- Agronomic: corn, wheat
- Vegetables: radishes (culinary)
- Crop Production: conservation tillage
- Education and Training: extension, on-farm/ranch research
- Production Systems: general crop production
- Soil Management: soil analysis, soil chemistry, soil physics, soil quality/health
A comparison of an oilseed radish cover crop in a corn rotation to a corn rotation without a cover crop in Wisconsin determined that a radish cover crop appears to have several benefits for some producers. Although fall radish nitrogen (N) uptake can be quite substantial, no N credit was determined. However, spring soil nitrate samples did suggest a N credit. Data gathered from a soil penetrometer indicated differences in soil compaction in the upper profile early in the season, as well as differences in the deeper profile later in the season.
Radish has become a popular cover crop option throughout the United States within the past decade. Among its touted benefits are N scavenging, compaction reduction, and pest suppression. To date, much of the radish cover crop research has been conducted in the Mid-Atlantic region of the United States. Weil and Kremen (2007) demonstrated that brassicaceous cover crops took up more N in the fall than rye, which was the standard N capture cover crop in Maryland. Brassicaceous cover crops also rapidly depleted soluble N from soil profiles in the fall which reduced spring N leaching. These results were confirmed by other studies (Kristensen and Thorup-Kristensen, 2004; O’Reilly et al., 2012; Dean and Weil, 2009). Kristensen and Thorup-Kristensen (2004) proposed that radish was effective at capturing N from deep soil layers due to its deep root system that increased in root intensity from the soil surface to 1.5 m depth. Other common cover crops, such as ryegrass and rye have shallow roots concentrated at the soil surface. It is important to note that in most of the University of Maryland experiments, the authors applied 56 kg ha-1 of N to ensure adequate growth on sandy textured soil. It is unclear if this extra N is necessary for non-sandy soils.
A seminal paper by Dean and Weil (2009) demonstrated the benefit of radish on N uptake and reduction of residual nitrate. Averaged across 3 years, radish had greater dry matter production and captured more N in fall than rye shoots, a popular and widely-used cover crop. However, in this study, the authors applied 56 kg ha-1 of N at the same time of radish planting to ensure adequate growth on sandy textured soil. It is unclear on non-sandy soils if extra N is required for adequate growth, although in most Wisconsin systems, radish will be applied after a manure application. A more recent paper from Chen and Weil (2011) showed that radish planted in the fall prior to corn planting resulted in a significant yield increase compared to plots with no cover crop. Similarly, results reported by O’Reilly et al. (2012) showed that planting radish as a cover crop increased yield in sweet corn when compared to no cover crop, especially in plots where less than optimal nitrogen rates were applied. This may also suggest there is a N credit for radish. Dean and Weil (2009) provide a summary of eight other studies that have shown that radish has at least some level of positive impact on capturing soil nitrate or improving crop yields.
The effects of radish were also seen during the subsequent crop’s growing season. Both corn and soybean plants produced more dry matter and had greater tissue N when following radish as compared to no cover crop (Weil and Kremen, 2007; O’Reilly et al., 2012). In late summer, soil moisture sensors showed more rapid infiltration of water into the soil after rains in the subsoil of plots previously planted in radish (Weil and Kremen, 2007). Williams and Weil (2004) suggested that the root channels left behind by radish provided the subsequent crop (soybean, in this case) roots with low resistance paths to water contained in the subsoil. Radish planted in the fall before corn resulted in a significant corn yield increase compared to plots with no cover crop (Chen and Weil, 2011). These results are similar to those reported by O’Reilly et al. (2012) who demonstrated that planting radish as a cover crop increased yield in sweet corn, especially in plots with less than optimal N; however, a N credit was not determined. Dean and Weil (2009) showed that radish decreased nitrate-N concentrations in soil pore water on fine textured soil compared to the control. Soybean yields were also significantly greater following radish cover crop treatments when compared to other brassicaceous cover crop treatments as well as the no cover crop treatment (Williams and Weil, 2004). Other than O’Reilly et al. (2012), there was a lack of studies that evaluated a potential N credit for radish, which is possible based on the amount of N in the whole plant biomass (119 kg N ha-1 according to Dean and Weil, 2009) and a favorable C:N ratio for net mineralization (20:1 according to Clark, 2007).
Researchers have investigated the impact of radish on soil resistance and soil compaction as well. In 2007, Weil and Kremen used minirhizotron images to confirm that soybean roots penetrate hard plow pans by following channels made by radish the previous fall. Chen and Weil (2010) found that under high and medium soil compaction, radish had significantly more roots penetrate the soil than either rapeseed or rye. This conclusion was found to be especially true in soil with high clay content, which has significant implications for clay type soils found in Wisconsin. Corn planted after radish also had more deep roots under high soil compaction than rye or no cover crop (Chen and Weil 2011). Weil and Kremen (2007) proposed that this could be due to the fact that soil cores taken in late summer revealed about 10 times more corn roots in plots previously in radish than in plots previously in no cover crop. A recent study by Chen et al. (2014) showed that radish also had greater air permeability than other cover crops as well as the no cover crop treatment due to an increase in roots into compacted soils.
Root lesion nematodes (Pratylenchus spp.) (RL) are the third most economically damaging nematode in the world for agricultural crops, behind root-knot and cyst nematodes. This is due to their wide host range (more than 400 crop plant species), as well as their wide environmental distribution (Davis and MacGuidwin, 2000). Pratylenchus spp. are the most common nematode pest of corn in the Midwest (MacGuidwin and Bender, 2012; Tylka et al., 2011). In 2012 the Wisconsin Soybean Marketing Board expanded their testing program to include other pest nematodes at no charge so that growers would be able to monitor their total nematode populations. Of the 315 samples collected, 96% tested positive for RL nematode. Out of the samples that tested positive, 20% were above the damage threshold and were distributed throughout WI (MacGuidwin, 2013). Root lesion nematode damage is often misdiagnosed as nutrient deficiencies and can even cause yield loss without any visible aboveground symptoms (A3646; Davis et al., 2015).
Nematode suppression from brassicaceous plants occurs due to the glucosinolate compounds (organic compounds that contain sulfur and nitrogen) and the enzyme myrosinase contained in their tissues (Brown and Morra, 1997). It is the degradation products of glucosinolates reacting with myrosinase that are toxic to soilborne organisms (Donkin et al., 1995). These degradation products include isothiocyanates (ITCs), thiocyanates, and nitriles, although ITCs are considered the most toxic (Brown et al., 1991). Glucosinolates and myrosinase are physically separated in Brassica plants; therefore, it takes some form of physical disruption of the plant tissue to bring the two compounds together. Since this is a hydrolysis reaction, water is required for the reaction to become active and release ITCs (Matthiessen and Kirkegaard, 2006). Several studies have shown that ITCs can suppress plant-parasitic nematodes (Mojtahedi et al., 1991; Zasada and Ferris, 2003, 2004). It is believed that this process is part of plant defense against insects and pathogens (Matthiessen and Kirkegaard, 2006).
Brassicaceous cover crops are well known for their potential for pest nematode suppression. Both Brussels sprout and horseradish plant material amendments reduced citrus nematode survival by 39 and 59%, respectively, as compared to the control (Zasada et al., 2003). Rapeseed grown for two months, then incorporated into the soil, was more effective at reducing nematode population density than the control (Mojtahedi, et al. 1991). In a field experiment, both radish and radish intercropped with oats demonstrated lower reproduction rates for Pratylenchus brachyurus (Chiamolera et al., 2012). Radish reduced plant-parasitic nematode populations by 55.7% when compared to a no cover crop control (Wang et al., 2009). Out of the 11 radish varieties tested, oilseed radish had the highest average content of glucosinolates (Ciska et al., 2000). Zasada and Ferris (2004) demonstrated that brassicaceous amendments added to the soil based upon glucosinolate profiles could be applied to achieve consistent and repeatable nematode suppression. High concentrations of ITCs in the soil after brassicaceous biomass incorporation indicated that brassicaceous plants would have a higherpotential to suppress nematodes (Hansen and Keinath, 2013).
Utilizing brassicaceous cover crops for nematode suppression is a broad-spectrum tactic; all soilborne nematodes are targeted. Therefore, plant-parasitic nematodes such as soybean cyst nematodes (Heterodera glycines) (SCN), which are not corn pests, would also be suppressed. Only soybean is a host for SCN, but the eggs remain viable and susceptible to nematode suppression even when soybean is not present (such as when the field has rotated from soybean to corn). Soybean cyst nematodes, like RL nematodes, can also greatly reduce crop yield; SCN decreased soybean yield in the U.S. more than any other pathogen from 1996 to 2007. In 2007, SCN infection led to a reduction in soybean yield of 94 million bushels (Wrather and Koenning, 2009).
Currently, no published papers investigate the biofumigant effect of Brassicaceous cover crops on soybean cyst or root lesion nematodes specifically. A paper by Gruver et al. (2010) examined the effect of radish on free-living nematode community composition, but the results were inconclusive. The authors state that while radish had unique impacts on nematode communities, these impacts appeared to be associated more with quality of organic matter inputs rather than biofumigation. Thus, there is enough evidence to support further research of radish as a cover crop in Wisconsin.
Some research has been conducted in the North Central region, including the first two years of this project. The beginning of this project focused solely on the uptake and release of N in the cropping system. Moving forward into the third year, it has become evident that important information about radish as a cover crop is missing, and this project hopes to fill in some of these gaps. Regardless, the bulk of the research on radish as a cover crop has been performed in the Mid-Atlantic region. While the results have demonstrated significant benefits of using radish as a cover crop, these data are specific to the Mid-Atlantic region, their management practices, and their soil. A greater range of research is needed in the North Central region on our soils under our management practices in order to provide more conclusive results. This expanded research would further inform North Central farmers and land managers on how to protect their soil and water in the future.
There is a great need for further research on cover crops in Wisconsin in particular for soil and water conservation. In some cropping systems, there is a window of opportunity for a cover crop to be planted from mid-to-late summer to early fall. Radish as a cover crop would fit into these summer time slots well, providing soil and nutrient conservation benefits. For example, in these cropping systems, the soil is typically left bare after the winter wheat is harvested. Planting radish in this time slot would therefore help prevent soil erosion. Also, even though it is important to note that radish will most likely not establish as quickly as a rye (a popular cover crop), radish does produce a rather dense canopy. This canopy reduces raindrop impact on the soil surface. But in order for radish cover crops to be utilized successfully, they need to be fully researched to understand their proper use across many different conditions. There is clearly not enough data on radish as a cover crop to make recommendations for use, and this puts the agronomic community at a disadvantage. There is a need for smart cover crop use here in the Midwest as many growers are participating in NRCS conservation programs.
Objective #1. Determine the potential nitrogen credit from radish as a cover crop.
Objective #2. Discern the effect of radish on soil resistance.
Objective #3. Determine the effect of radish on nematode populations.