Cropping system effects on weed population processes

Heggenstaller, Andrew
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Crop rotation is thought to represent a strategy for managing weed populations with reduced herbicide requirements, but understanding of the effects of crops and management practices on weed population processes is limited. Field experiments were conducted in 2003 and 2004 to characterize the demography of Abutilon theophrasti (velvetleaf) and Setaria faberi (giant foxtail) in a conventionally managed 2-yr (corn/soybean) rotation, and in 3-yr (corn/soybean/triticale+red clover) and 4-yr (corn/soybean/triticale+alfalfa/alfalfa) rotations, managed with 72% and 79% lower herbicide inputs, respectively. In the first experiment, rates of weed seedling recruitment, seedling survival and adult plant fecundity were determined for each phase of each rotation and used to calculate annual rates of weed population change. Over the two-year study period, A. theophrasti populations declined or remained stable in all three rotation systems. Despite greater rates of seedling survival and fecundity in corn and soybean in the 3-yr and 4-yr rotations, A. theophrasti population increase was prevented in these systems due to low fecundity in triticale and low seedling survival and fecundity in alfalfa. Setaria faberi populations remained stable in the 2-yr rotation, increased in the 3-yr rotation in both years, and increased in the 4-yr rotation in 2003. In the second experiment, removal of Abutilon theophrasti and Setaria faberi seeds by predators was measured during 27 trials conducted over the course of two cropping seasons. Seasonal patterns of seed removal were crop-specific and complementary. In corn and soybean, seed predation was low in spring, high in summer and low in autumn. In triticale+legume intercrops seed predation was high in spring, low in summer and moderate in autumn. In alfalfa, seed predation fluctuated from high to low, matching the harvest cycle of the crop. Measurements of crop canopy light interception were positively correlated with rates of seed removal for both A. theophrasti (r = 0.54; P <0.001) and S. faberi (r = 0.25; P <0.001), suggesting that vegetative cover promotes weed seed predation. The results of this study indicate that rotations including phenologically dissimilar crops with disparate management regimes can provide opportunities for managing weed populations with reduced herbicide requirements.