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Corn Breeding: Types of Cultivars

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Populations, Open-pollinated Varieties, and Synthetics

A population is a group of plants that are mating with each other (i.e., intermating). At many loci, more than one kind of allele will be present in the population. Thus, unlike an inbred or a single-cross hybrid, each plant in a population will have a unique set of genes and will be genetically different from all other plants in the population. Collectively, however, each new generation of a population will share certain characteristics it inherits from the previous generation. That is, each population will have a set of characteristics that distinguishes it from other populations. An example of a popluation in nature would be all the mice living in a barn.  They mate among each other, but rarely or not at all with mice living elsewhere.

Open-pollinated varieties and synthetics are types of corn populations. Until the introduction of double- and then single-cross hybrids, farmers grew open-pollinated varieties. Unlike hybrids, seed production of these varieties requires no controlled pollinations. In fact, open-pollinated varieties were so named because seed for the following year’s crop was obtained by saving seed from the current year’s crop. This seed was from open-pollinated plants.

Hundreds of different open-pollinated varieties were developed by farmers in the United States during the 19th and early 20th centuries by selecting for different plant characteristics in different environments. Some of the more famous of these were Krug, Lancaster Sure Crop, Leaming, Midland, and Reid (Figure 5).

Fig. 5:  Lancaster Sure Crop (left) and Reid (right) are open-pollinated varieties of corn.
(UNL, 2004)

These and other open-pollinated varieties were the source material from which the first inbreds were developed. Typically, these inbreds were developed by successive generations of self-pollination (Figure 1).

Most inbreds today are developed not from open-pollinated varieties but rather from F2 populations. Usually, the two inbred grandparents of these F2’s are related, so the typical single-cross hybrid from which a F2 is developed is not one that is sold as a commercial product.

Synthetic is a term often used to describe a population that is developed by allowing cross-pollination to occur for several generations among a number of different cultivars, such as inbreds. A F2 is a two-line synthetic. Probably the most famous corn synthetic is the Iowa State Stiff-Stalk Synthetic. This synthetic was developed at Iowa State University in the 1930’s and 1940’s by crossing together for several generations 16 inbreds that at the time were considered to have above-average stalk strength. Using various selection methods, this population has been undergoing improvement at Iowa State University for many decades.

A population may be genetically improved in several ways. First, the performance of the population itself may be improved (this is referred to as per se performance). Also, the performance of a hybrid in which the population is one of the parents could be enhanced (this is called cross or testcross performance). Finally, the probability of obtaining an elite inbred by repeated self-pollination of individual plants from the population may increase.

Public corn breeders have developed many improved populations. Some were improved primarily for grain yield, whereas for others a different trait, such as lodging resistance or resistance to a particular insect pest or disease organism, was the primary trait improved by selection. In some cases the focus was on increasing per se performance, whereas other times the focus was on cross performance.

Populations are expected to have vigor that is better than inbreds but less than single-cross hybrids. However, in low-input environments in which grain yields are lower, the actual difference in performance between populations and single-cross hybrids becomes less. Also, for some traits other than yield, such as nutritional quality of the grain, some populations may be superior to most commercial hybrids.

Seed of many populations developed at public universities is available to other plant breeders as well as to farmers. The populations improved by selection for per se performance should be better than open-pollinated varieties that have undergone little or no selection. No special pollination is required to produce seed of a population. Also, a farmer or group of farmers could use the same principles as used by university corn breeders to continually improve a population.

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