Investigating Causes of Corn Yield Decreases Following Cereal Rye Winter Cover Crop
In Iowa, lack of experience with cover crops and the predominance of the cornsoybean rotation, late harvests, early planting and long cold winters have limited adoption of cover crops. The Iowa Science Assessment of Nonpoint Source Practices identified cover crops as an effective means of reducing the transport of nitrate and phosphorus from row cropland. Future research needs included cover crop management techniques adapted to Iowa to limit the risk to corn yield reduction.
Although work on other potential cover crops is underway, winter cereal rye currently is the only species known to consistently grow well enough statewide to provide water quality benefits. For farmers, a drawback from considering use of winter rye or other grass cover crops is that occasionally they can cause a corn yield decrease the following year.
One potential cause of lower yields is a decrease in corn population or plant vigor caused by soil-borne fungal pathogens. This may occur because grass cover crops can be hosts for some of the same pathogens that infect corn seedlings. After herbicide treatment, pathogen populations may increase rapidly on the roots of the dying cover crop plants. If corn is planted soon after grass cover crops are terminated, the soil inoculum levels of these pathogens will be very high just as the corn is germinating and emerging. If environmental conditions are favorable for the pathogens to infect corn plants (i.e. cold and wet), these pathogens may reduce plant population, slow and reduce growth and decrease final yield. When planting conditions are warm and dry, or when background inoculum levels are low, the pathogens may have little or no effect.
Controlled-environment lab studies will be conducted under cold, wet conditions that should enhance infections. Field experiments will be planted as early as possible, to increase the likelihood of cold, wet soil conditions. Management practices such as seed-applied fungicides and timing of cereal rye termination before corn planting will be examined to see how those affect soil pathogens, corn growth and yield.
In both field and controlled environment experiments, investigators showed that winter rye cover crops terminated with glyphosate shortly before planting corn increased pathogen levels on dying rye roots and on new corn roots, disease incidence on corn roots was higher, and there were reductions in corn population, seedling growth, and yield in the field experiments. Increasing the interval between rye termination and corn planting decreased the negative effects and helped to explain why this has been a common recommendation when planting corn after a winter rye cover crop. Seed fungicide treatments did not completely eliminate the effects. Future investigations will look at management practices such as different cover crop species, new seed fungicides or combination, and spatial arrangement of winter rye cover crop rows relative to corn rows as a way to reduce the risk of negative effects on corn growth.
Work continued in the laboratory to identify Fusarium and Pythium strains isolated from corn roots from the field experiments. Statistical analysis of data from both growth chamber and field experiments continued. Members of the research team have been working to publicize results to date via two manuscripts, a news release and participation in numerous workshops and meetings where cover crops and this work were discussed or presented. This phase of the project is nearing completion and work is shifting to a new proposal and associated field experiments.
As occurred in 2014, seed treatment did not substantially reduce seedling root infection or improve seedling growth with or without a rye cover crop. Seed fungicide treatments did result in higher early plant populations for both cover crop and non-cover crop treatments. At harvest in 2015, seed treatment did not affect final population, barrenness, grain moisture or yield. Increasing the interval between terminating the rye cover crop and corn planting generally resulted in improvement of most measured parameters. The three treatments terminated at eight days or less before planting had less growth and more barren plants than the treatments terminated 17 and 25 days before corn planting. Root infection incidence and severity tended to be higher when rye was terminated fewer than nine days before corn planting.
The second year of this field experiment examined seeding root infection, plant population, corn growth and yield when a cereal rye cover crop was terminated at 25, 17, eight, or three days before planting, or two days after planting, compared with corn without a rye cover crop. In general, the three treatments that were terminated at eight days or fewer before planting had lower plant populations and growth than the treatments terminated 17 and 25 days before corn planting. It is sometimes observed that soil fungal pathogens of corn seedlings play some role in the poor growth of corn following a cereal rye cover crop. Additionally, the interval between cover crop termination and corn planting seems to affect both disease incidence and reduced growth of corn seedlings, which explains why a 14-day interval often has been recommended.
A field experiment examined corn growth and yield when a cereal rye cover crop was terminated at 25, 14, 10, or 3 days before planting, or one day after planting, compared with corn without a rye cover crop. Corn was harvested in October with an instrumented combine to determine grain yield. Rye terminated 10, 14 or 25 days before corn planting did not significantly reduce yield. However, grain yield of the rye treatments terminated three days before corn planting and one day after planting were significantly less than the no-cover-crop treatment. Another four treatments examined corn seed planted three days after terminating rye with and without seed fungicides. Preliminary conclusions are that current seed treatments are not effective on corn root infections following cereal rye cover crops. Rye cover crop plots were established for field experiments in 2015.
Both controlled environment chamber and field experiments are underway to study cover crop management techniques. Current controlled environment experiments are examining the difference in corn growth and root infection of seedlings following plants killed with glyphosate herbicides and those killed by clipping the plants below the crown at different times. Field experiments continue with measurements of plant growth stages and plant population taken at multiple dates. Whole plant samples were harvested to determine above-ground biomass and ear weight, cob weight, seed weight, kernel number and weight per seed. Stalk rot evaluations also were made at plant maturity.
To study cover crop management techniques, two field experiments were laid out and planted with rye cover crops in October 2013. The first compared corn seed treated with a combination of several seed fungicides with untreated corn seed in plots with and without a rye cover crop. The combined fungicide seed treatment also was tested in a controlled environment chamber experiment conducted over the winter. The second experiment compared treated corn seed planted without a cover crop to corn planted after a cereal rye cover crop killed at planned intervals before corn planting. In both field experiments, corn percent emergence, root rot ratings, root pathogen isolation and shoot and root dry weights were collected at six weeks after corn planting. Preliminary results showed 50 percent of the corn radicles (primary root) sampled were visibly infected even when corn did not follow a rye cover crop and the seed was treated with fungicide.
To study cover crop management techniques, 54 field plots were established in October 2013. A rye cover was successfully planted and overwintered in the plots, although growth was not as much as normal because of the cold winter and late spring. Two field experiments are underway. One compares corn seed treated with a combination of several seed fungicides with untreated corn seed in plots with and without a rye cover crop. The other compares treated corn seed planted without a cover crop to corn planted after a cereal rye cover crop killed at planned intervals before planting. Corn percent emergence, root rot ratings, root pathogen isolation, and shoot and root dry weights will be collected six weeks after corn planting. Two experiments in controlled environment chambers that match the field experiments have been completed. Initial results show cereal rye terminated three days before corn planting reduced emergence, shoot growth and radicle length compared to corn planted without rye. Fungicide treatments improved shoot growth and emergence of corn following terminated cereal rye, but radicle growth was not improved substantially.
Cover crop management techniques are the focus of one project. Fifty-four field plots were established in October and a rye cover planted for experiments and measurements that will take place this spring. In addition, two experiments in controlled environment chambers are underway, and simulate the field experiments.