Testing Methods For Nitrates In Forages

When making harvesting or feeding decisions for forages that have potentially accumulated nitrates, our human tendency is to want immediate answers. A representative sample sent for laboratory analysis is by far the best test of nitrate toxicity. However, while shipping and laboratory turnaround times are really very good, it will take several days to get results. Two types of quick tests exist, and it is important to understand the limitations of these tests when considering their use.

Quick Tests

Diphenylamine Quick Test– This solution of concentrated sulfuric acid is available from some county extension offices. Appropriate precautions must be taken in handling and storing the solution. This solution is most useful for evaluating standing green forage. The green stem can be split open and when drops of the solution are placed on the stem, a color change occurs when nitrate is present. Because nitrates accumulate in the base of the plant, testing from the base upwards until no color change occurs can be helpful in determining a harvest plan. An Oklahoma State University study comparing the diphenylamine test to a standard laboratory test indicated a large percentage of false positives (46%), this dropped to 24% if two stems were positive. The rate of false negative readings was 5.1%. In this study, the relative color change after 10 seconds had a low relationship (r=0.38) to the laboratory nitrate values. A Montana State University field study found 71% of samples correctly categorized with 23% false positives and 6% false negatives.

Test strips – Nitrate test strips have primarily been used to test nitrates in water or soil or to assess the need for additional fertilization in growing plants. Various types of test strips or kits are available. Some require you to press the test strip to the moist surface of a cut plant, or squeeze sap from the plant with pliers, a garlic press or hydraulic press, and others suggest a dried and ground sample. Testing may require additional solutions and several steps to get to the final step of a color change (red-violet dye). Reading the color change with a reflectometer or other tool provides more consistent results than the human eye. Smart phones have been tested as a possible way to read the color change but there are some consistency issues across phones and platforms.  Researchers at Montana State (Meccage et al.) compared the test strips to laboratory values and found 71% correctly categorized, 13% false positive and 16% false negative. If using this type of test be sure to understand any expiration date on the materials and appropriate storage conditions for unused portions.

Sample collection for nitrate testing

Nitrate concentrations within a given field can be highly variable. In one Kansas study, nitrate concentration of 23 large round bales of sudan from a single field averaged 2764 ppm, but ranged from 1525 to 6250 ppm on an as-fed basis. This is similar to other reports of “hotspots” within fields and wide variability.

When collecting samples for nitrate determination more samples are needed to represent the potential variation. In some cases, segregating and testing bales from different locations in the field may be warranted. The rule of thumb is to sample 10-20% of bales of a given forage lot for nutrient analysis. A forage lot should represent the same field, cutting, maturity and harvest condition and usually is less than 100 tons. When nitrate toxicity is a concern, sampling 20% of bales would be a minimum and some have suggested to sample up to 40%.  Knowledge of actual field conditions should be used in planning for sampling.

When collecting samples from a large round bale use a bale corer, start from the wrapped side (with net wrap or twine) of the bale and core toward the center.  Approximately 75% of the hay is in the outside 18” of the bale and sampling in this direction will maximize the number of layers within the bale that are sampled to get a representative sample.

If sampling standing forage, the sample collected should represent the grazed or harvested portion. No need to sample the base of a corn stalk unless cattle will be forced to eat that far down on the stalk. Sampling standing forage is more awkward. One approach is to sample every 50 to 100 feet in diagonals across the field. Chop up samples into smaller pieces, mix well in a large bucket or tub. Use a quartering method to create a subsample for analysis.