When tillage is reduced or eliminated, the growing environment selects for different types of weeds, and anticipating these changes in the weed community is critical to developing a successful weed management plan. Table 11.1 lists weeds that can be problematic in conservation tillage systems. In reduced tillage, it is common to see a shift to small-seeded annual weeds. Annuals suited to the reduced-tillage environment have seeds that do not need to be buried to germinate and typically produce large numbers of seed. In conventional systems, small-seeded annuals are typically controlled with seed burial or pre-plant-incorporated herbicides. Other annual species prevalent in conservation systems are adapted to germinating in cooler temperatures and shaded areas, a typical environment in reduced tillage, especially in row middles. Annual weeds tend to grow rapidly and compete with a crop if not controlled. However, with reduced tillage, weed seeds remain on the surface instead of being buried as in conventional tillage systems. This may reduce the number of viable weed seeds since they are exposed to many factors that lessen viability.
Perennial weeds are more likely to become a problem in conservation tillage systems. In conventional systems, tillage disrupts perennial weed growth or buries the weeds too deep for regrowth. Although perennials often grow slower than annuals, they can be more difficult to control with available herbicides. Fortunately, the majority of perennials reproduce vegetatively, using stolons, rhizomes, roots, crowns and bulbs rather than reproducing by seed. This leads to patches of weeds that can be targeted for control.
Other management practices used in conservation systems, such as cover crops, also cause a change in the types of weeds present. Cover crops help suppress weed growth while they are actively growing, and with adequate biomass production the cover crop mulch can continue to provide some early-season weed control for the cash crop . Thus, weeds that normally germinate and grow during the period of active cover crop growth are reduced due to competition with the cover crop. Cover crop residue is less effective than actively growing cover crops but can still suppress weed germination and early growth of weeds that germinate and grow during the cash crop season. Perennial weeds are generally not affected by annual cover crops.
Cover crops may also affect weed populations through allelopathy. During decomposition, some cover crops release allelochemicals, which have the greatest impact on germinating seeds, seedlings and young plants by retarding their growth, causing visible damage to roots or shoots, or even killing them outright. Because not all cover crops produce allelochemicals, and because not all weeds are impacted, a shift in weed populations can occur. Allelopathic effects strong enough to contribute significantly to weed control in field conditions have been documented for rye and other winter cereal grains, sorghum and sorghum-sudangrass hybrids, lablab beans, rapeseed, buckwheat and subterranean clover (Putnam and Tang, 1986; Rice, 1995; Boydston and Hang, 1995). Again, perennial weeds are generally not affected by cover crop allelochemicals.