Annual forage crops tend to accumulate greater amounts of nitrates than perennial forages. These crops are usually planted into well fertilized, manured or recently plowed grassland or pasture. Nitrate concentrations vary in different parts of the plant. The highest levels are found in the lower stems. Concentrations in the leaves and flowers are lower and the seeds or grain kernels are usually nitrate free.
Specific species can become nitrate accumulators if appropriate conditions exist.
The common problem crops and weed species are presented in the following table. Crops hurt by frost or hail have a reduced photosynthetic capacity. After frost or hail, the roots are usually unaffected and are able to supply the same amount of nitrate to the upper plant as prior to the injury. The upper plant is not able to use the nitrate as efficiently and it accumulates in the stem and leaves. Nitrate levels remain high until new tissue growth is able to utilize the nitrate that is present.