Weed suppression is one possible benefit of including cover crops in crop
rotations. The late spring planting date of dry beans allows for more growth
of cover crops in the spring. We assessed the influence of cover crops on
weed dynamics in organic dry beans and weed seed persistence. Medium red
clover, oilseed radish, and cereal rye were planted the year before dry
beans; a no-cover-crop control was also included. After cover-crop
incorporation, common lambsquarters, giant foxtail, and velvetleaf seeds
were buried in the red clover, cereal rye, and no-cover control treatments
and then retrieved 0, 1, 2, 4, 6, and 12 mo after cover-crop incorporation.
Dry beans were planted in June and weed emergence and biomass measured.
Eleven or more site-years of data were collected for each cover-crop
treatment between 2011 and 2013, allowing for structural equation modeling
(SEM), in addition to traditional analyses. Cereal rye residue increased
giant foxtail and velvetleaf seed persistence by up to 12%; red clover
decreased common lambsquarters seed persistence by 22% in 1 of 2 yr relative
to the no-cover-crop control. Oilseed radish and incorporated cereal rye
rarely reduced weed densities. When red clover biomass exceeded 5 Mg
ha−1, soil inorganic N was often higher (5 of 6 site-years),
as were weed density and biomass (5 and 4 of 12 main site sample times,
respectively). Using SEM, we identified one causal relationship between
cover-crop N content and weed biomass at the first flower stage (R1), as
mediated through soil N at the time of dry bean planting and at the stage
with two fully expanded trifoliates. Increasing cover-crop C : N ratios
directly reduced weed biomass at R1, not mediated through changes in soil N.
Cover crops that make a significant contribution to soil N may also
stimulate weed emergence and growth.