WEED-CONTROL AND YIELD ARE EQUAL IN CONVENTIONAL, REDUCED, AND NO-TILLAGE SOYBEAN (GLYCINE-MAX) AFTER 11 YEARS

Field experiments were conducted from 1979 to 1989 to determine the in fluence of conventional, reduced, and no-tillage systems and different herbicide combinations on weed species and population, weed control, and soybean injury, population, and yield. In no-till (NT) non-treated plots, there was an abrupt shift from horseweed as the dominant early spring emerging weed to gray goldenrod in 1985. Following its initial observation, gray goldenrod became the dominant species within 2 yr, with giant foxtail as the only other species observed in these plots. Giant foxtail was the dominant weed species from 1980 to 1989 in conve ntional till (CT) and reduced-till (RT) plots. There also was a shift in the frequency of occurrence and in density of several broadleaf wee d species during the 11-yr study. Most herbicides provided excellent c ontrol of all weeds in all tillage systems, especially those that incl uded POST herbicides. There was little difference between glyphosate a nd paraquat in controlling weeds present at the time of planting in NT . PRE herbicides caused 2 to 9% soybean injury with slightly greater i njury occurring in CT and RT than in NT. The POST broadleaf herbicides did not significantly increase soybean injury. There were no differen ces in soybean population or yield among the herbicide treatments rega rdless of tillage. There also was no difference in soybean population or yield in NT compared with CT when averaged over all herbicide treat ments.