Establishment of common milkweed (Asclepias syriaca) in corn, soybean, and wheat

The establishment of common milkweed in corn, soybean, and wheat and rotations of corn-soybean and soybean-wheat was determined in field studies at Rosemount and Morris, MN. Common milkweed seedling density 115 d after initial seedling emergence was 500, 15,000, and 31,300 seedlings ha^-1 in corn, soybean, and wheat, respectively, in 1990; 500, 7,300, and 15,600 in 1991 at Rosemount and 2,900, 26,500, and 37,000 in 1990; and 2,200, 6,700, and 8,100 in 1991 at Morris in the year of seedling establishment. Common milkweed seedling defoliation by monarch butterfly larva was 1, 31, and 0% at Rosemount and 20, 78, and 0% at Morris for corn, soybean, and wheat, respectively. In the 2nd and 3rd rotational years, corn-soybean rotations had fewer shoots ha^-1 115 d after shoot emergence than soybean-wheat rotations. Seedling densities 115 d after initial seedling emergence were greater in corn-soybean rotations where soybean rather than corn was the rotational crop and in soybean-wheat rotations where wheat rather than soybean was the rotational crop immediately following common milkweed seeding. Conventional tillage treatments had lower common milkweed shoot density than conservation tillage in all years of all rotations except in the initial count, when soybean was the first rotational crop in which shoot density was greater in conventional than in conservation tillage treatments. Cyanazine, imazethapyr, or bromoxynil applications in corn, soybean, or wheat rotational crops, respectively, reduced common milkweed shoot density.