Evapotranspiration and water use efficiency in maize-soybean crops in the US Midwest

Abstract:

Evapotranspiration from maize and soybean crops is an important component of terrestrial water balance in the US Midwest. In this study we examine water use in continuous maize (corn) vs. maize-soybean rotations, with cover crops planted in some years. From 2010-14, we continuously measured growing season evapotranspiration (ET) based on daily drawdown of soil moisture content using TDR (time-domain reflectometry) probes installed throughout the root zone. Treatments included continuous maize (CM), continuous maize with cover crops (CMC) and maize-soybean rotation with cover crops (MSC), all grown without irrigation in a temperate humid climate (Michigan, USA). Cover crops were planted in the autumn after harvest of the main crop and harvested in spring prior to planting of the next main crop during 2012-2013 (2013) and 2013-2014 (2014). Four study years (2010, 2011, 2013 and 2014) had normal growing season rainfall (568, 555, 445, and 472 mm) while 2012 was an extreme drought season with a growing-season rainfall deficit of ~50% (210 mm below average). Growing season ET in CM, CMC and MSC during years of normal rainfall averaged 517, 433, and 443 mm, respectively, compared to 455, 374 and 304 mm in the 2012 drought year. Cover crop ET was inconsequential to the subsequent main crops due to abundant rainfall in the spring periods; soils held as much water as they could at the transition from cover crops to main crops. Grain yield in years of normal rainfall for CM, CMC and MSC averaged 12.6, 8.4 and 7.8 Mg ha^-1, respectively, compared to 4.9, 4.0, and 4.0 Mg ha^-1 in the 2012 drought year. Maximum biomass in years of normal rainfall averaged 38, 30 and 21 Mg ha^-1 compared to 19, 13, and 13 Mg ha^-1 in the drought year. Water use efficiencies, defined as ratio of maximum standing-stock biomass to growing season evapotranspiration, were 74, 69, and 47 kg ha^-1 mm^-1 for CM, CMC and MSC in years of normal rainfall, while values in the drought year were 41, 34 and 46 kg ha^-1 mm^-1. Compared to CM, our results suggest that ET and biomass were reduced when maize was grown with cover crops either continuously or in rotation with soybean. However the reduced yield of maize with cover crops cannot be ascribed to water competition with the cover crops; rather, the later planting dates following cover crops likely explain the reduced ET and yields.