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V5 cornfield in northern Pennsylvania Photos by Melissa Bravo
EVALUATING CORN: One way to search out problems in your fields, such as this V5 cornfield in northern Pennsylvania, is to simply walk through the field, take a notebook and write down what you see.

Herbicide injury or fertility issues? What to look out for

It’s a good idea to take a notebook out to the field and write down what you see.

A few weeks ago, I was taking plant tissue analysis in no-till corn strips on clay-loam soils for pre-sidedress nitrogen testing. The corn was V5 to V6 stage.

This corn has seen an abundance of hot weather and very little moisture since planting. My eyes immediately noticed red clovers not terminated by the spray program; an odd reddish tint on the leaf margins. I filed that bit of knowledge away and kept on going.

Then I noticed clover plants with irregular yellow markings. By the time I was done, I had crisscrossed several strips of corn, established alfalfa-clover stands and a new seeding, and I noted several of these yellow clovers along with an occasional alfalfa plant showing white and bronze speckles. After a quick phone call to get an update on herbicides and potash applications, I went back and took a closer look.

Many of you likely do the same thing. Something in the field catches your eye as not normal and you send a query out to your favorite social media group. I often find myself audibly and emphatically disagreeing with the replies because I need more information.

Close up of irregular yellow markings on clover IRREGULAR CLOVER: Are these yellow markings on this clover a sign of herbicide uptake injury or is it a fertility issue?

So how can we differentiate herbicide injury from soil nutrient deficiency in multicrop farm field strips?

Herbicide or nutrient issues?

While there are dozens of brand names and premixes for use in corn and soybeans, the symptoms of herbicide uptake injury to susceptible grasses and broadleaves can be grouped within the nine mode-of-action categories and further differentiated symptomatically according to the known characteristics of the chemical family and associated active ingredients after uptake by a particular crop.

Knowing the precipitation, soil classification, soil temperature and crop growth stage are also critical.

Recognizing the symptoms of herbicide uptake vs. fertilizer deficiency takes time but can be done. The easiest way is to keep a notebook and write down what you see, press a representative sample into the page, and look up the active ingredient of the herbicide sprayed and what is most characteristic about injury to susceptible plants. Then, compare that to fertilizer deficiency symptoms such as phosphorus (purpling), potassium (white speckles), nitrogen (pale green-yellow) and essential micronutrients (leaf margin discolorations, bronze speckling, chlorotic spots, cupping).

Keep in mind that excessive insect damage from leaf and root feeders, not to mention fungal pathogens and rusts, will add to the difficulty.

A table outlining the differences between herbicide injury and fertility issues

I learned how to look for signs of soil fertility symptoms by attending crop clinics, participating in collegiate contests hosted by regional weed science societies, and working as a certified crop adviser scouting row crops, hay, forestry and right-of-way vegetation for 20 years.

In the middle of this pandemic, you may have to become your own teacher until the ag universities open for public events. Or work with independent agronomists, who by the nature of their sole proprietorship are still out and about and covering a lot of ground.

If the herbicide injury is severe consider the lingering residual effect for fall planting decisions.

If a soil nutrient deficiency is suspected, take root depth soil samples from different fields after harvest and send in for macro- and micro-analyses.

Bravo is a certified crop adviser for Meadow Lake Crop Consulting Services in Lawrenceville, Pa.

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