Fallow Field Prep

ROCKVILLE, Md. (DTN) -- Fields that were flooded or lay fallow in 2019 may require extra management for the 2020 spring planting season.

The two top concerns will be weed control and nutrient availability issues, particularly a phenomenon called "fallow syndrome" in cornfields. Brace for high weed seedbanks and the potential need for extra nutrients, scientists told DTN.

Soil compaction may also be a problem if growers moved equipment through prevented planting fields during wet periods. See more on managing soil compaction this spring from DTN here: https://www.dtnpf.com/….

WEEDS GONE WILD

Last year, a record 20 million acres went unplanted -- but they didn't stay bare. Weeds, nature's greatest opportunists, were quick to fill empty fields.

There's a definite hierarchy of potential weed problems based on your 2019 field management, noted Purdue University Extension weed scientist Bill Johnson.

"Management was all over the board," he recalled. "It ranged from doing absolutely nothing, to tilling them a couple times, some spray applications and some cover crops. But my guess is the majority did not have a lot done to them."

Those who were unable to do any kind of weed control will face massive weed seedbanks from aggressive summer annual weeds like Palmer amaranth and waterhemp going to seed, Johnson warned. Female plants of these species can produce hundreds of thousands of seeds each, especially when they face little competition from crops.

Even timely herbicide applications may not be sufficient, given the volume of weed seed some growers could be working with, Johnson added. "Remember that efficacy is a numbers game," he said. "Even 98% control can look like zero if you have a huge seedbank out there."

Pre-emergence applications with residual herbicides included will be more important than ever for all growers, including those using new herbicide-tolerant systems, Johnson warned.

"I'm hearing that because of low commodity prices and now this complete pandemic chaos, some growers want to take pre-emergence passes out and go back to postemergence only," he said. "I see us being driven down that path once again, and I'm worried we're going to wear this new herbicide-tolerant technology out in five years if we get back on that path."

Even growers who took the time to till or spray fallow fields early in the summer to control weeds could face higher populations of certain species, such as grassy weeds like crabgrass, yellow foxtail, or barnyardgrass, which come up later in the summer.

When spraying grassy weeds such as barnyardgrass or junglerice, some weed scientists are seeing herbicide antagonism between auxin herbicides such as dicamba and 2,4-D, and grass herbicides such as glyphosate and clethodim. See the DTN story here: https://www.dtnpf.com/….

Marestail might also be a greater threat for growers who only managed a single early season herbicide application on fallow ground, since this aggressive weed species often drops seeds in August and September, Johnson added.

Fall herbicide applications can control any marestail that germinates immediately in the fall, but up to 50% of those seeds typically germinate in the spring. Growers who missed fall applications will deal with even heavier marestail populations.

"Over the winter, any marestail that has germinated goes into dormancy," Johnson explained. "About 80% dies, but the ones that make it through the winter are very hardy, start growing early and are tougher to kill."

Even growers who were able to plant cover crops on prevented planting fields in the summer or fall should be prepared to monitor weed pressure. While a cover crop such as cereal rye can provide enough biomass to suppress some winter annual weeds, some marestail may still make it through -- and overwintering biotypes can be more difficult to kill. In those cases, adding a herbicide like 2,4-D, saflufenacil (Sharpen) or dicamba to your glyphosate cover crop termination pass would be wise, Johnson said.

For more on terminating cover crops effectively this spring, see this DTN story: https://www.dtnpf.com/….

WATCH NUTRIENT AVAILABILITY IN CORNFIELDS

Growers with flooded and fallow fields in 2019 may face a phenomenon known as fallow syndrome, added Dan Kaiser, University of Minnesota soil scientist.

The problem is most concerning for growers planting corn in 2020. Corn roots depend on soil microorganisms called mycorrhizae to help the plant uptake nutrients early in the season.

Without a plant host actively growing on a field, those mycorrhizae populations can decline and be initially unavailable the next spring. That leads to symptoms of nutrient deficiencies in young corn crops, particularly phosphorus and zinc, Kaiser said. The syndrome can cause purpling, indicative of phosphorus deficiency, as well as stunted corn plants.

Not all prevented planting fields may be at risk since many plants, including weeds, can act as a host for these mycorrhizae, Kaiser said. "If you have anything growing on those fields, colonization of roots can occur normally," he said.

Cover crops can also be a host for these mycorrhizae, but not all species. In particular, brassica species such as tillage radish are not a host and can lead to fallow syndrome if they are the only cover crop on a fallow field.

For growers at risk for fallow syndrome, Kaiser said the best options are to apply in-furrow fertilizer that includes as much phosphate as you can safely apply, such as 5 gallons per acre of 10-34-0.

"You can need up to 30 to 40 units of phosphorus to prevent this syndrome, but it's hard to do that safely with in-furrow placement," he noted. Adding a quart of 9% chelated zinc would also be helpful, he said.

There are inoculant products on the market that deliver mycorrhizae directly into the root zone to replace a depleted field, but there is not much independent data on the effectiveness of these tools, Kaiser noted.

See more on managing fallow syndrome here: https://blog-crop-news.extension.umn.edu/…

Growers might also want to contact local soil fertility experts for information on whether or not they can expect a nitrogen credit in the soils of their fallow fields, Kaiser added.

For example, in Minnesota, growers can often expect a nitrogen credit up to 75 pounds, due to nitrogen mineralizing in the soil over the summer, with no growing crop to take it up, Kaiser said.

"Some soil types have a tendency to do this, so I would check out the local guidance on how to properly credit fallow situations," he said.

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow her on Twitter @Emily_Unglesbee

(PS/AG)

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