Improved Cover Crop Options for Corn Belt Farmers

Fall Cover Crop Aboveground Biomass

When fall cover crop stand counts and aboveground biomass harvest were determined at Ames, Lewis, and Sutherland in 2014, the grasses were at two-leaf stage, the Brassicaceae cover crops were at early cotyledon stage, and vetch shoot length was approximately 2 in. Biomass harvest was taken in late fall, at the time of the first killing frost. Biomass harvest did not occur at Boone because the cover crops did not emerge until the following spring.  Biomass harvest did not occur at Lewis in fall 2013 because biomass accumulation was negligible with grasses barely emerged.  Soybean harvest occurred approximately 1-2 weeks later than in most years due to wet field conditions, which limited available growing degree days for cover crop development due to later planting.  Despite cover crops being planted at each site one or two days after combine harvest of soybean, postharvest heat unit accumulation was limited, resulting in poor fall cover crop biomass accumulation at all sites (Table 3). Cover crops had at most three to four weeks to germinate, emerge, and grow before the first hard killing frost (Table 2).  Johnson et al. (1998) documented a mean of 393 pounds per acre of fall aboveground biomass when oat and winter rye cover crops were overseeded into soybeans in August in Iowa.  The most productive cover crop in our study produced 15% of the fall cover crop biomass of the Johnson et al. (1998) study.  Early maturity soybean cultivars which can be harvested earlier, or intercrop cover crop seeding by either aerial application or a high-clearance tractor with drop tubes, may allow cover crops to achieve significantly more biomass accumulation and improved winter survival of canola and hairy vetch cover crops.

Cover crops differed for fall biomass accumulation with rye-camelina mix producing the greatest amount of aboveground biomass, 61 pounds per acre (Table 3). Rye monoculture and rye mixtures did not differ in biomass production.  Triticale associated cover crops produced slightly less biomass than rye-associated cover crops.  Triticale monoculture and triticale mixtures did not differ in biomass production. These results would lead to the conclusion that fall biomass production was not greater in mixtures than in a monoculture under the conditions and cropping systems of this study.  In general, the treatments that included a grass accumulated at least twice as much biomass as the sole crop Brassicaceae treatments (Table 3).  Hairy vetch monoculture accumulated 4 pounds per acre, the least biomass of all cover crops and cover crop mixtures, producing 90.1% less biomass than monoculture rye (Table 3).  Apparently, hairy vetch must be planted earlier in the fall to allow the accumulation of more growing degree days to increase potential winter survival (Samarappuli et al. 2014).  It is also possible that the hairy vetch genotype used in this study may not have sufficient winter hardiness to tolerate harsh Iowa winters with limited snow cover, as occurred in this study.

Table 3.  Fall aboveground biomass cover crop associated response variables. Cover crop biomass accumulation, cover crop carbon accumulation, cover crop nitrogen accumulation, cover crop C:N ratio for 3 Iowa sites 2013-2014.

* Significant at P ≤ 0.05; ** Significant at P ≤ 0.01; *** Significant at P ≤ 0.001.

^† Means followed by different lower case letter within a column in a set are significantly different at P ≤ 0.05 by the least square means test.  

Fall Cover Crop Stand Density

Cover crop entries differed for stand density; rye-camelina had the greatest stand density.  Monoculture rye and monoculture camelina produced stand densities that were 43% and 63% of the stand density of rye-camelina mixture. Rye mixtures and triticale mixtures which included camelina produced the greatest stand densities.  These results lead to the conclusion that fall cover crop stand densities can be significantly greater in a mixture than in a monoculture when two productive monoculture cover crops are planted as a mix, as long as seeding rates of either are not greatly decreased. 

Fall Cover Crop C and N Accumulation and C:N Ratio

Cover crop entries differed in their aboveground C and N accumulation and C:N ratio (Table 3).  Entries that included a grass accumulated the most C and N (Table 3).  Among entries, the C:N ratio was generally lowest for the Brassicaceae and vetch sole crops (Table 3).  Cover crop residues that remain on the soil surface and have a high C:N ratio decompose and release accumulated N more slowly than low C:N ratio cover crop residues (Quemada and Cabrera, 1995).  Despite the statistical significance among species for fall cover crop biomass, C, and N accumulation, cover crop growth was nominal compared to other areas of the United States, different cropping systems, or earlier cover crop planting dates that accumulate more heat units following cover crop planting and the onset of winter (Kuo et al., 1997a,b; Sainju et al., 2005; Sainju and Singh, 2008; Poffenbarger et al., 2105). The most productive cover crop in our study produced 2-10% of the fall cover crop biomass of six different cereal forage crops, which were seeded in August in winterhardiness zones 4b and 5a in Wisconsin (Maloney et al., 1999).  Finney et al. (2016) documented that when cover crops were seeded after oat harvest in August in Pennsylvania (winterhardiness zone 6b) fall cover crop biomass accumulations of winter rye and winter rye mixtures all produced at least 10 times greater biomass accumulation than the most productive cover crop in our study.

Spring Cover Crop Aboveground Biomass

The majority of turnip and vetch plants winterkilled and the spring turnip and vetch biomass collected probably came from hard seed, which did not emerge in the previous fall.  In southern Minnesota (winterhardiness zone 4b), hairy vetch planted into fallow ground in early to mid-September had survival rates of 0-73% across 12 ecotypes sourced from differing locations in the United States (Harbur et al., 2009).  They also documented that all 12 hairy vetch ecotypes winterkilled at one out of six locations across three years of the study, and the average hairy vetch survival rate was approximately 50% (Harbur et al., 2009). Our study had lower hairy vetch survival rates and was planted much later than the Harbur et al. (2009) study. We are not aware of optimal fall developmental growth stage for winter survival of hairy vetch.   

Sitro canola, Claremore canola, barley, and oat all winterkilled and produced no spring growth.  Oat does not survive the winter in Iowa (Johnson et al., 1998), so it was expected it would winterkill. Gusta and O’Connor (1987) documented that barley at the two-leaf stage does not survive temperatures below 14°F. In our study the barley had just reached two leaf stage in the fall and winter temperatures were far below 14°F.  Rife and Zeinali (2003) documented that canola rarely survives temperatures below 10°F.  Canola generally doesn’t survive winter unless it has reached rosette stage or has at least 6 fully developed leaves (Great Lakes Canola Association, 2016).  In this study, canola plants only reached cotyledon stage and none survived the harsh winters. Temperatures dropped below -20°F on multiple occasions at all sites during the 2013-2014 and 2014-2015 winters, with no snow cover during several of these events.  It should be noted that the rye-canola mix contained no canola biomass in the spring due the winterkill of canola (Figure 2). Camelina survived winter well in our study, similar to other reports from winterhardiness zones 3b and 4a in the upper Midwest (Gesch and Cermak, 2011; Gesch and Archer, 2013; Gesch et al. 2014).  

Cover crop entries influenced spring cover crop aboveground biomass accumulation (Figure 1).  Rye-vetch mixture was composed of 97% rye biomass and produced the most biomass of all cover crops, but it was not significantly different from any other rye-associated treatment or the three-species mixture of triticale-camelina-vetch (Figure 1).  Rye and mixtures with rye accumulated the most biomass with a mean of 676 pounds per acre across the five rye treatments.  Spring winter rye biomass accumulation was very similar to another recent Iowa study in which ‘Wheeler’ winter rye produced 642 pounds per acre of biomass over a three-year study (Pantoja et al., 2015).  Conversely, Kaspar and Bakker (2015) reported mean aboveground biomass yield of 1990 pounds per acre from six winter rye cultivars grown in Iowa over a different four-year period, 2006-2009, a period with greater growing degree days (GDD) accumulation than observed during the period of our study.  In a five-year study from 2005-2009, Kaspar et al. (2012) reported average spring biomass accumulation of 1170 pounds per acre for winter rye.  As for other annual crops, winter rye biomass accumulation is influenced by environment, cultivar, date of termination, and numerous other factors.  

At cover crop termination, rye and triticale mixtures had produced similar biomass to their corresponding monocultures (Figure 1).  Both rye and triticale were highly competitive with the intercropped Brassicaceae and vetch (Figure 1).  Rye production was greater than 79% and triticale production was greater than 58% of the total biomass in two- and three-way mixtures (Figure 1).  These findings are consistent with other studies which have found that cover crop mixes do not produce more biomass than the most productive cover crop monocultures (Finney et al., 2016; Poffenbarger et al., 2015; Wortman et al., 2012a).  Other studies have confirmed that rye often dominates biomass production in a mixture (Clark et al., 1994; Finney et al., 2016; Poffenbarger et al., 2015).  Poffenbarger et al. (2015) examined cover crop planting ratios for hairy vetch-winter rye mixtures and recommended a hairy vetch:winter rye seeding rate of 27:34 ratio in order to maximize nitrogen content of the cover crop and maintain a relatively low C:N ratio for a cover crop, which was planted in Mid-September to Mid-October in Maryland after a soybean cover crop that was terminated in late August.  In the same study, if rye was proportionally greater than a seeding rate of 27:34 ratio of hairy vetch:winter rye, rye biomass dominated the mix for three out of four site years (Poffenbarger et al., 2015).  Winter rye/hairy vetch seeding ratio in our study was 45:11 ratio, which would give a strong advantage to the winter rye. Triticale monoculture produced 50% as much biomass as the mean of the rye-associated cover crops (Figure 1).  It should be noted that our study only included a single cultivar of winter rye, winter triticale, camelina, hairy vetch, turnip, spring barley, and spring oats so our results may not be representative of each species. Camelina monoculture produced 41% as much biomass as the mean of rye-associated cover crops (Figure 1).  Camelina and vetch produced less biomass in a mix than when grown as a monoculture (Figure 1).  Turnip and vetch produced 15% and 10% as much biomass as rye-associated cover crops, respectively, and had sparse stand density due to lack of overwintering. 

Spring Cover Crop C and N Accumulation and C:N Ratio

Rye-associated cover crops accumulated the most aboveground C and N with a mean of 281 pounds C per acre and 19 pounds N per acre across the five rye-associated treatments (Figures 2,3).  Rye mixes did not accumulate more C or N than a rye monoculture (Figures 2,3).  Due to their increased biomass production, winter rye cover crops have the potential to contribute more soil organic carbon and accumulate more soil N than the other cover crop treatments included in this study. Kuo et al. (1997a) also documented similar results to our study, demonstrating that winter rye had greater potential to increase soil organic carbon than hairy vetch, canola, or Austrian winter pea [Pisum sativum L. ssp. arvense (L.) Poir.].  On average, approximately 20-23 pounds N per acre is lost from Iowa corn-soybean farm ground every year through nitrate leaching (Christianson et al., 2012). Nitrogen fertilizer recommendations in Iowa assume that nitrate which is present in fall soil samples is lost via leaching or denitrification (Sawyer, 2013). In our study, the five treatments which included a winter rye cover crop accumulated a mean of 19 pounds N per acre in aboveground biomass (Figure 3b), which should reduce the N being lost from Iowa farm ground in the months of October-April when crops are otherwise not growing in Iowa fields. In a three-year field study at four Iowa locations, Pantoja et al. (2015) documented that winter rye cover crops accumulated 19 pounds N per acre, identical to what we report. Camelina and vetch accumulated 36% and 10% as much C and 60% and 13% as much N, respectively, as the mean of the five rye-associated treatments. Camelina and vetch accumulated much less C and N when grown in a mix compared to a monoculture (Figure 3), likely due to competition from rye and triticale.

The C:N ratio of rye-associated cover crops was greater than for all other cover crops and cover crop mixtures, except for triticale monoculture and triticale-vetch. The C:N ratio did not differ between rye monoculture and rye mixtures.  Two out of three triticale mixtures had significantly lower C:N ratios than all of the rye-associated cover crops.  Camelina monoculture had a lower C:N ratio than all rye and triticale-associated cover crops.  Turnip had the lowest C:N ratio (Figure 4), but this was comparatively unimportant as turnip produced limited biomass from seeds that germinated in the spring (Figure 1). 

The C:N ratio for rye mixtures were somewhat surprising as we expected rye mixtures to have lower C:N ratios than rye monoculture as previously documented by Sainju et al. (2005). Sainju et al. (2005) planted a fall seeded rye and hairy vetch biculture in Georgia at 50% of the rye and 68% of the hairy vetch monoculture rates. Sainju et al. (2005) documented rye monoculture C:N ratios of 29, 57, and 40 and rye-hairy vetch mixture C:N ratios of 10, 32, and 11 across a three-year study.  We had assumed the presence of camelina and vetch with low C:N ratios, would decrease the C:N ratio of the mix.  But this was not the case; probably due to the fact that rye accounted for 79% or more of the biomass in rye mixtures, and triticale accounted for 58% or more of the biomass in triticale mixtures.  Our results may have been different if seeding ratios would have been adjusted to greatly decrease the seeding rate of rye and triticale, while increasing the seeding rate of hairy vetch and camelina.

The C:N ratio is often a major factor in determining cover crop rate decomposition and N release (reviewed in Blanco-Canqui et al., 2015).  All cover crop entries included in this study had C:N ratios less than 14:1, in large part because their growth and development was low due to lack of accumulated heat units prior to their termination for planting corn. Weinert at al. (2002) documented that cover crops with C:N ratios below 16:1 were unlikely to cause N immobilization; therefore, we can assume that although C:N ratio probably affected decomposition rates, cover crop entries in our study did not immobilize soil N. Due to differences in cover crop C:N ratio, cover crop residue N release may have been slower for the rye-associated cover crops, intermediate for the triticale-associated cover crops as well as the vetch, fast in the camelina and turnip, and very fast in the cover crops which did not overwinter. The assumption that winterkilled cover crops released N very fast is supported by Weinert et al. (2002), who documented that cover crops which winterkill release and leach N more quickly than cover crops which overwinter. 

Soil Growing Degree Days

Over the 34-day period following corn planting, accumulated soil growing degree days were similar at corn seeding depth for rye, rye-camelina, rye-camelina-vetch, and the no-cover crop control (Table 4).  A high percentage of the accumulated rye biomass present at cover crop termination remained on the soil surface, but this did not influence soil growing degree days.  Other studies have found that cover crop residues can reduce spring soil temperatures, total seed germination, and negatively influence crop establishment in no-till systems while moderating temperature extremes by decreasing day time high temperatures and increasing night time low temperatures (reviewed in Blanco-Canqui et al., 2015).  None of these effects were observed in this study, perhaps due to limited biomass production or relatively high soil residue cover in the no cover crop treatment.  Biomass accumulation may have been insufficient to impact radiation interception. It is also possible that rye transpired increased levels of soil water and created a drier soil than the control.  Dry soil has the potential to heat and cool more quickly than wet soil (Licht and Al-Kaisi, 2005). Drier soil in the rye treatment may have compensated for decreased solar interception caused by the rye residues shading the soil surface. 

Table 4.  Spring cover crop (CC) associated response variables not affected by cover crop treatment. Mehlich-3 P at cover crop termination, Mehlich-3 K at cover crop termination, weed density in cover crop at cover crop termination, weed density in V2/V3 corn for 5 Iowa locations 2014-2015. Accumulated soil growing degree days (GDD, corn planting to 34 days after planting) for Boone location 2015.

^† Cumulative Growing Degree Days °C with T_min=41°F, T_max=86°F.

^‡ na, not applicable, data not collected. ^§ Not significant.

Spring Soil Nutrients

Cover crops did not influence soil extractable Mehlich-3 P and K concentrations from 0-6 in at the time of cover crop termination and had similar P and K concentrations to the no cover crop control (Table 4). Mean soil P and K concentration was 18 and 147 ppm, respectively.  However, cover crops differed in their effect on soil nitrate concentration (Figure 3).  Rye, rye mixtures, triticale-camelina, and triticale-camelina-vetch cover crops produced greater biomass than the other cover crop treatments and consequently, soil at 0 to 6 in had significantly lower soil nitrate concentrations than soil in the no cover crop control (Figure 3).  Conversely, soil nitrate concentrations where cover crops produced limited or no spring growth did not have significantly different soil nitrate concentrations from the control (Figure 3).  At the time of cover crop termination increased cover crop biomass was negatively correlated with soil nitrate concentration in this study (results not presented).  In another study conducted in Iowa, Pantoja et al. (2015) documented similar results, finding that a winter rye cover crop had decreased soil nitrate levels at the time of spring cover crop termination.

Weed Community

Weed density and weed community associated with cover crops before cover crop termination was similar among cover crop entries and the control (Table 4).  Mean weed density taken at the time of cover crop termination was 5.5 weeds ft^-2.  Weed community at the time of cover crop termination across all site years contained 42.4% shepherd’s purse [Capsella bursa-pastoris (L.) Medik], 31.0% clover (Trifolium spp.), 7.9% neckweed (Veronica peregrina L.), 6.7% field pennycress (Thlaspi arvense L.), 2.7% West Indian black nightshade (Solanum ptycanthum Dunal), and 9.3% composed of 12 other species.

Weed density and weed community associated with corn was taken approximately 22 days after corn planting and results were similar among cover crop entries and the control (Table 4).  Mean weed density associated with corn was 8.4 weeds ft^-2.  Weed community associated with corn was 83.6% tall waterhemp [Amaranthus tuberculatus (Moq.) Sauer], 5.0% shepherd’s purse [Capsella bursa-pastoris (L.) Medik], 2.3% velvetleaf (Abutilon theophrasti Medik), and 9.1% composed of 24 other species. Weed density among cover crops and the no cover crop control was similar for both sampling events of weeds within cover crops before cover crop termination and also within the growing corn at an average of 22 days after corn planting (Table 4).  Weed suppression by cover crops is often determined by the quantity of cover crop biomass produced (Teasdale, 1996; Finney et al., 2016; Smith et al., 2015), which is highly dependent on the accumulation of heat units.  Few heat units were accumulated between soybean harvest in the fall and corn planting in the subsequent spring, resulting in limited biomass accumulation of cover crops.  Finney et al. (2016) reported that cover crop biomass production which exceeded 4126 ± 454 pounds per acre was the plateau at which nearly 100% weed suppression occurred. The cover crops included in our study achieved far less biomass production than the Finney et al. (2016) study which is a probable explanation for why cover crops had limited influence on weed density or weed community. Smith et al. (2015) documented that different functional or family classifications of cover crops did not serve as directional filters, increasing or decreasing certain weed species, and that cover crop mixtures did not provide better weed suppression than cover crop monocultures. We had hoped to test our cover crops in a similar fashion as the Smith et al. (2015) study, but the cover crops in our study did not significantly influence weed community.  Cover crops have a great deal of variation in their influence on weed suppression (Teasdale, 1996; Blanco-Canqui et al., 2015). Brassicaceae cover crops have been reported to suppress weed emergence in greenhouse settings due to the production of toxic glucosinolate breakdown products, (Haramoto and Gallandt, 2004) but these breakdown products show little evidence for being effective in the field (Haramoto and Gallandt, 2005; Teasdale, 1996).  Our results support the hypothesis that glucosinolate breakdown products do not suppress weeds in the field, at least for the amount of biomass and glucosinolates accumulated. 

 Corn Population

Cover crops did not influence corn population as measured in V2-V3 corn; mean corn population was 30,500 plants acre^-1 (Table 5).  At the one environment where soil temperature was monitored in the seed zone, Boone in 2015, cover crops did not influence corn stand density (results not presented).  Pantoja et al. (2015) documented a 5% reduction in corn population following a winter rye cover crop with similar biomass production as our study. Pantoja et al. (2015) reported that the corn population reduction may have been due to incomplete rye residue removal from the corn row and early season insect feeding on corn plants.  These problems did not occur in our study.  Many climatic factors including soil moisture, soil temperature, and growing degree days contribute to early season corn development so it is difficult to determine exactly why these two studies report different results for the influence of cover crops on corn stand density. 

Table 5.  Spring corn associated response variables not affected by cover crop treatment. Corn population ×1000, soil volumetric water content (VWC; 0-3 in depth at corn planting, 0-3 in depth at V6 corn, 0-8 in depth at V6 corn, 0-3 in depth at R1 corn, 0-8 in depth at R1 corn), corn grain yield at 15.5% moisture for 4 Iowa locations 2014-2015.

^† VWC 0-3 in at planting, data only for 3 locations, all in 2015. ^‡  Not significant.

Volumetric Soil Water Content

 Cover crops did not influence soil VWC at 0-3 in depth at corn planting, 0-3 in depth at V6, 0-8 in depth at V6, 0-3 in depth at R1, or 0-8 in depth at R1 stage corn (Table 5). Cover crops influenced VWC at 0-8 in depth at corn planting (Figure 4).  Although VWC at 0-8 in depth at corn planting did not differ between most cover crop entries (rye-vetch mixture was the only entry which was significantly different from the control), a generalization can be made that the cover crops with higher levels of spring biomass accumulation resulted in lower VWC in comparison to the other cover crops and the no-cover crop control (Figure 4).  Rye-associated cover crops and triticale mixtures generally had lower VWC than the control at 0-20 cm depth at corn planting (Figure 4).  Rye monoculture and rye mixes were similar for VWC (Table 5, Figure 4).  Monoculture triticale had significantly greater VWC than triticale mixtures at 0-20 cm depth at corn planting, and it is unclear why this occurred.  Gesch and Johnson (2015) reported that camelina used less water than soybean in Minnesota.  Nielsen et al. (2015) documented that cover crop monocultures and mixtures used similar amounts of soil water and were similar for water use efficiency (Nielsen et al., 2015).  Daigh et al. (2014) documented that even during drought in 2012, rye cover crops either did not affect or increased soil VWC.  It should be noted that although VWC was reduced at 0-8 in at corn planting, is it unlikely corn was limited by water availability.  All sites received average or above average spring precipitation. Differences in VWC between treatments could generally be described as wet versus very wet soils at four out of five research sites at the time of corn planting.

Corn Leaf Chlorophyll

Cover crops influenced corn leaf chlorophyll content at both V6 and R1 stage corn.  At V6, chlorophyll content for rye-associated cover crops was significantly lower than the no-cover crop control (Figure 5).  Corn following the triticale-camelina-vetch mixture was the only other treatment which had significantly lower chlorophyll content than the control.  SPAD chlorophyll readings indicate leaf greenness and N content (Costa et al., 2001) so it might be concluded that rye cover crops decreased corn N accumulation.  This is reasonable to assume as rye-associated cover crops had the highest N accumulation and C:N ratios, leading to slower N release, and lowest soil nitrate concentrations in soil (Figure 4, Figure 5).  We observed that corn leaves were visibly less green in some plots which followed a rye cover crop.  This would suggest that soil N, which was originally taken up by the rye, may not yet have been returned to the soil in an available form by stage V6. The N was probably retained in slowly decomposing rye residue on the soil surface, a phenomenon that has been observed in previous research (Quemada and Cabrera, 1995).  

At R1 stage corn, corn leaf chlorophyll content was lower when following rye-canola, rye-camelina, rye-camelina-vetch, and triticale-camelina-vetch cover crops than corn that followed the no-cover crop control (Figure 6).  Corn, which followed these four cover crops, also had lower corn leaf chlorophyll content than corn, which followed the no-cover crop control at V6.  Lower chlorophyll content throughout the growing season in the aforementioned cover crops was probably a result of increased cover crop uptake of soil N, higher C:N ratios of cover crop residues, and a slow release of N from cover crop residues which limited corn N accumulation through R1 stage corn.  These results indicate that reduced corn leaf N content may occur as a result of increased cover crop growth even when N application rates of 150 pounds N acre^-1 are exceeded.  Alternately, rye may have had an allelopathic effect on corn due to benzoxazinones (Schulz et al., 2013).

One possible alternative explanation to the lower leaf greenness readings in this study would be to attribute the nitrogen deficiencies to corn root fungal disease damage, which may have occurred in corn which followed a rye cover crop (Bakker, et al., 2015; Kaspar et al., 2015; Acharya et al., 2017) They determined that corn seedlings which followed a winter rye cover crop sometimes exhibited visible fungal disease symptoms on the primary and seminal roots.  If corn root pathogens more readily infect seedling roots following a rye cover crop, they could potentially decrease the plants’ nitrogen uptake capability regardless of the nitrogen fertilizer rate applied.  In this study we did not investigate fungal disease symptoms on corn roots.

Corn Grain Yield

Cover crops did not influence corn grain yield; mean corn grain yield was 226 bushels acre^-1 across all site years and treatments (Table 5).  Additionally, although the differences were not significant the winter rye and rye-canola cover crop treatments had an average corn yield of 226 and 229 bushels acre^-1, respectively, compared with 223 bushels ha^-1 for the no cover crop control.  This is somewhat surprising as both V6 and R1 chlorophyll readings suggested that corn was consistently N limited following rye-canola, rye-camelina, rye-camelina-vetch, and triticale-camelina-vetch cover crops (Figures 5,6).  The R1 corn SPAD readings have been documented to be highly positively correlated with corn yield (Rorie et al., 2011).  This lack of N limitation expressing itself in reduced corn yield may be due to late season cover crop decomposition and release of N, which was previously tied up in cover crop biomass.  It is possible that cover crop decomposition between R1 and corn harvest released adequate corn available N to reduce the severity of original N limitation at V6 and R1 stage corn.  However, alternate explanations are that the variability of the yield measurements was large enough that small differences in yield could not be detected or that factors other than N affected yield.

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