Shifts in weed community composition and abundance along an intercropping and soil disturbance intensity gradient

Background/Question/Methods

In many ecosystems soil seed banks are an important source of recruitment and crucial components of plant community dynamics. In annual row cropping systems, weed seed bank expression can be promoted by soil disturbances such as tillage, and suppressed by practices such as intercropping. This experiment created a gradient of tillage and intercropping intensity within an established organic pasture, so as to determine how these factors interact to affect weed expression. Six different management treatments, replicated four times in a randomized complete block design, were established in summer 2012 at the University of New Hampshire Organic Dairy Research Farm in Lee, NH. Two control treatments, a pasture control (an established mixture of alfalfa and grass) (P), and a full-tillage conventional organic feed-grain rotation (not intercropped)  (CT), were compared to four intercropping system treatments differing in disturbance intensity: full-tillage with annual legume (crimson clover) inter-seeded following feed grain emergence (CTI); strip-tillage establishment into pasture (ST); minimum-tillage establishment into pasture following mowing and undercutting (MTU); and minimum-tillage establishment into pasture following mowing only (MT). Weeds that emerged from the seed bank were measured at peak biomass (mid-August), sorted to species, dried to constant biomass, and weighed. 

Results/Conclusions

Total weed biomass increased with tillage intensity, with higher weed biomass in conventionally tilled treatments compared to the minimally tilled intercrop treatments (p = 0.0071), while weed species richness was highest in the strip-tilled treatments compared to the uniformly tilled treatments (p = 0.0061). Weed community composition also differed across treatments, with Amaranthus spp., Chenopodium album, Digitaria sanguinalis, and Setaria viridis dominating the conventionally tilled treatments and perennial pasture species dominating the non-crop plant community in the minimally tilled treatments. In contrast, corn biomass was significantly higher in the conventionally tilled treatments than in the minimally tilled treatments (p = <0.0001). At the community-level, total stand productivity (corn, pasture, and weeds) was highest in the uniformly tilled treatments. These results suggest that intercropping into established pasture using conservation tillage methods can decrease weed abundance; however trade-offs exist between disturbance intensity, maintenance of desirable pasture species, and crop yield.