Demographics of Amaranthus palmeri in annual and perennial cover crops

Palmer amaranth (Amaranthus palmeri S. Watson) is the most problematic weed of cotton cropping systems in the Southeast US. Heavy reliance on herbicides has selected for resistance to multiple herbicide mechanisms of action. Effective management of this weed may require the integration of cultural practices that limit germination, establishment, and growth. Cover crops have been promoted as a cultural practice that targets these processes. We conducted a two-year study in Georgia, USA, to measure the effects of two annual cover crops (cereal rye [Secale cereale L.] and crimson clover (Trifolium incarnatum L.), a perennial living mulch (Durana® white clover, (Trifolium repens L.), and a bare ground control on A. palmeri population dynamics. The study was conducted in the absence of herbicides. Growth stages were integrated into a basic demographic model to evaluate differences in population trajectories. Cereal rye treatments suppressed weed seedling recruitment (seedlings seed-1) by 19.2 and 13 times, and living mulch 12 and 25 times more than the bare ground control. Low recruitment was correlated positively with low light transmission (above canopy PAR/below cover crop PAR) at the soil surface. Low recruitment rates were also negatively correlated with high survival rates. Greater survival rates and reduced adult plant densities resulted in greater biomass (g plant-1) and fecundity (seeds plant-1) in cereal rye and living mulch treatments in both years. The annual rate of population change (seeds seed-1) was equivalent across all treatments in the first year but was greater in the living mulch treatment in the second year. Our results highlight the potential of annual cover crops and living mulches for suppressing A. palmeri seedling recruitment and would be valuable tools as part of an integrated weed management strategy.