During droughty conditions, certain forage species that we feed livestock may become unsafe for animal consumption. Two problems that producers might face during these dry weather patterns are nitrate accumulation and prussic acid formation.
Nitrate Accumulation - Regardless of the form of nitrogen applied via fertlizer (ammonium or urea) the form of nitrogen primarily taken up by plants is nitrate. Soil microorganisms convert ammonium nitrogen to nitrate nitrogen. Nitrates are extremely soluble in water and are easy absorbed by plant roots along with moisture in the soil.
Three conidtions that cause nitrate accumulation:
1. During times of high temperatures and adequate soil moisture, plants can undergo a process called photorespiration during which carbon dioxide is released into the atmosphere rather than being stored in the plants as a carbohydrate.
2. When the soil contains nitrate nitrogen but little soil moisture, nitrates beome highly concentrated in the water that plants take up.
3. Herbicide injury can limit the conversion and assimilation of nitrates in plants.
Some common forages that can accumulate toxic levels of nitrates include: barley, corn, flax, millet, sudangrass, and johnsongrass.
Prussic acid in forages can also pose a threat to animals. Prussic acid can build up in plants during the following 5 conditions:
1. Grazing conditions are poor and stems do not develop properly.
2. Recent hay harves or grazing causes slow and stunted growth of new plant tissue.
3. Nitrogen fertilizers are over-used or there are other soil fertility or nutrient imbalances.
4. Plants develop new growth after prolonged drought.
5. Plants are injured by herbicides, frost, hailr, or other events.
The list of forages that can accumulate prussic acid include: pearl and foxtail millet, sudangrass and sudangrass hybrids, sorghum-sudangrass hybrids, johnsongrass, and grain sorghum.
Two laboratories located on the campus of Texas A&M University can analyze forage or other plant samples for nitrates and prussic acid. It is best to send samples of suspected forages to these labs via UPS, FedEx or LoneStar to ensure they arrive directly at the lab of choice as quickly as possible. The Texas AgriLife Extension Service Soil, Water, and Forage Testing Laboratory can be reached at 979-845-4816. The Texas Veterinary Medical Diagnostic Laboratory can be reached at 979-845-3414. Call either lab to determine the best way to collect and submit your sample.
Do not feed any suspect forages or plants to livestock until you receive your results and are presented with feeding recommendations from the lab.
For more information on these two problems, read this publication (E-543) "Nitrates and Prussic Acid in Forages: Sampling, Testing, and Management Strategies" by Provin and Pitt. e-543 nitates and prussic acid in forages.pdf
