THE BASICS OF CORN BREEDING: INBREDS AND HYBRIDS

Corn, a tropical crop, originated 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. The ultimate goal of corn breeding in Canada is to improve the adaptation of corn to temperate and short season environments. Improved adaptation results in higher yields 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. To achieve this requires the expertise of breeders, entomologists, pathologists, physiologists and many other specialists. There are five major steps in the development of a commercial corn hybrid:

1. identification of appropriate parental germplasm;
2. development of superior inbreds from the parental populations;
3. evaluation of the inbreds in experimental hybrid combinations;
4. identification of a superior hybrid combination(s); and
5. multi-location testing of the pre-commercial hybrid(s).

Once developed, extensive seed production and marketing of the hybrid is required.

To understand these processes, a basic knowledge of the structure of the corn plant and general breeding processes is required. Corn has separate male and female flowering parts (Figure 1). The tassel produces pollen and is the male flower; the ear is the female flower. A typical hybrid corn ear consists of several hundred kernels attached to the cob (rachis) and is surrounded by modified leaves that form the husk. Each kernel starts as an ovule and has its own silk that grows out of the husk at the top of the ear.

When the tassel is fully emerged from the upper leaf sheath, pollen shed begins from the middle of the central spike of the tassel and then spreads out over the whole tassel. Pollen grains are borne in anthers that open up under appropriate weather conditions. Pollen shed from the tassel usually begins 2-3 days before silk emergence and can continue for several days afterward. Pollen grains are very light and can be carried considerable distances by the wind. Pollen shed will stop when the tassel is too wet or too dry. Pollen grains are only viable for 18-24 hours.

The silks are covered with fine, sticky hairs that catch and anchor the 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 12-28 hours. Under good conditions, all silks will emerge and be ready for pollination within 3-5 days. There is usually more than enough pollen since a given tassel can produce up to 5 million pollen grains (roughly 5,000 per silk), so problems generally occur when there is poor synchronization between silk emergence and pollen shed. The greatest impact on grain yield occurs when environmental conditions are unfavourable during pollination time.

Corn, because it has separate male and female flowering parts, is a naturally cross-pollinated plant. Kernels 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 often achieved by conducting hand-pollinations. As soon as ear shoots are visible in the leaf axils of a plant, a small paper 'shoot-bag' is placed over the ear shoot (Figure 2); this allows the ear to continue growing and the silks to emerge but prevents any pollen from falling on the silks. When the tassels are shedding pollen, a paper bag is placed over the tassel and stapled or paper-clipped at the base of the tassel to trap any pollen that is shed. The next day the tassel bag is removed and quickly placed over a covered ear that has silks, taking care to quickly remove the shoot-bag on the ear. Alternatively, a see-through glassine bag can be placed on the tassel and shaken to allow pollen to collect in one corner of the bag; this corner is then ripped off and the pollen is deposited on the silk (Figures 3a, 3b) and then the whole ear is rapidly covered with a brown paper tassel bag. With both techniques, the tassel bag is pulled around the stalk, stapled and shaken to assist the pollen grains in falling on the silks (Figure 3c). A plant is 'self-fertilized' (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 (Figure 4). A plant is 'cross-fertilized' or 'crossed' when the pollen from a tassel is placed on the silks of a different plant (Figure 4). Of the millions of hand pollinations made by corn breeders, only an extremely small amount ever result in a superior inbred that will be used in a commercial hybrid.

From 1850-1910, North American corn breeders developed higher yielding open-pollinated corn varieties. The plants in a given field were allowed to shed pollen and no silks were covered, 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 plants were gradually improved for agronomic traits, but were variable in plant height, ear height, maturity, etc….due to the continuous cross-pollinations. In the 1920's, the concept of hybrid vigour (heterosis) was discovered. If corn plants were continually self-pollinated for six or more generations, the plants became smaller (due to a decrease in vigour) but more uniform for all traits. Selection can be made every generation for specific traits such as pest resistance, plant or ear type, ear size, etc. This produces what is called an inbred line or inbred. The development of an inbred can be accelerated with the use of winter nurseries in warmer southern climates.

An inbred is genetically uniform for all traits and will breed true to form. Hybrid vigour is demonstrated two inbred lines from unrelated backgrounds are cross-pollinated (Figure 5). The offspring of such a cross will have a larger ear and will be a more robust plant. It is also uniform for most traits. There are many theories to explain the phenomenon of hybrid vigour, but few are adequate to explain everything. If an ear of hybrid corn is self-pollinated, the resulting progeny will be not be genetically uniform, will be variable in height and have lower yields. This is why farmers must buy their hybrid corn seed each year and should not keep the seed from a field of hybrid corn to plant next year.

Development of inbreds takes about 75% of the breeding resources in a corn breeding program. The largest effort is spent evaluating whether or not the inbred when crossed to another inbred which is called a 'tester', will produce a hybrid with high yield and good agronomic characteristics. This is called evaluating the combining ability of the inbred. The cross or hybrid is called a 'testcross'. The field performance of this testcross is extensively evaluated in replicated multi-location field trials. Inbreds with superior testcross performance are retained and advanced to the next generation. It would be great if selection could be done at the inbred level since expenditures for testing could be reduced considerably if the performance of the inbred on its own could predict the performance of the hybrid testcross. This happens for some traits such as earliness, plant height and disease resistance but unfortunately not for yield. Besides having good combining ability, an inbred line must be easy to maintain and to cross if hybrid seed production costs are to be kept low, and a consistent production of high yields of uniformly good quality seed is to be maintained.

Pure stocks or breeder's seed of inbred lines used in commercial hybrids must be maintained by hand-pollination. For production of hybrids, inbred seed can be increased in isolated fields which must be planted a minimum of 200 m from any other corn crop. For each commercial hybrid, large quantities of seed of its parental inbred lines and the cross must be produced to ensure an adequate supply of seed is available for growers. Hybrid seed is produced by planting the 'female' and 'male' inbred lines together in a field. 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 seed yield and the male has good pollen production. The ratio of female to male rows varies with different seed companies. Differential planting dates can be used to ensure synchronization between male and female flowering. Female rows are mechanically and/or hand-detasseled shortly after the tassel has emerged from the uppermost leaf sheath and before any female plants begin to shed pollen. At harvest, commercial seed corn fields are usually harvested by a picker/husker and the husked ears are sorted to remove off-type ears and trash. The seed is dried, shelled, cleaned and graded. Germination tests are also carried out and the seed is treated with a fungicide before packaging.

Today, more than 80% of corn grown in North America is a single-cross hybrid of this nature. The remaining 20% consists of double, three-way and modified (related-line parents) crosses. Corn yields have increased in North America from approximately

1.3 Mg/ha in 1930 to 8.7 Mg/ha in 1994 or approximately 0.078-0.110 Mg/ha per year. This steady increase is due to improved hybrids, increased use of fertilizers, better weed control, higher plant densities and better management.