two unrelated inbred lines are crossed to form a hybrid, the resultant seed produces plants with restored vigor and a significantly higher yield than either of the two parents. This is known as “hybrid vigor,” and it is this vigor that is exploited in hybrids and makes hybrid varieties useful to farmers.

The most common types of hybrids in maize are single-cross, three-way, and double-cross hybrids. A single-cross hybrid is made by crossing two inbred lines; a three-way hybrid is made by crossing a single-cross hybrid with an inbred line; while a double-cross hybrid is made by crossing two single cross hybrids. Two other types of hybrids are top-crosses and varietal crosses. A top-cross hybrid is made from an open-pollinated variety crossed with an inbred line, while a varietal cross is a hybrid of two unrelated open-pollinated varieties. On the assumption that the best genetics and agronomic management are applied, general statements may be made about the advantages of different hybrid types. Single-cross hybrids are generally higher-yielding than other hybrid types. They are very uniform in appearance because every plant has the same genetic make-up, but the seed yield of a single-cross hybrid is lower than other hybrids because the female is an inbred line.

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Consequently, the seed of single-cross hybrids is the most expensive, but this is usually acceptable because of the high yield potential of the seed. Three-way and double-cross hybrids use a single-cross hybrid as a female, and so the seed yield is high. In fact, with double-crosses, the seed yield can be the highest of all hybrid types because a high female: male ratio may be used, given the vigor and abundant pollen production of the single-cross male. Thus, double-cross hybrid seed is the least expensive, but the resulting hybrid crop is more variable and the grain yield is usually less than that of a three-way or single-cross hybrid. This is because farmers in these settings aim at higher yields and can afford to pay the higher price, given that the seed cost is a lower percentage of total costs than in low-yield situations. 

Varietal cross hybrids and top-cross hybrids are also used in some African countries or where seed markets require low-cost products. The advantage of using a varietal cross hybrid is that the male parent produces abundant pollen that enhances the seed set. In addition, there may be better synchrony between male tasseling and female silking, because of the greater variability in the two parents. By the same token, varietal cross or top-cross hybrids are the least uniforms and least productive in farmers’ fields, among the different types of maize hybrids.

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