Mass Selection for Grain Yield
Does the low heritability of single-plant grain yields explain the absence of any improvement for grain yield in United States corn from the 1870’s to the 1930’s (Figure 2)? Is mass selection for grain yield a total waste of time?
Certainly, the low heritability of single-plant grain yield was part of the answer, but probably not the whole answer. As noted in the first lesson of this series, Corn Breeding: Lessons From the Past, some farmers selected for traits in their open-pollinated varieties that had little to do with grain yield. For example, the Reid family not only selected for shorter ear shanks but also for yellow kernel color and against a reddish color. Length of ear shank may have been related to grain yield, although this is not certain. But it is highly doubtful that grain color had any correlation to grain yield. This selection for beauty over substance reached a peak in the corn shows. The more traits that are selected simultaneously, often the more difficult it becomes to significantly alter any one trait.
A second factor minimizing the gain for yield that U.S. farmers realized was that selection almost always was done visually and by judging the weight by feel. The 20 ears shown in (Figure 3) all differ in ear weight. Can you correctly rank them for weight without a scale? You probably can separate the ears into a group of high yielders and a group of low yielders, but is it possible to identify the best two or three ears? Gauging their weights by feel may improve your accuracy, but weighing the ears on a good scale would ensure correct identification of the heaviest ears. Even in this case, however, there is a problem. Do you know what it is?
In measuring ear weight, you are collectively measuring dry grain weight, dry cob weight, and water content of both grain and cob. Likely, you are interested only in dry grain weight. Selecting for ear weight could easily result in selecting primarily for later maturing plants that have higher moisture contents. Thus, farmers practicing mass selection with visual and hand selection of desirable ears had limited success. [Shelling and drying of the grain from the ears shown in Figure 3 revealed that the three ears with the greatest dry grain weight were ear #’s
A third factor limiting usefulness of mass selection for grain yield is the absence of pollen control. By selecting open-pollinated ears and by assuming that pollen dispersal in the field was random, only the female parents of the next generation are being selected. Some male selection can occur if it is possible to select against weak, undesirable plants prior to pollination by detasseling them. However, even this limited level of pollination control rarely was practiced by 19th and early 20thcentury American corn farmers in their open-pollinated varieties.
A fourth factor that likely contributed to ineffectual mass selection for grain yield from the 1870’s through the 1930’s was lack of environmental control. Effectiveness of estimating genetic value by evaluating phenotypes depends upon the environment being the same for each plant. In a field of corn there may be substantial variation for plant-to-plant spacing, soil type and/or condition, and other environmental factors. Variation in soil type and many other environmental factors often is spatially related; that is, variation of these factors is likely to be greater in a large space rather than in a small space. For this reason, it is better to select 100 total ears by selecting one plant from 50 plants growing in a small space and repeating this process 100 times than to select 100 ears from 5000 plants growing in a larger space. This procedure is known as blocking a field to control environmental variation.
In Figure 4a there are 500 plants (each 'x' represents a single plant) with no blocking. Therefore, performance of each plant is compared to all other plants. These comparisons are affected by environmental variation that occurs in the entire field.
In Figure 4b, there are again 500 plants, but now they are arranged in five blocks with 100 plants each. Performance of each plant is compared only to those of other plants in the same block, and consequently these comparisons are affected only by the environmental variation that occurs within each block. If this variation is significantly less than the environmental variation that occurs in the entire field, which often is true, then blocking improves the effectiveness of mass selection. The topic of blocking is discussed more thoroughly in a subsequent lesson of this series.