WASHINGTON (AP) _ Soybean plants that grow up to seven feet tall could provide feed for dairy cows that produces more protein and needs less nitrogen fertilizer than corn, says a researcher.

The experimental plants, bred from long-neglected hay-type soybeans, ''open the way to a new dairy silage for sustainable farming systems. I think they'll find a useful niche, especially in the mid-Atlantic region,'' says Thomas Devine, a geneticist with USDA's Agricultural Research Service.

Silage is plant material - mainly leaves and stems - that is ''pickled'' by natural microorganisms to yield long-lasting, nutritious feed, Devine explained in a recent edition of Agricultural Research magazine.

Plants grown for silage are cut, slightly dried, chopped, then enclosed in a chamber such as a bunker. After natural microorganisms use up the chamber's oxygen, other microbes produce lactic acid that pickles the forage.

Soybean silage varieties based on his experimental strains could be available in about three years, Devine said. He will begin testing a few of the best experimental lines for yield and nutrient value in Iowa, Pennsylvania and Virginia this spring.

''Corn is the number one silage, but soybeans potentially can produce more protein,'' he said. And soybeans are a legume, which need less commercial nitrogen fertilizer than corn.

''Symbiotic bacteria on soybean roots capture nitrogen from the air, and the roots slowly release it as they decay,'' Devine said. ''After harvesting soybean silage in summer, a farmer could plant a small grain, like barley or wheat, that would use this 'free' nitrogen.''

Soybeans first became popular in the United States as a hay crop, Devine noted, with 70 percent of the 1924 crop used for that purpose.

But that use dropped to 3 percent by 1964, with the rest used for grain. The drop in hay use happened, Devine said, because ''quality was erratic. The plants dried slowly and rain often leached out nutrients.''

The few farmers who do grow soybeans for silage have to plant varieties bred for their grain production and therefore produce much less foliage.

Devine began in 1976 crossing a hay soybean with grain types having resistance to diseases and pests.

The hay type was described as a vigorous, leafy but little used strain supplied by Virginia Polytechnic Institute and State University. Devine tests the plants in cooperative studies with David Starner of Virginia Tech and Elwood Hatley of The Pennsylvania State University.

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WASHINGTON (AP) - The 1993 U.S. rye production is estimated at 10.3 million bushels, down 13 percent from the previous year.

The average yield was 27.1 bushels per acre, down 2.3 bushels per acre from 1992, said a 1993 summary of crop production by the National Agricultural Statistics Service.

Area harvested was 381,000 acres, down 6 percent from last year. Planted acres were estimated to total 1.49 million, down 6 percent from a year ago.

''This is the smallest planted area on record,'' the report said.