The hot dry conditions throughout our region have raised concerns about the potential for nitrate toxicity in corn chopped for silage. Nitrates have the potential to accumulate in a corn plant under any stressful conditions hinder plant growth. There are many guides, articles, and fact sheets available that discuss the interpretation of the lab data and sampling procedures for corn that has already been chopped, but there is little guidance for sampling the corn prior to harvest.
The most important step in collecting a sample from a standing corn field is that the sample must be representative of the portion of the plant that will be harvested. That means cutting it at the same height as the chopper. Nitrates accumulate primarily in the lower stalk section, so a few inch difference can have a significant impact on your results. Second, the plants that are collected need to be representative of the condition of the field. For example, if a quarter of the field is performing poorly as compared to the rest of the field, a quarter of the plants collected for the sample need to be from that section, three quarters from the good area of the field. A sample should consist of a minimum of 15 plants to best represent the average of the whole area being sampled. The sample also needs to be collected as close to harvest as possible, because nitrate levels can change quickly due to changes in the weather.
Prior to sending the sample to the lab, the plants need to be chopped and thoroughly mixed. This is best accomplished with a lawn chipper shredder. Once all the plants are chopped and mixed, collect a 1-gallon zip top bag subsample to be shipped to the lab for analysis.
Please note that a Corn Stalk Nitrate Test (CSNT) and a feed nitrate test are very different in the sample collection and will give you very different results. A CSNT involves collecting only an 8-inch section of the lower stalk around black layer. This test is used to evaluate the effectiveness of a nitrogen program and does not necessarily represent a potential for nitrate toxicity.
For more information please see our A&L Great Lakes fact sheet, Nitrate Toxicity in Feed.
Another excellent resource is from the University of Wisconsin Extension, Nitrate Poisoning in Cattle, Sheep, and Goats.
The Ohio State University Extension has also the topic of corn silage harvest in their most recent issue of the C.O.R.N. newsletter.
For any additional questions regarding feed nitrate testing and sampling, feel free to contact your A&L Great Lakes Laboratories agronomist or call the laboratory directly as 260-483-4759.