Agriculture and Agri-Food Canada Research Scientist, Ottawa Research and Development Centre
Corn, a tropical plant, was first domesticated approximately 8,000 years ago in Central America. Many different types of corn evolved with the help of indigenous people who were the first corn breeders.
Today, the ultimate goal of corn breeding is to improve the adaptation of corn to temperate and early season environments. Improved adaptation means higher yield and better quality. The development of a new corn hybrid is a slow and costly process. New hybrids must possess improved yield, standability, pest resistance and tolerance to various stresses. This means that the expertise of breeders, entomologists, pathologists, physiologists and many other specialists are required. Corn grain yields have increased in North America from approximately 1.3 t/ha (0.6 T/ac) in 1930 to 8.7 t/ha (3.9 T/ac) in 1994 or approximately 0.08-0.1 t/ha per year. This steady increase is due to a combination of improved hybrids, increased use of fertilizers, better weed control and higher plant densities.
There are five major steps in the development of a commercial corn hybrid:
1) selection and development of appropriate source germplasm
2) development of superior inbreds
3) testing of inbreds in experimental hybrid combinations
4) identification of a superior hybrid combination
5) multi-location testing of the pre-commercial hybrid
Finally, extensive seed production and marketing of all new hybrids is required.
To understand how a new hybrid is developed, a basic knowledge of corn pollination and breeding processes is required. The corn plant has separate male and female flowering parts (Fig. 1). The tassel is the male flower and produces pollen; the ear is the female flower. A typical hybrid corn ear consists of several hundred kernels attached to the cob or rachis and surrounded by a group of modified leaves called the husk. Each kernel starts as an ovule and has its own silk which grows out of the husk at the top of the ear.
When the tassel is fully emerged from the upper leaf sheath, pollen-shed will begin, usually from the middle of the central spike of the tassel and then spreading out over the whole tassel. Pollen grains are produced in anthers which open up under appropriate weather conditions. Pollen, which is only viable for 18-24 hours, is very light and can be carried considerable distances by the wind. Pollen shed from the tassel usually begins 2-3 days before silk emergence and can continue for several days thereafter, but will stop when the tassel is too wet or too dry.
The silks are covered with fine, sticky hairs that catch and anchor pollen grains. Within minutes after landing on the silks, the pollen grain germinates and a pollen tube grows down the silk to fertilize the ovule or potential kernel. This usually takes 12 to 28 hours. Under good conditions, all silks will emerge and be ready for pollination within 3 to 5 days. Unfavourable environmental conditions during pollination can have a great impact on grain yield. Since there is usually more than enough pollen (a given tassel can produce up to 5 million pollen grains), problems generally occur when there is poor synchronization between silk emergence and pollen shedding.
Corn with its separate male and female flowering parts is a naturally cross-pollinating plant. This means that ovules can be pollinated by pollen from neighbouring plants. Therefore, care must be taken in a breeding program to ensure that pollen from the appropriate tassel fertilizes ovules on the appropriate ear. This is usually achieved by hand-pollinating. As soon as ear shoots are visible in the leaf axils of a plant, a small paper ‘shoot-bag’ is placed over the shoots; this allows the ear to continue growing and the silks to emerge but prevents any pollen from falling on the silks (Fig.2).
When pollen shed begins, a paper bag is placed over the tassel and stapled at the base of the tassel to trap the pollen. The next day the tassel bag containing pollen is removed and quickly placed over the silks of a covered ear after removing the protective shoot-bag (Fig. 3). The tassel bag is pulled around the stalk, stapled and shaken so that the pollen grains fall on the silks (Fig. 4). A plant is ‘selffertilized’ (also referred to as selfing or inbreeding) when the pollen from a tassel is placed on the silks of the ear of the same plant (Fig. 5). A plant is ‘crossfertilized’ or ‘crossed’ when the pollen from a tassel is placed on the silks of a different plant. Of the millions of hand pollinations made by corn breeders, only a handful result in a superior inbred that will be used in a commercial hybrid.
| Fig. 3 Transferring pollen from tassels of male parent
to silks of female parent.
|Fig. 4 Maintaining inbred lines.|
Between 1850 and 1910, North American corn breeders developed higher yielding corn varieties by open-pollination. In this procedure, plants were allowed to shed pollen without covering silks, resulting in a mix of cross and self pollinated kernels on each ear. The best plants would be selected and their ears (usually the largest ones in the field) would be kept to use as seed the next year. The resulting populations were gradually improved for agronomic traits, but were very variable in plant height, ear height, maturity, etc., due to the random cross-pollinations.
In the 1920’s, the concept of hybrid vigour (heterosis) was discovered. If corn plants are self-pollinated for six or more generations, the plants become smaller and less vigorous due to inbreeding depression, but their traits become more uniform. At every generation, selection can be made for specific traits such as pest resistance, plant or ear type, ear size, etc. This repeated inbreeding produces an ‘inbred’ line of corn. We can save breeding time by getting two generations per year using winter nurseries in warmer climates.
An inbred is genetically uniform for all traits and will always breed true to form. Hybrid vigour occurs when we crosspollinate two inbred lines from different unrelated backgrounds (Fig. 6). The offspring of such a cross will have a larger-yielding ear and will be a more robust plant. It is also uniform for most traits. There are many theories to explain hybrid vigour, but this phenomenon is still not well understood. Note that if an ear of hybrid corn is self-pollinated, the resulting progeny will be variable in yield as well as in other traits. This is why farmers must buy their hybrid corn seed each year and should not plant the seed from a field of hybrid.
Development of inbreds takes about 75% of the effort in a corn breeding program. Most of the effort is spent evaluating inbreds by crossing to another inbred, which is called a tester, to see if it will produce a desirable hybrid. The process is called evaluating the combining ability of the inbred. The cross is called a testcross. The field performance of this testcross is extensively evaluated in replicated multi-location trials. Inbreds with superior testcross performance are advanced to the next generation. If we could select at the inbred level, i.e. if the performance of the inbred on its own could predict the performance of the hybrid testcross, we could considerably reduce expenses. In fact, this can be done for some traits such as earliness, plant height and some disease resistance but, unfortunately, not for yield. It is important to note that the seed sold to farmers is produced on small inbred plants. Therefore, besides having good combining ability, an inbred line must be easy to maintain and to cross in order to keep seed costs down.
The inbred lines used for commercial hybrids must be maintained by hand-pollination, a painstaking process (Fig. 4). For production of hybrid seed, inbred seed is planted in fields isolated from other corn by at least 200 m (600 ft). Hybrid seed is produced by planting the ‘female’ and ‘male’ inbred lines together in a field (Fig. 7).
The choice of which inbred to designate female and which to designate male depends on the ear and tassel characteristics of each; usually the female has higher yield and the male has better pollen production. The ratio of female to male rows varies among seed companies. Differential planting dates can be used to ensure synchronization between male and female flowering.
Female rows are detasseled mechanically or by hand shortly after the tassels have emerged from the uppermost leaf sheath and before they begin to shed pollen (Fig 8). This ensures that all pollen is from the male parent. Commercial seed-corn fields are normally harvested by a picker-husker and the husked ears are sorted to remove off-type ears. The ears are dried and shelled and the seed is cleaned and graded by size. Finally, germination is tested and the seed is treated with a fungicide before packaging.
Today, 80% of corn seed grown in North America is single-cross hybrid as described above. The remaining 20% of hybrids are double, three-way and modified (related-line parents) crosses. Three-way cross hybrids have only one inbred parent and are somewhat cheaper to produce.