Source: Nitrates in Livestock Feeding, R. Rasby et a. 2007 (pdf)
Drought and Nitrate Toxicity
Drought conditions can cause high nitrate concentrations in plants. However, some moisture must be present in the soil with nitrate for absorption and accumulation. If the major supply of nitrates for the plant is in the dry surface soil, very little nitrate will be absorbed by plant roots.
In plants that survive through drought, nitrates often are high for several days following the first rain.
The amount of nitrate found in plants varies by plant species, stage of growth, plant parts, and timing of harvest.
Corn grown in drought conditions can potentially contain nitrates. The majority of the nitrates will be in the lower 8 inches of the stalk. Raising the chopper height to 6 to 8 inches will reduce the amount of nitrates in the silage. Ensiling drought damaged corn can reduce nitrates in the silage 40 percent to 60 percent. Before feeding drought damaged corn silage, allow it to go through at least a 21-day fermentation period before feeding.
If drought-stressed forage crops are part of your feeding strategy, learn about the causes and symptoms of nitrate toxicity, how you can manage nitrates in your feed source, and get a laboratory test of your feed.
Testing Feeds for Nitrates
The cost of testing for nitrates is inexpensive so get representative samples of questionable feeds analyzed by a laboratory before feeding. When sampling suspected silages or green chop for nitrates, take representative grab-samples from at least six areas of the feeding face of the pit or mound. Mix the grab-samples and sub-sample an amount to fill a plastic bag that can be sealed at the top. Compress the air out of the bag and seal. The sample is now ready to send to the laboratory for analysis. Send samples early in the week to avoid weekend delays in delivery of the samples to the laboratory.
For suspected forages being put into an upright silo, take grab-samples for three successive days, then sub-sample and transfer to a plastic bag as mentioned above. Samples should be frozen between days or whenever kept in storage.
Long stem hay should be sampled using a hay probe. Sample bales or stacks that represent the suspected hay. For baled hay, probe about 20 different bales and for hay stacks, sample each stack in six different areas to obtain a representative sample. Transfer the sample to a plastic bag, compress the air out and seal before sending the sample to the laboratory.
In some situations it may be possible to identify which bales or forages are most likely to contain the highest nitrate levels due to knowledge of the factors described earlier that affect nitrate concentrations. In these situations, it sometimes is wise to collect samples specifically from the suspect forage to determine the highest concentration of nitrate that livestock may be exposed to.
It is difficult to obtain a representative sample from pastures suspected of nitrates that cattle are grazing. Cattle are selective in the plants and plant parts they consume, and a clipped sample will not represent what is actually being consumed. It is recommended you not test grazed forages for nitrate, but manage the grazing of such forages to reduce the problems due to nitrates, as mentioned above.
Most commercial feed laboratories will analyze feeds for nitrates. Contact your local extension educator to obtain information regarding laboratories in your area that test feeds for nitrates.