CROPPING HISTORY, TILLAGE, AND HERBICIDE EFFECTS ON WEED FLORA COMPOSITION IN IRRIGATED CORN

A change from conventional tillage to a conservation tillage system can lead to shifts in weed species composition. A weed species shift can result in the emergence of weeds tolerant of existing weed management practices. An understanding of crop production effects on weed species shifts can lead to development of improved weed management strategies. Research evaluated the effects of primary tillage (moldboard and chisel plowing), row cultivation, and herbicide input level on weed species changes over a 5-yr period in three irrigated cropping sequences. The cropping sequences consisted of continuous corn (Zea mays L.) for 5 yr (CN), pinto bean (Phaseolus vulgaris L.) for 3 yr followed by corn for 2 yr (PB), and sugar beet (Beta vulgaris L.) for 2 yr followed by corn for 3 yr (SB). Over the course of the 5-yr study, total weed density increased from 1 to 245 weeds m-2 in PB, from 100 to 209 in SB, and from 2 to 190 in (N cropping sequences in chisel-plowed treatments, while weed density increases in moldboard-plowed treatments were negligible. General observation of each cropping sequence indicated that during the final year, green foxtail [Setaria viridus (L.) P. Beauv.] was most prevalent in the CN sequence, redroot pigweed (Amaranthus retroflexus L.) and hairy nightshade (Solanum sarrachoides Sendtner) in the PB sequence, and redroot pigweed and green foxtail in the SB sequence. However, weed species differences due to cropping sequences were evident only in treatments receiving chisel plow primary tillage. Further, the increase in redroot pigweed density due to cropping sequence and chisel plowing was diminished by high herbicide input levels. Row cultivation also had an influence on the weed species composition when compared to uncultivated plots.