Vegetative Cover Crops as a Nitrate Reduction Strategy for Tile Drainage

Jeff Vetsch

Over the last 30+ years, the use of nitrogen BMP’s has been the primary strategy for reducing nitrate loss in tile drainage water. A research study was initiated in 2016 to evaluate the potential of cover crops and university recommended N rates for corn as management practices to reduce nitrate loss in tile drainage water. The objective of this research was to measure the effects of two vegetative covers [winter hardy (cereal rye) and winter terminating (blend of annuals)] at various N rates on the following: 1) tile water flow, NO3-N concentration, and NO3-N loss in tile drainage water and 2) corn and soybean yields, nitrogen uptake and NUE. Cover crops were overseeded (broadcast) in early Sep (R6 in soybean and R5 in corn) each year beginning in 2016.

These research data were greatly influenced by weather during each growing / drainage season. Warm Sep and Oct in 2016 and Apr in 2017 were ideal for cover crop germination and growth, especially cereal rye that was terminated on 17 Apr. In 2017, FW NO3-N concentrations and flow-adjusted losses were 70 and 20% less with cereal rye and annual blend than no cover, respectively. At the greatest N rate (150 lb N/ac) corn grain yields in 2017 were statistically similar among the three cover crop treatments; however, at the 2016 MRTN rate for corn following soybean (120 lb N/ac) grain yields were reduced compared with 150 lb N/ac in both the no cover and cereal rye treatments. Even though a cold Apr in 2018 (13° F below normal) hindered rye growth, FW NO3-N concentrations and flow-adjusted losses were about 20% less with cereal rye than no cover. In 2018 (soybean), NO3-N concentrations and losses increased as N rate for corn in 2017 increased; however, NO3-N concentrations were quite low (<4 mg/L) and annual losses averaged only 10 lb/ac across the 120 and 150 lb N/ac treatments in this wet year with 12 inches of tile drainage. A wet and cold fall in 2018 and spring in 2019 resulted in very little cover crop growth.

Cover crops did not affect FW NO3-N concentrations, NO3-N losses or corn grain yields in 2019. Nitrate-N concentrations and losses were greater with fertilized treatments (120 and 150 lb N/ac) than the control. Corn grain yields and N uptake increased with increasing N rates in this very wet year with 48.5 inches of annual precipitation and 14 inches of tile drainage. A warm 2020 with near normal growing season precipitation resulted in 6.7 inches of tile drainage and very low (< 4 mg/L) FW NO3-N concentrations and minimal loss. A rye cover reduced annual mean FW NO3-N concentrations about 30% (only 0.6 mg/L) and flow-adjusted NO3-N loss in 2020. Generally, soybean yields were not or minimally affected by treatments in this study. However, legacy effects from past studies affected yields in one treatment.

This study has shown a cereal rye cover crop can reduce NO3-N in tile drainage water if weather permits adequate cover crop growth. However, rye may interact with corn production requiring a greater N rate to optimize yield. These data suggest annual blend covers that are terminated by cold temperatures in late fall in Minnesota have little value for mitigating nitrate in tile drainage water.