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Ignoring some data may aid crop counts.

Ignoring some data may aid crop counts

Disregarding some data may actually help farmers and commodities brokers predict crop yields more accurately. A new study of Illinois corn suggests measures of soil moisture during key preharvest periods lead to better predictions of seasonal yields than do similar data representing the entire growing season.

Midseason estimates of final yields now rely on the most current value for the Palmer Index, a year-to-date soil-moisture measure. The Illinois study suggests predictions might improve if they instead depended on Palmer Indexvalues recorded at the end of periods during which crops peak in sensitivity to moisture changes. A year of discussion with nine Illinois agricultrue experts led the researchers to determine that, for corn, these critical phases are planting and anthesis/grain fill--when corn reproduces and individual kernels fill with edible starch. In Illinois, where the corn-growing season runs from mid-April through October, planting typically takes six weeks including all of May, and anthesis/grain fill usually lasts three or four weeks in July and August.

The study, led by Scott A. Isard of the University of Illinois at Urbana-Champaign and William E. Easterling at the Illinois State Water Survey, focused on corn yields from 1960 through 1983 in 12 counties. The researchers estimated known annual yields using end-of-season Palmer Index values and compared those results with estimates based on values from the end of the two critical periods. They found the latter method, which neglects several months' worth of data, more effective. They also discovered, to their surprise, that weather conditions during harvest "don't seem to play a major role in the seasonal [corn] yields," says Easterling, now at Resources for the Future, a Washington, D.C., research group.

Scheduled for publication early next year in the JOURNAL OF CLIMATE, the study's results show that the cumulative growing-season estimates accounted for about half the true fluctuation-from-normal of drought-year yields, while the new technique accounted for about 75 percent. "The remaining variation in yields," says Easterling, "is probably due to things other than weather, such as insects, diseases, fertilizers, pesticides and agronomic practices."

Easterling says complex computer models simulating crop growth eventually will replace Palmer Index methods. Meanwhile, Isard plans to apply his team's technique in estimating Illinois' yearly corn yields since 1983, seasons he says had "less stable weather" than those from 1960 through 1983.
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Author:Knox, Charles
Publication:Science News
Date:Nov 12, 1988
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