WASHINGTON (AP) _ Instead of buying fresh potatoes to boil, bake or fry, Americans are turning more to potato products that have been frozen, dehydrated, canned or chipped, an Agriculture Department report says.

Moreover, according to the analysis, U.S. potato production is steadily shifting from the Northeast to irrigated areas of the Pacific Northwest.

The annual per capita consumption of potatoes remained about the same at slightly less than 122 pounds from 1970 to 1985. But fresh potato consumption dropped almost 20 percent during the period, from 61.4 pounds to 49.3 pounds.

The per capita consumption of processed potato products rose a corresponding 20 percent, from 60.4 pounds to 72.5 pounds.

The biggest gainer during the 1970-85 period was frozen potato products such as french fries. Overall, per capita use of such products rose 45 percent, from 28.9 pounds to 42.1 pounds.

Eugene Jones, an assistant professor of agricultural economics at Ohio State University, said in his report for the department's Economic Research Service that rising consumer income affects the mix of potato consumption.

According to his study, per capita fresh potato use falls 4.2 percent for each 10 percent rise in income. By comparison, consumption of frozen potatoes climbs 8.1 percent for each 10 percent growth in per capita income. The consumption of chips and dehydrated potatoes is unaffected by income growth.

Jones said that frozen potato consumption rises 2.2 percent, or about one pound per person, with each 10 percent growth in fast-food sales, which also can mean declines in the use of fresh potatoes.

Fresh potatoes are bought more as a retail grocery item, while frozen potatoes are more often marketed through restaurants, particularly fast-food places. However, he said, the current trend toward baked potatoes at fast-food establishments ''could diminish the negative effects of eating out on fresh potato consumption.''

Another influence on the use of potatoes by American consumers has been the growing proportion of women in the U.S. labor force, rising from 36 percent in 1970 to 43 percent in 1985.

''Less time is available for in-home food preparation when women participate in the labor force,'' Jones said. ''Therefore, pre-prepared potatoes are purchased rather than fresh. Fresh potato consumption declines 1.6 percent with each 10 percent growth of women in the labor force.''

Jones said the growth of frozen potato products between 1970 and 1985 coincided with the expansion of potato production in the Pacific Northwest, with processors' preference for the Russet Burbank potato apparently an underlying factor.

Processors favor the Russet Burbank variety for freezing and dehydrating because of its high solids content and elongated shape, which makes it desirable for producing long french fries, he said.

''The Pacific Northwest appears to have a production advantage in Russet Burbank production,'' Jones said. ''The variety requires moderate amounts of water consistently applied throughout its growing season. The irrigated Pacific Northwest growing areas meet this need well.''

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WASHINGTON (AP) - Agriculture Department scientists are using hormones and artificial light to reset the biological clock of sheep, making they think it's time to mate just about any month of the year.

Normally, sheep mate in the fall and produce lambs in the spring, says animal physiologist James A. Fitzgerald in the current issue of Agricultural Research magazine. The following fall, the lambs, fattened to about 120 pounds each, go to market. As a result, almost 800 million pounds of lamb hit the market at about the same time.

By changing the mating clock, ewes could provide lambs year-round, says Fitzgerald, who works at the USDA's sheep experiment station at Dubois, Idaho.

As it stands now, Fitzgerald and his colleagues are getting some flocks to breed at different times, with 60 percent to 70 percent of the ewes giving birth in the off-season.

John N. Stellflug, another Dubois scientist, is studying melatonin, a hormone that the human body releases during the short days and long nights of fall and winter. Melatonin is suspected as the cause of ''winter blues'' - a condition accompanied by feelings of exhaustion, depression and irritability.

But Stellflug said the same biological messenger that turns people off ''can turn sheep on sexually.''

''Ewes receiving the hormone in the spring will become ready to breed in spite of the lengthening days,'' he said.

Stellflug said the hormone cannot be used for sheep until it is approved by the Food and Drug Administration.

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WASHINGTON (AP) - China is importing ''an unprecedented high level'' of soybean oil, apparently to take advantage of low world prices, according to an Agriculture Department report.

The department's Foreign Agricultural Service said China's soybean oil imports in 1986-87 are expected to total about 400,000 metric tons, up from 290,000 tons last year. The forecast for 1987-88 is for imports of 350,000 tons. A metric ton is about 2,205 pounds.

Most of the oil is coming from Brazil, Argentina and Spain, the agency said in its recent report.