Effectiveness of oat and rye cover crops in reducing nitrate losses in drainage water

Much of the NO3 in the riverine waters of the upper Mississippi River basin in the United States originates from agricultural land used for corn (Zea mays L.) and soybean (Glycine max [L.] Merr.) production. Cover crops grown between maturity and planting of these crops are one approach for reducing losses of NO3. In this experiment, we evaluated the effectiveness of oat (Avena sativa L.) and rye (Secale cereale L.) cover crops in reducing NO3 concentrations and loads in subsurface drainage water. The oat fall cover crop was broadcast seeded into living corn and soybean crops before harvest in late August or early September and was killed by cold temperatures in late November or early December The rye winter cover crop, which had already been used annually for four years, was planted with a grain drill after corn and soybean harvest, overwintered, grew again in the spring, and was killed with herbicides before main crop planting. These treatments were evaluated in subsurface-drained field plots with an automated system for measuring drainage flow and collecting proportional samples for analysis of NO3 concentrations from each plot. The rye winter cover crop significantly reduced drainage water NO3 concentrations by 48% over five years, but this was less than the 58% reduction observed in its first four years of use. The oat fall cover crop reduced NO3 concentrations by 26% or about half of the reduction of the rye cover crop. Neither cover crop significantly reduced cumulative drainage or nitrate loads because of variability in cumulative annual drainage among plots. Both oat and rye cover crops are viable management options for significantly reducing NO3 losses to surface waters from agricultural drainage systems used for corn and soybean production.

► Nitrate losses in agricultural drainage water impact surface water quality in the USA.
► Both a fall oat cover crop and a rye winter cover crop reduced nitrate concentrations in drainage water.
► The relative reduction of nitrate concentrations by the fall and winter cover crops was related to their relative growth.
► No significant effect of cover crops on cumulative drainage or nitrate load was detected, but drainage was highly variable.