'Bioherbicide' snuffs out competition.

Weed scientist Donald L. Wyse and his colleagues have bred what they describe as the first intentionally designed biological herbicide. Called the "smother" plant, this short-lived green mulch quickly carpets a field with a dense mat of vegetation that chokes out yield-threatening weeds.

"We wanted something that had rapid germination, didn't get too tall and did not compete very long," explains Robb De Haan, who worked on the project at the University of Minnesota in St. Paul. The researchers got what they wanted by crossing two subspecies of Brassica campestris - a species whose members include turnips, Chinese cabbage and bok choy.

In initial field test last year, the researchers broadcast seeds of the new cultivar over rows of planted corn. The resulting broad-leaved carpet reached a height of 8 inches and stayed green for about five weeks. More important, Wyse notes, it protected the corn from about 80 percent of the weeds - an effect comparable to that of many chemical herbicides - without diminishing crop yields. In this year's field trials, the team seeks to verify that weed-smothering efficiency and determine whether the novel herbicide itself might become a weed in the fields where it's sown.