Post-emergent herbicide timing key in corn production | 6/7/2018 | Staff
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Weed control in corn is important to profitability, but producers need to be aware of herbicide application timing, said Dr. Jourdan Bell, Texas A&M AgriLife Extension Service agronomist in Amarillo.

"We know post-emergent herbicide applications are necessary for season-long weed control," Bell said. "However, it is important to follow post-emergent corn herbicide labels, which define the latest growth stage for which herbicide applications can be made without causing crop injury."

Herbicide - Applications - Growth - Stages - Crop

She said herbicide applications past the recommended growth stages can result in significant crop injury, so it is important producers understand and recognize each of the stages.

"Some herbicide labels also provide a recommended height for the last herbicide application as plant height often corresponds with a particular vegetative stage."

Bell - Environments - Irrigation - And/or - Precipitation

However, Bell said, in high-input environments with abundant irrigation and/or precipitation and fertility, internode distances can expand at a greater rate than new leaves.

"Consequently, plant height does not always correspond to the correct vegetative stage," she said. "So, it is important to accurately stage the corn crop before making post-emergent herbicide applications."

Bell - Production - Environments - Water - Temperatures

Bell also warned stressful production environments with limited water or cool temperatures can slow corn growth, which can result in magnified crop injury because the plant does not metabolize the herbicide quickly enough to avoid injury.

For labels that provide both plant height and growth stage, the applicator should follow the more conservative recommendation, she said.

Stages - Leaf - Collar - Method - Leaves

The vegetative stages are described using the leaf collar method. Leaves are counted from the lowermost first rounded-tip leaf to the uppermost leaf with a leaf collar, which is the connection between the leaf blade and the leaf sheath, she explained.

"The key features to identify are the leaf collar and the first round tipped leaf," Bell said. "While the individual stages are important, it is key to be able to identify the leaf collar and first true leaf to...
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