Nitrate accumulation in drought-stressed corn

Recent rainfall events may further increase concerns of nitrate poisoning in drought-stressed corn.

Farmers attempting to salvage the drought-stressed corn crop by chopping and feeding or ensiling need to be aware that feeding this material to cattle carries the concern of nitrate poisoning.

Nitrate accumulation within the plant

During a typical growing season, nitrate nitrogen taken up by plants is quickly converted into amino acids and, later, proteins. This process of reducing nitrates occurs in green leaf tissue and often happens fast enough that the level of nitrate is not high enough to be toxic. During conditions of prolonged moisture stress or drought, high temperatures and low humidity, or large amounts of nitrate or organic matter in the soil, nitrate will accumulate in plants.

The accumulation of nitrate will be accentuated if heavy nitrogen fertilization occurred prior to experiencing drought conditions, a scenario that may resonate with many farmers this year as dry conditions began across a large portion of Michigan just as corn was being sidedressed. However, the most critical time period for nitrate accumulation in drought-stressed corn may be the first week or two following the return of significant rainfall.

The first significant rainfall to moisten the upper soil profile will stimulate microbial activity, causing a flush of nitrate nitrogen to be released into the soil system. As corn plants then begin to recover from drought conditions, the plants take up nitrate at rates greater than they are fully able to assimilate, resulting in the accumulation of nitrate within the plant. This process of intense nitrate uptake and accumulation may require up to two weeks following significant rainfall to fully assimilate nitrate in the plant. In order to minimize the risk of nitrate poisoning, a general rule of thumb is to wait at least two weeks before harvesting corn.

Feeding high nitrate, green-chopped corn or silage is a threat to livestock. Ensiling chopped corn will reduce the accumulated nitrates by one-third to one-half, but this process also takes time to complete and is often difficult to predict. Farmers considering any of these options should have their plants tested to determine the degree of nitrate accumulation.

Table 1. Guidelines for feeding forages to livestock based on nitrate nitrogen and nitrate concentrations

Nitrate nitrogen NO3-N

NO3

Feeding recommendation

———ppm———-

< 1,000

< 4,400

Not toxic

1,000-2,000

4,400-8,800

Limit feed to less than 50 percent of the ration dry matter

2,000-4,000

8,800-17,600

Limit feed to less than 25 percent of the ration dry matter

Do not feed to pregnant cattle

> 4,000

> 17,600

Toxic, do not feed

To convert from percent dry matter basis to ppm, multiply the percentage by 10,000. For example, 0.5 percent dry matter multiplied by 10,000 would equal 5,000 ppm.

Green-chopping or silage may not be the only option

If drought-stressed corn fields were not pollinated and will not be producing any grain, farmers may wish to consider tilling the entire field in preparation for winter wheat planting. If this is the case, first be sure there are no residual herbicide carryover issues to consider. If sufficient pre-plant or sidedress nitrogen were applied to the corn crop, no autumn nitrogen applications should be required for the wheat crop. Farmers should then continue with their typical spring soil fertility programs for wheat.

Sampling and testing for nitrates

The Michigan State University Soil and Plant Nutrient Lab can help determine the nitrate content of your plant material. Be sure to obtain a representative sample from the entire field as field variability in soil fertility, soil moisture, degree of drought stress and stage of plant growth will all influence nitrate levels.

Testing for nitrate content should preferably be done prior to cutting or chopping. Collect 20 plant samples from the field area of interest. The highest concentrations of nitrate will occur in the stalk portion of the plant, specifically the lower one-third. Growers may wish to leave this portion of the plant in the field. Collecting plants from harvest height will also allow a more accurate reading of nitrate.

Chop and mix the plant material and remove about a gallon of material to be dried. An alternative method is to take grab samples from previously chopped material. Several grab samples should still be taken and mixed to form a composite sample. Again, mix samples together and remove about a one-gallon sample for drying. Dry samples before sending them in a paper bag to the laboratory. Sealed plastic bags may increase the risk of sample rotting. Moist samples should be delivered directly to the lab.

After drying, send 1 to 2 quarts of plant material to the lab for analysis. Attempt to reduce the amount of time between sampling and arrival at the laboratory as nitrate can be lost from the wet plant material at room temperature or greater. Samples can be sent directly to:

Soil and Plant Nutrient Laboratory
Department of Plant, Soil and Microbial Sciences
Plant and Soil Science Building
1066 Bogue St., Room A81
East Lansing, MI 48824-1325.

Provide name, address, phone number and county of residence when sending in samples. Fee for analysis is $12 per sample. Enclose a check payable to Michigan State University for the appropriate amount.

Additional information:

Dr. Steinke’s work is funded in part by MSU‘s AgBioResearch.