Spring seeding of green manure cover crops expands the opportunity for their use in northern climates where there are limited opportunities to plant covers in the fall before snowfall. In regions characterized by cooler climates, cash crops typically occupy fields for the majority of the growing season, which limits the ability to plant cover crops in the fall, particularly as options for overwintering grasses and legumes are minimal. Spring seeding allows for greater choice in the types of cover crops that could be used, expanding the ability to integrate species that are not winter-hardy. For certain cool-season cover crops, spring planting (March–April) allows adequate time for cover crops to establish and serve as a supplemental N source, resulting from either sequestration of existing soil N resources or fixation of atmospheric N by legumes. The potential performance of cover crops within these windows may be limited to soils with high sand content (which will allow for more rapid drying and warming in the spring) and crop rotations that allow for a later planting date of the main crop (e.g., crops that are harvested within 90 d of planting).
The Central Sand Plains of Wisconsin is a landscape in which spring-seeded green manures could be used to provide benefits to irrigated cropping systems. This region has high nitrate concentrations in groundwater (Bero et al., 2014; Kraft et al., 1999), resulting, in part, from high-value, high-N-demand crops grown under pivot irrigation. Sweet corn is a common crop in this region and is typically harvested 90 d after planting (DAP), allowing for a range of planting dates in the spring and summer months. Current fertilizer guidelines for sweet corn are 168 kg·ha–1 N, but significant amounts of N leaching can occur if crop uptake is not synchronized with plant-available N in irrigated sandy soils (Yuan et al., 2017).
On sandy soils, cover crops can be used both to reduce N leaching (Snapp et al., 2005) and to supply N to the following cash crop (Andraski and Bundy, 2005). Past research on spring-planted legume and grass cover crops in the Central Sand Plains has shown varying N benefits for sweet corn grown after cover crop integration (Johnson et al., 2012; West et al., 2016). Investigating the potential benefits of field pea (Pisum sativum) as a spring-seeded cover crop preceding sweet corn in the Central Sands, West et al. (2016) and Johnson et al. (2012) determined no N benefit from field pea in this system. Beyond field pea, other legume cover crops have not been tested in this region for potential spring biomass production or N contribution to the cash crop production season. Grass green manures such as oat may also supply N to subsequent crops; Andraski and Bundy (2005) showed that oat supplied N to field corn in two of three growing seasons.
Beyond the degree of spring growth, other management factors may impact the N contribution of the cover crop. Cover crop termination methods and subsequent effects on N uptake by the cash crop have not been well studied. West et al. (2016) and Johnson et al. (2012) ended field pea cover crop through mechanical tillage. This aggressive termination technique, which incorporates the cover crop residue into the soil, may have increased decomposition of the plant material and mineralization of N, leading to asynchronous N availability, with N uptake and potential leaching losses. It is likely that chemical termination, without mechanical incorporation, would be more beneficial on sandy soil as a result of the more extended decomposition of the residue and subsequent N mineralization.
Outside of the Central Sands, other cover crops such as CV and BC have provided promising benefits of increased cash crop yields resulting from supplied N credits. When grown as a full-season crop, CV has been shown to increase wheat yields compared with red clover (Bullied et al., 2002). On a sandy loam soil in Winnipeg, Canada, Thiessen Martens et al. (2005) measured an N credit of 29 to 45 kg·ha–1 after spring-planted CV. Ross et al. (2009) measured 84 to 110 kg·ha−1 N in their spring-planted BC after 14 to 16 weeks of growth, resulting in an increase in subsequent barley yield. For studies that have evaluated the benefit of cover crops on sweet corn yields, results have been quite variable, showing no effect (Isse et al., 1999) and neutral to positive effects (Bhardwaj, 2006; O’Reilly et al., 2012). In the case of O’Reilly et al. (2012), total sweet corn aboveground biomass (AGB) was greater with oat and oat and oilseed radish cover crops compared to no cover crop; however, this biomass increase did not translate to an increase in marketable sweet corn yield.
Evaluation of other legume cover crops is warranted, as is their effect on crop yield and N fertilizer equivalent. The goal of the study was to determine the N credit of spring-planted grass and legume cover crops (oat, oat + BC, CV, and BC) for sweet corn on irrigated sand, as measured by differences in agronomically optimum N rates determined by nonlinear regression. Specific study objectives were 1) to measure spring-planted cover crop growth and N content, and 2) to determine the impact of spring planted cover crops on sweet corn yield.
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