Weed Biomass Accumulation and Weed Seedbank Composition in Marsden Farm 2014.

Cropping system diversification through the use of extended rotations and integration of physical and chemical control tactics have been suggested as effective ways to suppress weeds while reducing reliance on herbicides. We used data from a long-term, large-scale cropping systems experiment conducted in Boone, IA, to test this hypothesis. Treatments included three rotation systems (2-yr: corn/soybean; 3-yr: corn/soybean/oat+red clover; 4-yr: corn/soybean/oat+alfalfa/alfalfa) managed with two herbicide regimes (conventional and low). The low herbicide regime used banded applications over corn and soybean rows and interrow cultivation, which reduced herbicide inputs 62% and 94%, respectively. Oat, red clover, and alfalfa were not treated with herbicides. Twenty-five weed species and genera (7 monocots and 18 dicots) were found in fall 2014; the most prevalent were Setaria faberi and S. glauca, Eriochloa villosa, Amaranthus rudis, Chenopodium album, and Taraxacum officinale. Total weed biomass was comparable between the 2-yr and 4-yr rotations in corn and soybean phases, regardless of herbicide regimes (16 ± 11.6 kg ha^-1, mean ± SE). Oat phases of the 3-yr and 4-yr rotations and the alfalfa phase of the 4-yr rotation contained the greatest amount of weed biomass (95 ± 6.0, 291 ± 33.2, and 97 ± 24.4 kg ha^-1, respectively). Averaged all crop phases of each rotation system, weed seedbank population density in the soil to 20 cm depth was unaffected by herbicide regime (p=0.65), but strongly influenced by rotation system (p=0.0002), being highest in the 3-yr system (5567 ± 872.8 seeds m^-2), intermediate in the 4-yr system (2626 ± 306.9 seeds m^-2), and lowest in the 2-yr system (1320 ± 237.0 seeds m^-2). These data indicate that integration of physical and chemical tactics provided effective weed control in corn and soybean; small grain and forage crops that were not cultivated or sprayed had greater weed infestation.