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Adaptation of crops to different climatic regimes: the case of wheat and corn
Expansion of a crop into a new region often requires that the crop be adapted to a new climatic regime. Here we describe how hard red winter wheat and dryland corn have undergone such adaptation.
Hard red winter wheat--Hard red winter wheat has accounted for about half of all wheat produced in the United States. The figure below shows how much the production zone for hard red winter wheat expanded from 1920 to 1980 (76). Once limited primarily to Nebraska and Kansas the crop is now grown as far north as the Canadian Prairie Provinces and as far south as the Rio Grande River. This process of expansion has occurred even during times of hardship in the farm economy (such as the prolonged drought and economic depression in the 1930s and the surplus production and depressed crop prices in recent years).
Through the efforts of crop breeders and agronomists hard red winter wheat has been effectively adapted to colder temperatures and drier conditions. The crop is now grown in northern locations that are about 6 deg.F (3.5 deg.C) cooler and 15 percent drier than where growth was possible in 1920. The southward expansion of the crop has not been as strong as the northward spread. Still, average annual temperatures at the current southern boundary of the crop are almost 3.5 deg.F (2 deg.C) higher than they are at any location in the crop zone of 1920. The expansion in the hard red winter wheat range has come about from steady improvements in productivity made possible by the development of improved wheat varieties and farm-management practices (42).
The development and adoption of semi dwarf varieties in the 1940s (varieties whose stalks support heavier, grain-laden heads) boosted wheat productivity (21). Continued breading efforts since the 1940s have resulted in the great diversity of wheat varieties now being used by U.S. farmers. The progression to greater varietal diversity over time (see figure) has been associated with better adaptation of wheat to local growing conditions. Breeding for disease resistance helped the expansion to the south. Selective breeding for cold-hardy varieties of hard red winter wheat helped the expansion of wheat to the north.
Improved farming practices, especially the use of nitrogen fertilizers, better soil-moisture management practices, and large self-propelled machinery, have increased the productivity of wheat growers. The practices of stubbling-in (i.e., direct seeding of winter wheat into untilled fields immediately after harvest of the previous crop) and snow trapping (i.e., using snow fences to collect snow on fields) have reduced the risk of winterkill and permitted an expansion of the crop northeastward into Canada s western agricultural Provinces (86).
The past performance of the research community in developing new ways for wheat to overcome climatic constraints suggests the enormous capacity of the community to respond in the future. For example, as a consequence of breeding programs, the genetic diversity of hard red winter wheat is increasing; this greater genetic diversity should provide the raw material for further progress in crop development (19). This is but one example of the promise for future progress in adaptive agricultural research.
Dry land corn--Perhaps even more remarkable than the spread of hard red winter wheat into the Canadian Prairie Provinces is the recent adaptation of dryland corn to that same region. Farming systems in the semiarid northern Great Plains have historically suffered from overdependence on a narrow range of crops, especially wheat (56). This overdependence made the region vulnerable during times when wheat prices were depressed. Recognition of this problem caused farmers, working in concert with the local agricultural research establishment, to seek an alternative crop.
The Lethbridge Research Station devoted 8 years of research to adapting corn to the climate of southern Alberta (56). Relative to regions of the United States that produce significant quantities of dryland corn, southern Alberta is drier, the frost-free season is shorter, cumulative seasonal warmth is lower, and day length (period of daylight) is longer. The long day length can delay flowering, and the short growing season then provides little time for maturation.
In response to these challenges, plant breeders at Lethbridge have developed hybrids that have reduced sensitivity to day length and a short juvenile phase, so that the tassel starts to grow within a week of plant emergence. Moreover, breeders have successfully selected for varieties with a short interval between the opening of the corn tassels and the production of silk, which appears to give corn plants increased tolerance to drought. In dryland trials, corn yields from these new varieties are competitive with those of barley and wheat (56). These results clearly illustrate how directed research (i.e., the desire to diversify cropping systems in southern Alberta) can overcome major climatic constraints on crop production.
Rapid introduction of new crops: the case of soybeans
Climate change may necessitate widespread and relatively rapid shifts in the types of crops currently grown in the United States. How easily such a shift could be accomplished will depend on the available pool of crops that will flourish under the changing climate and on their production costs and markets. The expansion of soybeans into U.S. agricultural production, especially since World War II, is a vivid example of the rapidity with which the Nation's production systems can be modified to accommodate a new crop.
Soybeans have been cultivated in the United States since the early 1800s, although most were used for forage until the middle of this century (71). In 1920, there was no measurable acreage planted in soybeans in the Corn Belt States. Acreages planted in soybeans remained quite low until just before World War II (34). At that time, the United States Imported over 40 percent of the soybeans that were used domestically.
During World War II, a growing demand for margarine created a market for soybean oil (34). Soybeans rapidly began to compete with other oil seeds, and cropland was shifted into soybeans. In the midwestern United States, increases in soybean production came at the expense of corn, wheat, and oat production. For the South, the soybean was a savior, replacing cotton as cotton prices plummeted in the wake of declining world demand. By 1949, the United States became a net exporter of soybeans. In less than 30 years, soybeans had become a major cash crop for U.S. farmers.
Since World War II, continuous growth in the livestock and poultry industries has further increased the demand for the high-protein soybean meal. By 1982, more than one-third of the cropland in the Corn belt was planted in soybeans(4). The increase in midwestern soybean acreage between 1949 and 1982 is shown below. The rapidity of the spread of soybeans in the United States is significant for assessing the prospects of a successful shift to alternative crops under climate change. It demonstrates the capacity of the farming sector to convert equipment, management, and marketing to grow and process a new crop in a short period of time. It also shows the willingness of farmers to experiment with a new crop as the crop of preference (cotton, in the South) became uneconomical.
There are limits, however, to the usefulness of the U.S. soybean experience as an analogy to the shifting of crops to adapt to climate change. A major incentive to growing soybeans was the rapid growth of demand for oil and meal worldwide. The combined attributes of oil-bearing seeds and high-protein residual meal gave soybeans a clear advantage over competing crops. There do not appear to be crops waiting in the wings that could generate the kind of market that soybeans did. On a smaller scale, new crops may provide alternatives to farmers. For example, several drought-tolerant crop species, such as paloverde, jojoba and mesquite, may be useful in dealing with increasingly scarce water in the southwestern United States (58, 102). These crops have low water requirements and produce harvestable quantities of valuable botanochemicals and other plant products.
