Nitrates can accumulate in a variety of forages and weeds.
The species most likely to accumulate high levels include corn, sorghum, sudangrass, sorghum x sudangrass hybrids, johnsongrass, pigweed, and cereal grains (e.g. oats, wheat, etc.).
Nitrates accumulate when plants are pulling nitrates out of the soil but are not growing enough to convert these nitrates to amino acids and protein molecules. However, when the plant resumes normal growth, the nitrates will be converted to protein.
Hay from any of the species listed above that was baled during drought conditions should be tested before feeding to ensure it is safe.