SDSU Study: Plant Nitrates Increase with Fertilizer N

Brookings, South Dakota (July 28, 2006) - South Dakota State University data from the drought of 2002 shows a clear increase in nitrates in the lower part of the plant as the fertilizer nitrogen rate in corn increases.

SDSU Extension Soil Fertility Specialist Jim Gerwing said that's important information for producers to keep in mind this year if they're considering using drought-stressed corn for forage.

It's crucial that producers sample and test for nitrates before considering using drought-damaged corn as forage. Since soils and corn growth vary greatly across fields, samples from more severely stressed areas should be collected and analyzed separately from less stressed areas of the field, Gerwing added.

Gerwing said 2002 data from the Southeast Research Farm near Beresford shows that even the nitrogen rate recommended for that site, 90 pounds an acre, resulted in elevated nitrate levels in the lower one-third of the plant.

"Nitrates were higher with increased nitrogen fertilizer rates," Gerwing said.

The middle third of the plant didn't have high nitrates until a very high N rate (400 pounds) was applied, Gerwing added. The top third of the plant did not have elevated nitrates, regardless of fertilizer nitrogen rate.

The SDSU test evaluated nitrate levels with no added nitrogen fertilizer; at 90 pounds added nitrogen fertilizer; at 200 pounds; and at 400 pounds. The previous crop was soybeans and the nitrate soil test was 56 pounds/acre.

"Corn in that location in 2002 was drought-stressed by extremely hot and dry conditions in June and July, but had tasseled and was trying to put out an ear - I called it silking to early blister when we sampled," Gerwing said. "Plants were about 5 to 7 foot tall to the top of the tassel."

The main lesson from the 2002 data, Gerwing said, is that producers should test drought-stressed corn for nitrates before considering using that corn for forage, and keep in mind that your levels may be higher if you applied high rates of nitrogen fertilizer. Ask your county Extension livestock or agronomy educator for more information.

Contact:

JIm Gerwing
(605) 688-4772