Nitrate-N distribution within drought-stressed corn plants

During a prolonged drought, the bottom one-third of the stalk accumulates relatively large quantities of nitrate-N compared to other plant parts.

Corn continues to struggle in mid-Michigan counties with the prolonged drought and high temperatures. Some fields that recently received rain look tall and somewhat recovered, but close inspection shows very small ears and barren plants. Severe drought interferes with normal pollination and kernel development contributing to corn ears.

Under normal growing conditions, nitrate-N is reduced and incorporated into amino acids that are used to make proteins. The major site of nitrate reduction is the green leaf. These amino acids are then distributed to developing kernels. However, during an extended drought, when kernel development and protein synthesis are slowed down, the soluble nitrate-N begins to accumulate in the plant tissue. It is interesting to note the variability in the nitrate-N within the plant (Table 1) and the disproportionate quantity in the bottom third of the stalk compared to other plant parts.

Table 1. Nitrate-N levels in drought-stressed corn
(Source: Walsh and Schulte, 1970, University of Wisconsin)

Corn plant part

PPM nitrate-N

Leaves

64

Ears

17

Upper 1/3 Stalk

153

Middle 1/3 Stalk

803

Lower 1/3 Stalk

5,524

Whole plant

978

This shows the remarkable ability of the plant to store and utilize excess nitrate-N should normal growing conditions resume later. Unfortunately, the rainfall was too late on many corn fields near Charlotte, Mich. Using such corn for feeding cattle also carries a potential risk of lethal nitrate levels. A cornstalk sample recently submitted to the MSU Soil and Plant Nutrient Lab showed a nitrate-N concentration of 7,550 ppm, which is in the toxic range. The nitrate-N levels generally surge after a drought-ending rain as the rain water will move the soil nitrates closer to the root.

Corn farmers who are considering harvesting corn for silage versus harvesting for grain should refer to the following MSU Extension articles for useful information and guidelines, Nitrate accumulation in drought-stressed corn plants and Harvesting drought-damaged corn for silage. Also, prior to harvesting for silage, please check with the USDA Farm Service Agency and crop Insurance personnel about compliance with farm program provisions.

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