application of a common, otherwise nontoxic substance would cause the smother plant to senesce. Another possibility would be the incorporation of a lethal gene into the smother plant so that it dies after four to five weeks of growth, unless a spe cific management tool is utilized. Producers would then have a way to control the growth of the smother plant and obtain maximum benefit from its use.
The Brassica smother plant, along with other cover crops that have been evalu ated, does not have any direct economic value of its own. Consequently, its use can cause little, if any, main crop yield loss if it is to be an economically viable weed control method. Development of smother plants that suppress weeds for the first four to six weeks of the growing season and then can be harvested and marketed may be an important element in the feasibility of weed control methods utilizing plant interference.
The development of smother plants designed for specific crop production condi tions may be necessary to facilitate smother plant use. Currently, corn and soybeans can be treated with one, or a combination, of many available herbicides. Producers select herbicide treatments based on their production practices, the weed species present in their fields, and the cost of the treatment. Smother plant weed control technology is still in its infancy. It is likely that a range of smother plants, each adapted to a particular crop species, soil type, and weed spectrum, will need to be developed before smother plants can be a reliable, widely adopted weed suppression tool.
Herbicides and tillage are the predominant means of weed suppression in industri alized countries, but use of these methods often results in environmental degrada tion. Biological weed management through plant interference may be a viable al ternative. Winter annual and perennial cover crops or smother plants have been evaluated for their ability to suppress weeds, but the species investigated were not winter hardy in the Upper Midwest United States, and most were competitive with the main crop. Development of effective spring-seeded annual smother plants would provide producers with a nonchemical weed control option, and smother plant use could also reduce soil erosion. Research on a Brassica species smother plant for weed control in corn (Zea mays L.) is described as an example of the de velopment and use of spring-seeded annual smother plants. Crosses were made be tween rapid cycling, short-statured Brassica campestris entries and locally adapted Brassica species. A population derived from a dwarf Brassica (Brassica campestris "CrGC 1-21") by Chinese cabbage (Brassica campestris ssp. pekinensis "Chinese mastsus") cross was chosen for field evaluation. Field grown dwarf Brassica smother plants flowered three weeks after emergence, had a maximum height of 33 cm, and began to senesce five weeks after emergence. Field studies were conducted to deter mine the effect of the dwarf Brassica smother plant on corn. Corn silk emergence date at St. Paul in 1991 was not affected by the Brassica smother plant, but at St.