INFLUENCE OF WEED-CONTROL ON WEED POPULATION IN RIDGE-TILL CORN (ZEA-MAYS)

Field studies were conducted to determine the influence of annual herb icide treatments plus cultivation on weed populations and corn yields in ridge-till corn during a 3-yr period at Mitchell, NE, and a 7-yr pe riod at North Platte, NE. When the experiment was initiated at North P latte, no weeds were present before corn planting. It took 4 yr before triazine-resistant kochia became a problem before corn planting in pl ots treated with atrazine, but these were controlled by other operatio ns prior to corn harvest. In the cultivated check, green foxtail densi ties before harvest increased from 0 in 1985 to 32 plants 100 m(-2) in 1991. Annual applications of dicamba plus 2,4-D 10 d early preplant f ollowed by cultivation controlled triazine-resistant kochia and velvet leaf, but common lambsquarters, nightshade species, and green foxtail increased. Volunteer corn was controlled with cultivation. After 3 yr at Mitchell, the annual weed population increased 10-fold in the culti vated check. Thus, corn yields were reduced 64% with two cultivations compared with an annual early preplant application of dicamba plus 2,4 -D followed by alachlor plus cyanazine PRE and two cultivations. With two cultivations under low annual weed populations at North Platte, gr ain yield from the cultivated check treatment was not different from a nnual treatments of herbicides after 7 yr. Metolachlor plus atrazine o ccasionally caused a reduction in corn grain yields.