- Agronomic: corn
- Crop Production: cover crops, no-till, tissue analysis
- Natural Resources/Environment: carbon sequestration, soil stabilization
- Pest Management: allelopathy, competition, cultural control, mulches - killed, weed ecology
- Soil Management: nutrient mineralization, soil quality/health
The use of cover crops can decrease soil erosion, weed density, and nitrate leaching while improving soil quality. We investigated nine cover crops, winter rye (Secale cereale L.), winter triticale (× Triticosecale Wittm. ex A. Camus), two winter canola (Brassica napus L.), winter camelina [Camelina sativa (L.) Crantz], spring barley (Hordeum vulgare L.), spring oat (Avena sativa L.), turnip (Brassica rapa L.), and hairy vetch (Vicia villosa Roth), as sole crops and selected binary and trinary mixtures and their influences on subsequent corn (Zea mays L.) productivity. A control treatment of no cover crop was included. Cover crops were no-till drilled immediately after soybean [Glycine max (L.) Merr)] harvest. The study was a randomized complete block conducted in five environments over 2013-2014 and 2014-2015. Across environments, rye and rye mixtures produced the greatest spring aboveground biomass (676 lb a-1), carbon, and nitrogen accumulation, had some of the lowest spring soil nitrate concentrations, and generally produced the lowest corn leaf chlorophyll. Rye accounted for more than 79% of spring aboveground biomass accumulation in rye mixtures. Triticale and camelina monoculture produced approximately 50% less biomass than rye or mixtures with rye. Cover crops in monoculture and mixtures did not influence surface soil temperature, soil P or K concentrations, weed density, weed community, or corn yield. Cover crops had a limited influence on volumetric soil water content. Cover crop mixtures had no advantages over monocultures except for increasing fall stand density. Turnip and vetch had limited winter survival while barley, oat, and canola winterkilled.
The corn-soybean cropping system dominates the Midwest and is one of the most productive cropping systems in the world. The Midwestern state of Iowa often leads the United States in acres of corn and soybeans, with an estimated 13.7 million acres of corn (Zea mays L.) and 9.9 million acres of soybean [Glycine max (L.) Merr.] planted in 2014 (USDA, 2015). Although corn and soybeans are highly productive in Iowa, they are only grown for approximately five to six months of the year. For the remainder of the year most of the cropland in Iowa does not have any actively growing plants. Soil residue cover is often low, especially in systems which use fall tillage. The lack of growing plants and limited ground cover can result in soil erosion, nitrate leaching, decreased soil microbial activity, decreased accumulation of soil organic carbon, and increased weed density. Iowa corn-soybean cropland is losing approximately 20 to 23 pounds N per acre every year through nitrate leaching (Christianson et al., 2012) with the majority of this loss occurring because of a lack of actively growing plants in the late fall or early spring. Iowa cultivated cropland soil is currently being eroded at a rate of approximately 6.07 tons per acre every year through sheet and rill erosion with no decrease in erosion having occurred since 1992 (USDA, 2015).
The addition of cover crops to an agricultural system has great potential to decrease soil erosion, weed density, and nitrate leaching while increasing soil organic carbon (Kaspar et al., 2001; Teasdale, 1996; Strock et al., 2004; Dinnes et al., 2002; Villamil et al., 2006; Kaspar and Singer, 2011). Despite these benefits, approximately 1.9% of Iowa farm ground was planted to cover crops in 2015 (Lenssen, 2015). Of this area, the majority of hectares were planted to winter rye (Secale cereale L.). Winter rye is the most widely used cover crop in Iowa because it establishes easily, produces high quantities of biomass, germinates at approximately 34°F, produces vegetative growth above 38°F, is very winter hardy, and the seed is available and inexpensive (Snapp et al., 2005; Singer, 2008).
Establishment, overwintering, and growth of cover crops planted into standing corn/soybean or after corn/soybean harvest is a major limitation for the implementation of cover crops in the upper Midwest (Johnson et al., 1998; Wilson et al., 2013). Many cover crop species have been shown to be effective in the 7a winterhardiness zone (USDA ARS, 2016) of the mid-Atlantic (Clark et al., 1994), 6a to 7b winterhardiness zone of irrigated soils of Pacific Northwest (Weinert et al., 2002), 8a to 9a winterhardiness zones of southeastern (Sainju et al., 2005), and 9a to 10a winterhardiness zones of the irrigated western (reviewed in Snapp et al., 2005) United States. However, some of these cover crops would not survive the 5a winterhardiness zone that makes up the majority of Iowa (USDA, 2012), in part because of variable snow cover. Many of these cover crops would be far less productive in Iowa than in other areas of the country due to Iowa’s corn-soybean cropping system, shorter cover crop growing season, and lower heat unit accumulation from harvest to planting. Iowa’s colder climate limits viable cover crop options and the potential for successful establishment and growth of cover crops.
Recently, many new cover crops for Iowa have been widely promoted by cover crop seed companies, farm journals, and other commercial sources. Iowa Natural Resource Conservation Service (NRCS) has produced literature with a vast assortment of new cover crop options for Iowa (NRCS, 2013). Despite this interest, limited research has been done on these new cover crops to test their effectiveness in Iowa or the Midwest. Winter rye has been heavily researched and is the predominant cover crop planted in Iowa, but winter rye can sometimes negatively affect corn establishment, growth, and yield possibly due to water use, immobilization of soil nitrogen, interference with planter performance, or fungal disease (Munawar et al., 1990; Tollenaar et al., 1993; Duiker and Curran, 2005; Krueger et al., 2011; Kaspar et al., 2015). Oat (Avena sativa L.) is a potential cover crop for Iowa when overseeded into standing soybeans in August, but oat will not produce any spring growth because it does not overwinter in Iowa (Johnson et al., 1998).
Alternative cover crops such as hairy vetch (Vicia villosa Roth), rapeseed (Brassica napus L.), and white mustard (Sinapis alba L.) have been effective in warmer regions of the United States (Clark et al., 1994; Wilke and Snapp, 2008; Villamil et al., 2006, Weinert et al., 2002), winterhardiness zones 6a to 8b. Brassicaceae are increasingly being utilized as cover crops and can reduce weed populations due to allelophathic breakdown products from glucosinolates (reviewed in Haramoto and Gallandt, 2004). Conventional, non-genetically modified canola (Brassica napus L.) and camelina [Camelina sativa (L.) Crantz], both small-seeded annual Brassicaceae can overwinter in Iowa (Martinez-Feria, et al., 2016; Lenssen, personal observation). Recently, camelina was documented to be an effective fall seeded cash crop in west central Minnesota (Gesch and Cermak, 2011), a 3b winterhardiness zone. Camelina can survive harsh Minnesota and North Dakota (winterhardiness zone 4a) winters (Berti et al., 2015) and has been documented to produce the greatest seed yields when planted in early to Mid-October and then harvested in mid-July (Gesch and Cermak, 2011). They also reported that earlier seeding dates in September produced lower camelina seed yields.
Hairy vetch has sparked interest as an Iowa cover crop due to it’s potential to increase N supply to the following crop, which has been documented in warmer climates than the Upper Midwest (Clark et al., 1994; Sainju and Singh, 2008). Harbur et al. (2009) planted hairy vetch into fallow ground in early September and documented hairy vetch winter survival rates of 0.0% to 73.0% for 12 different hairy vetch ecotypes grown in southern Minnesota, a 4b winterhardiness zone. Hairy vetch ecotypes, which were sourced from Minnesota, had superior survival rates to hairy vetch ecotypes sourced from warmer climate areas (Harbur et al., 2009), demonstrating that hairy vetch seed source selection is an important component of improving winter survival. Hairy vetch mean aboveground biomass accumulation across two years and two locations was 1695 pounds per acre (Harbur et al., 2009).
Despite the interest in new cover crops and cover crop mixes for Iowa, few cover crops can survive the harsh Iowa winters, which limits their potential to accumulate biomass, prevent soil erosion and nitrate losses. Cover crops, and especially winter rye, effectively decrease soil erosion and nitrate losses in many environments including Iowa (reviewed in Kaspar and Singer, 2011; Kaspar et al., 2001; Kaspar et al., 2012). The use of a winter cereal cover crop such as winter rye can provide excellent biomass production and decrease soil erosion (Kaspar et al., 2001; reviewed in Kaspar and Singer, 2011) but can also create management challenges as winter cereal cover crops often result in N immobilization and limit N supply to the following crop (reviewed in Snapp et al., 2005). Release of N from cover crop residues is greatly increased when cover crops are incorporated as opposed to being left on the soil surface (Kuo et al., 1997b). Incorporation of Brassicaceae cover crops in the fall leads to increased soil N loss compared to spring incorporation (Weinert at al., 1997; Haramoto and Gallandt, 2004). Cover crop residue C:N ratio has been documented to be a good predictor of N mineralization and N residue retention (Quemada and Cabrera, 1995). High C:N ratio residues mineralize N at a slower rate and retain more N throughout the growing season, limiting the supply of soil available N to the cash crop (Quemada and Cabrera, 1995).
Cover crops have been documented to suppress weeds primarily through decreasing light transmittance to the soil (Teasdale, 1996). Teasdale et al. (1991) documented that when rye or hairy vetch cover crop residues covered more than 90% of the soil, total weed density was decreased by 78% as compared to a no-cover crop control in a sweet corn crop one month after sweet corn planting. In the same study Teasdale et al. (1991) also documented that increased cover crop biomass was positively correlated with decreased weed density and the relationship was linear. Other mechanisms for weed suppression, such as allelophathic effects from Brassicaceae cover crops, have been studied and are effective in the greenhouse (Haramoto and Gallandt, 2004), but show little evidence for being effective in the field (Haramoto and Gallandt, 2005). Teasdale (1996) reported that cover crop allelopathic effects are inconsistent and often difficult to document in field studies. When a cover crop produces adequate biomass and light interception, early season weed suppression in the crop can be observed (reviewed in Teasdale, 1996). Spring terminated cover crops rarely provide complete weed control later in the season (reviewed in Teasdale, 1996). Cover crops provide limited weed suppression when they are tilled into the soil and cover crops residues are not allowed to remain on the soil surface (Teasdale et al., 1996; Wortman et al., 2013). Wortman et al. (2013) documented that increasing the number of species in a cover crop mix did not decrease weed density or weed biomass in an organic sunflower-soybean-corn rotation when cover crops were planted in late March, terminated in late May, and weed sampling occurred approximately 30 days after cash crop planting.
Cover crops might negatively impact crop development as a result of decreased spring soil temperatures due to light interception and soil shading. Corn emergence rate is highly correlated with the accumulation of growing degree days and soil temperature (Schneider and Gupta, 1985). Increases in soil cover from crop residue in the corn row at the time of planting has been documented as a strong detriment to corn growth rates from the time of planting to V6 stage corn (Swan et al., 1987). Corn row residue coverage of 87% was documented to require an additional 48 growing degree days for corn to reach V6 stage compared to 8% corn row residue coverage (Swan et al., 1987). Increasing cover crop biomass may increase soil cover and reduce soil solar interception, but little research has been published on this topic. Cover crops can have both positive and negative effects on soil available water (Munawar et al. 1990; Liebl et al., 1992; reviewed in Miguez and Bollero, 2005; Unger and Vigil, 1998; Krueger at al., 2011). Cover crops can decrease early season soil available water through transpiration losses but also increase available soil water due to increased soil coverage from cover crop residue remaining on the soil surface (Liebl et al., 1992; reviewed in Miguez and Bollero, 2005). Increased soil water loss through cover crop transpiration could be desirable in areas where heavy, wet spring soils and frequent rainfall limit early season field operations (reviewed in Kaspar and Singer, 2011). Alternately, areas with course textured soils and limited rainfall may experience soil water deficits during the cropping season as a result of cover crop transpiration, if adequate rainfall does not occur after cover crop termination (Unger and Vigil, 1998; reviewed in Kaspar and Singer, 2011). The effect of cover crops on corn yield is highly variable and many contrasting results have been reported (Miguez and Bollero, 2005). Some studies documented that corn yield can be negatively influenced by certain cover crops in some years (Johnson et al., 1998; Krueger et al., 2011; Parr et al., 2011; Kaspar et al., 2012). Other studies documented that certain cover crops have no influence on corn yield in some years (Wortman et al., 2012b; Kaspar et al., 2012). Lastly, some studies documented that certain cover crops have a positive influence on corn yield in some years (Clark et al., 1994; Parr et al., 2011). A meta-analysis of 36 studies from the United States and Canada found on average a 21% increase in corn yield following a biculture winter cover crop, a 37% increase in corn yield following a legume winter cover crop, and no influence on yield of corn which followed a grass winter cover crop (Miguez and Bollero, 2005).
The objective of this study was to evaluate 16 potential cover crop treatments for Iowa, including two- and three-way mixtures. The effects of these 16 cover crops on (a) fall and spring cover crop aboveground biomass, carbon, and nitrogen accumulation, (b) spring soil temperature, (c) soil nutrients, (d) weed community and density, (e) corn population, (f) volumetric soil water content, (g) SPAD corn leaf chlorophyll, and (h) corn yield were examined.