Christopher LeMaster, Jr.
Livestock & Forage Agent
Cherokee, Spartanburg, & Union Counties
We don’t usually associate tall fescue with nitrate toxicity in the spring. However, the current D3 drought conditions have changed the equation. This is not only a “summer problem,” but cool-season grasses—such as tall fescue or winter annuals—can pose a risk under these specific conditions.
If you applied high levels of nitrogen this spring to catch a flush of growth that never came, the risk in your fescue fields is real. Without rain, the plant couldn’t convert that nitrogen into growth. It simply accumulated in the lower portion of the plant. Now that we are finally seeing some rain, that “stuck” nitrogen is being pulled into the plant at a rapid rate.
The Spike
It is a common misconception that the danger passes as soon as the grass turns green again. In reality, research shows that nitrate levels typically spike 2 to 3 days after a drought-ending rain. This is the most dangerous window for your livestock.
- The One-Week Rule: While the spike is most acute in the first 48 to 72 hours, we recommend waiting at least one week before grazing or harvesting. This allows the plant time to resume normal metabolism and begin “growing out” of that initial nitrate flush.
- Hay is “Locked In”: Nitrates do not dissipate in dry hay. Once your mower hits the field, you are locked in at that nitrate level forever.
- Silage/Fermentation: If you are putting up high-moisture forage or baleage, the fermentation process can reduce nitrate levels by 40% to 60%, but this should still be verified with a test.
Don’t Guess—Test
If you have fields with a high nitrogen history that are currently recovering from drought, I strongly recommend sampling the standing forage before you cut.
- Get a Representative Sample: Take “grab samples” from multiple areas of the field, specifically focusing on the lower portions of the plant where nitrates concentrate.
- Utilize the Lab: Bring your samples to your local Extension Office or send them directly to the Agricultural Service Laboratory. They can provide a precise analysis of nitrate-nitrogen levels.
- Check the Backlog: We may also consider testing outside of that initial one-week window. Once the plant has had time to work through the backlog of accumulated nitrogen, a follow-up sample can confirm if the forage has returned to an acceptable or safe range for harvest.
Next Steps
If your test results come back with an elevated nitrate level, we can work on a plan to safely manage that forage. Whether that means diluting the feed with other forages, adjusting your grazing strategy, or simply waiting a few more days for the plant to balance out, there are ways to keep your herd safe.
Have questions about how to pull a proper sample or interpret your lab results?
Reach out to your local livestock and forage agent today.
