Weed Seed Population Response to Tillage and Herbicide Use in Three Irrigated Cropping Sequences

Research was conducted to evaluate the effects of primary tillage (moldboard plowing and chisel plow- ing), secondary tillage (row cultivation), and herbicides on weed species changes in the soil seed bank in three irrigated row cropping sequences over a 3-yr period. The cropping sequences consisted of continuous corn for 3 yr (CN), continuous pinto beans for 3 yr (PB), and sugarbeets for 2 yr followed by corn in the third year (SB). A comparison between moldboard and chisel plowing indicated that weed seed were more prevalent near the soil surface after chisel plowing. The density of certain annual weed seed over the 3-yr period increased more rapidly in the seed bank after chisel plowing compared to moldboard plowing. Species exhibiting the most pronounced increase included hairy nightshade and stinkgrass in the PB cropping sequence and redroot pigweed and common lambsquarters in the SB sequence. Conversely, kochia seed density in the SB sequence decreased more rapidly in chisel-plowed plots. Row cultivation generally reduced seed bank densities of most species compared to uncultivated plots. Herbicide use in each cropping sequence produced a shift in the weed seed bank in favor of species less susceptible to applied herbicides. In particular, seed of hairy nightshade became prevalent in the PB cropping sequence, and seed of kochia, redroot pigweed, and common lambsquarters became prevalent in the SB sequence. Nomenclature: Corn, Zea mays L.; pinto beans, Phaseolus vulgaris L.; sugarbeets Beta vulgaris L.; hairy nightshade, Solanum sarrachoides Sendtner #3 SOLSA; stinkgrass, Eragrostis cilianensis (All.) Lulati # ERACN; redroot pigweed