Cover Crops Influence Nutrient Cycling, Yield and Diseases of Corn and Soybean

Dec 2015


Planting cover crops is an established practice that has been shown to limit nutrient loss and improve nutrient cycling in corn-soybean rotations, yet adoption in Iowa has been limited. What is needed is research into management approaches that will reduce costs and facilitate adoption of cover crops by farmers.


This project will document nitrogen accumulation in corn-soybean systems with and without cover crops; document pathogen infection levels of corn and soybean seedlings following cereal rye or camelina cover crops, plus the effect of infections on plant populations, growth and yield; determine when during germination and seedling development that corn is most susceptible to infection by pathogens; and examine the effect of different seed-applied fungicides and combinations of fungicides on corn and soybean root seedling infections following cereal rye and camelina cover crops.


A series of field and controlled environment studies will be conducted to document if choosing cover crop species to maximize beneficial rotation effects (i.e., camelina before corn, cereal rye before soybean) retains more nitrogen while simultaneously decreasing disease incidence. All phases of each rotation will be present each year. Seed treatment research will be done in controlled environments. To facilitate observing impacts of disease, field experiments will be planted as early as possible, to provide the cold and wet soil conditions that favor soil-borne pathogen infection of corn and soybeans. Management practices such as seed-applied fungicides on corn and soybeans will be examined to see how they affect soil pathogens and crop growth and yield.

Project Updates

Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.

December 2018


Key messages to share with Iowans about this research on nutrient management include:

A winter camelina cover crop before corn, with or without winter rye in a cover crop rotation system, can be a potential option to mitigate the negative effect of winter cereal rye on corn growth and development.

  • Researchers observed less corn seedling disease, improved corn seedling growth and less yield drag on corn following a cover crop of winter camelina.
  • However, corn seedling disease was greater following a cover crop of camelina compared to no cover crop.
  • Pythium populations were lower in corn roots following a cover crop of winter camelina compared to a cover crop of winter rye.
  • Both cover crops established better following soybean compared to corn because of earlier fall planting. Greater stand densities were measured for the winter rye cover crop than the winter camelina cover crop.
  • Biomass of winter camelina was much lower compared to the biomass of winter rye at termination.
  • No treatment effects of winter camelina or winter rye on soybean seedling disease, soybean seedling growth or soybean yield were detected.
  • Treatment effects were detected for Pythium clade B and clade F densities in seedling tissues. In corn, greater populations of clade B were detected compared to clade F, while in soybean, greater populations of clade F were detected and minimal clade B numbers were present.  Moreover, in corn, greater populations of Clade B were detected in seedlings following winter rye compared to seedlings following camelina.  Interestingly, the proportion of Clade F to Clade B was greater in corn following winter camelina compared to corn following winter rye.  

Nitrogen immobilization that occurs after cover crops may affect growth and development of corn.

  • C:N ratios and N uptake of winter rye shoots were much larger than those of winter camelina when each cover crop was terminated shortly before planting either corn or soybean.  Part of this effect was due to the greater growth of winter rye.
  • Corn seedling development following a winter rye cover crop was equivalent to corn seedling development following no cover crop when a nutrient solution was applied to the seedlings following winter rye.
  • Similarly, SPAD readings of chlorophyll content taken on corn seedlings following a winter rye cover crop to which a nutrient solution was applied were greater than readings taken on corn seedlings following a winter rye cover crop that were watered only.
  • These data suggest that nutrients also may be limiting to corn following a rye cover crop in the early part of the growing season, e.g. limited nitrogen availability associated with N removal from soil during rye cover crop growth, slow release of N from decomposing cover crop residues, and possible immobilization of soil N during decomposition of rye residues with high C:N ratios. Thus, corn growth may have been affected by limited nutrient availability in addition to seedling disease.  Furthermore, because these seedling diseases are known to primarily affect roots and root extension, it is likely that nutrient uptake is further compromised by poor root growth and function.  Further research into this area is being done in growth chamber and field trials in 2019.

Potential for improved cover crop management resulting from this study:

  • There is the potential of using a winter hardy brassica, like camelina, as a cover crop after soybean and before corn, and still using cereal rye after corn and before soybean
  • Understanding the seedling disease complex that develops on corn seedlings following a cereal rye may allow development of strategies like fungicide seed treatments, early termination of the cover crop, or precision planting of rye cover crops in inter-rows to mitigate the effect.
  • There is the potential to overcome some of the effect of cereal rye on corn seedlings with nutrient applications early in the growing season that mitigate the reduced nutrient availability and restricted root growth.

March 2018

Growth chamber experiments are underway to evaluate various questions related to the field plot portion of this project. In one experiment, the team compared disease severity and growth parameters of corn seedlings planted next to, and 4 inches away from, terminated rye crowns. Corn seedlings next to rye plants were shorter, had shorter radicles and greater radicle rot severity. Seminal disease severity was statistically greater in seedlings planted next to rye plants compared to seedlings planted further away. The effect of rye termination after corn planting on seedling disease severity and growth of corn seedlings is being analyzed. An experiment to assess the effect of different amounts of winter cereal rye cover crop biomass on seedling disease severity and growth of corn seedlings is in progress. These are all preliminary experiments. Further growth chamber and field research is required to test hypotheses and enhance understanding of the effect of a rye cover crop on corn.

December 2017

Corn and soybean yield data were collected and analyzed. Cover crop showed significant effect on corn yield. Corn yield was lowest whenever winter rye preceded a corn crop. Yield also was reduced following a cover crop of camelina compared to the no-cover-crop control. The effect of cover crop was significant on the number of harvestable corn ears. A greater number of ears were counted in the no-cover-crop control compared to cover crop treatments. There was no effect of cover crop treatment on the number of barren plants and final plant stand. Soybean yield preceding winter rye cover crop was greatest; the yield following a camelina cover crop was not different from the no-cover-crop control. No effect of cover crop was detected on final plant stand. No differences in corn stalk rot index were detected between cover crop treatments. Fall cover crops were sown Sept. 23 and Oct. 10 after soybean and corn harvest, respectively. Stand count data were collected in all plots Nov. 10. Cover crop stand densities were greater after soybean compared to after corn. Moreover, the stand density of winter rye was better than that of the camelina cover crop. Experiments under controlled environments settings to further understand the negative effect of winter rye cover crop on corn seedling growth and root disease are in progress. 

September 2017

Data from field trials were collected from both soybean and corn plots. Soybean plants were sampled for yield and grain carbon and nitrogen analysis. Mature corn plants were sampled for grain weight, number of kernels, kernel size, cob weight, stalk nitrate analysis from basal stalk, and carbon and nitrogen analysis of the stover and grain. Cover crops (winter rye, camelina, and hairy vetch with oats) were planted September 23 on corn and soybean plots for the 2018 field trials. Data from 2017 field trials are being analyzed. A manuscript on fungicide seed treatments to understand the corn seedling disease pathogen following a winter rye cover crop or winter fallow is underway.

June 2017

Corn and soybean seedlings following cover crop treatments (winter rye, camelina, and hairy vetch) were sampled and assessed for seedling growth parameters and root rot severity. No treatment effect was detected for both corn and soybean seedling growth. Root rot on corn seedlings was observed on the radicle and seminal roots. Disease severity was higher following rye than other cover crop treatments, however, it was not more than 10%. No root rot on soybean seedlings was found. DNA from corn and soybean root tissues were extracted and processed to quantify the density of Pythium Clade B and Clade F members. Pythium Clade B groups were detected predominantly in corn tissue, with more Clade B detected following a rye cover crop and camelina rotated with rye. In soybean, more Pythium Clade F members were detected, and a greater density of Pythium was detected following camelina and hairy vetch. 

March 2017

This quarter a growth chamber experiment was conducted to evaluate the risk of seedling disease of corn with proximity of the corn seedling to terminated winter rye cover crop plants. Results showed radicle and seminal root rot severity was greater the closer the seedling was to the rye plants. Also, more Pythium Clade B was detected in corn grown within the terminated rye compared to corn planted further away. No effect of distance between corn and rye was detected for Pythium clade F. This study will inform management practices for farmers to reduce occasional yield loss of corn following a winter rye cover crop. A calendar of projected dates for cover crop termination, planting and plant sampling and assessments was developed.

December 2016

Cover crops (winter rye, camelina, hairy vetch) stand count data collected in fall 2015 and 2016 were analyzed. In both years, fall cover crop establishment was better after soybean compared to after corn. Cover crop stand establishment after both corn and soybean was better in fall 2016 than in 2015, except for camelina after corn. Stand counts of cover crops planted into soybean residue were greater than cover crops planted into corn residue. The stand count of vetch was recorded only in fall 2016, with establishment better after soybean than corn. The percent dry weight of total carbon and nitrogen in cover crop shoot tissue collected in spring 2016 was measured and analyzed. Rye contained lower nitrogen (1%) compared to camelina (2.5%) and hairy vetch (2.7%). However, carbon percentage was greater in rye (42.5%) than in vetch (41.6%) and camelina (40.3%). Corn yield data were analyzed. Lower yield was observed in corn following rye compared to corn following other cover crops and no cover crop treatments. No differences in yield were observed in corn following camelina, hairy vetch, and no cover crop treatments. Soybean yield data are being analyzed. 

September 2016

Agronomic and disease data obtained from 2016 field trials are being analyzed. Preliminary analysis suggests: Early corn plant height was shorter and crop developmental stage delayed following rye compared to the no cover crop check or the other cover crop treatments; corn population and number of ears were not affected by cover crop species; and soybean population did not differ among cover crop species and the no cover crop check. Cover crop treatments (winter rye, camelina, and hairy vetch) were planted in preparation for 2017 field trials, and establishment on both corn and soybean plots is good. Growth chamber studies were initiated to evaluate changes in the Pythiumpopulation as rye roots decay.

June 2016

Winter rye dry shoot biomass following both corn and soybean was greater than other cover crops. No cover crop treatment effects were detected for corn stand count at both assessment dates, but soybean stand count was affected. Soybean stand count was greater following rye than camelina or hairy vetch, and was similar to the no-cover-crop control treatment. An effect of cover crop was detected on corn seedling growth and root rot incidence. Corn shoot dry weight was lower and seedling root rot incidence was greater following rye cover crop, compared to both camelina and hairy vetch. Bigger plants with less severe or no root rot or mesocotyl rot were observed in the no-cover-crop control treatment. Soybean seedling shoot height and shoot weight were not affected by the cover crop treatment. 

March 2016

Stand density of cover crops (camelina and rye) following the 2015 corn and soybean crops did not differ by rotation. Camelina stands thinned over the winter. Rye cover crop stands survived winter well, and had good biomass accumulation towards the end of March. Plots were marked in preparation for the 2016 field trial experiments.

December 2015

This project involves a series of field and controlled environment studies to document if choosing cover crop species to maximize beneficial rotation effects (i.e., camelina before corn, cereal rye before soybean) retains more nitrogen while simultaneously decreasing disease incidence. Camelina and rye treatments were planted Sept. 16, 2015. Fall conditions were favorable for establishment of both cover crop treatments. Due to a late frost and unusually warm fall, the camelina treatments bolted. It is unclear what this may mean in terms of winter survival and growth in the spring.