Leaving the ground bare greatly increases the risk of soil erosion, as well as fallow syndrome the following year.
Producers may leave the ground bare if challenging springs cause planting delays that lead them to take the prevented planting option for insurance purposes, or when extensive flooding and/or severe hail significantly damages standing crops.
Fallow syndrome can severely limit crop growth in soils where no crop or weed growth occurred the previous year.
How and when it occurs
When there’s no plant growth in an area for an extended period of time, populations of “good fungi,” called active arbuscular mycorrhizae (AM), are dramatically reduced because AM fungi need actively growing roots to survive. AM fungi assist in the uptake of phosphorus, zinc and other nutrients with limited mobility in the soil.
Fallow syndrome has also occurred in Minnesota when corn is grown following sugarbeet, because sugarbeet is not a host crop for AM.
Impact on crops
Fallow syndrome tends to impact corn and small grains more, with corn showing the greatest potential impact on grain yield from fallow syndrome. The potential risk for other crops, such as soybean, is low and special management isn’t required.
Areas at risk for fallow syndrome include:
Prevented plant acres, defined as a failure to plant an insured crop with the proper equipment by the final planting date.
Acres where severe hail or extensive flooding wiped out crops.
Drowned-out spots in fields.
Planting some kind of an annual crop in these areas can help maintain levels of AM fungi in the soil, reducing the risk of fallow syndrome the following year.
From a biological perspective, weeds could serve as a cover crop to help prevent fallow syndrome. However, the resulting seed production and contributions to the weed seedbank would increase future weed management issues.
Benefits of a cover crop
Planting a cover crop on a field’s prevented plant acres, hailed out areas or drowned-out spots can provide multiple benefits beyond addressing the issue of fallow syndrome.
For example, if nitrogen was applied but corn was not able to be planted, a cover crop that helps scavenge nitrogen might be a good choice.
Nitrogen that was already applied will transform to nitrate in the course of a growing season. Once in nitrate form, nitrogen can be lost by leaching or denitrification if there’s a wet fall or wet spring.
A cover crop can keep nitrogen in the field by taking up currently available nitrogen and converting it to organic nitrogen (plant material). This organic nitrogen would then need to be mineralized back to ammonium and converted to nitrate before there’s a concern for nitrogen loss.
By cycling nitrogen this way, it’s possible to protect the nitrogen investment currently in the field and reduce the chance of losing nitrogen to the environment.
In addition to protecting the nitrogen investment, cover crops may help maintain other nutrients and soil organic matter by reducing soil erosion risks.
Cover crops can vary in their effectiveness at fighting weeds, alleviating compaction and providing forage value. Note that brassicas, such as forage radish, are not hosts for AM.
Herbicide considerations with cover crops
There’s also the question of what cover crops you can plant if an herbicide was applied earlier in the growing season. Information is very limited – if not non-existent – regarding plant-back or rotation restrictions for various cover crops.
Be sure to check the labels of all herbicides applied where you wish to plant a cover crop. Crop injury ranging from minor to severe stand reductions could result. If you’re planting a cover crop to protect from soil erosion or prevent fallow syndrome, the grower assumes all risk if the cover crop is not on the herbicide label.
However, you must follow rotational restrictions if you plan to graze, feed or harvest a cover crop to ensure the safety of the food and feed chain.
Managing fallow syndrome
If it’s not feasible to plant any crop this growing season and corn or small grains will be planted next year, a banded application of phosphorus at planting next year may help alleviate fallow syndrome’s effects.
For corn, the best way to mitigate potential impacts is to apply banded phosphorus and chelated zinc directly on the seed as starter fertilizer. While research hasn't identified the exact rate to use for all circumstances, a normal application of 5 gallons per acre of 10-34-0 may be enough.
Under fallow syndrome, banding is important and recommended – even if soil test values for phosphorus are high. Broadcasting extra phosphorus and zinc hasn’t been shown to effectively treat the problem.
Reviewed in 2018