Hot, dry weather can trigger deadly accumulations of nitrates in summer annual forages — and fire the gun point-blank at cattle consuming too much of the tainted forages.
Glenn Selk has seen it happen more times than he’d like and the Oklahoma State University Emeritus Extension animal scientist urges cattlemen to test suspect forages before heading to the hay field.
“Primarily, we’re talking about summer annuals, especially in the forage sorghum family — sudans, sorgo-sudans, sorghum-sudans, millets and Johnsongrass. That’s where the vast majority of nitrate problems come from,” he explains, adding that he has seen nitrate issues on very rare occasions in grasses, usually due to extreme nitrogen application and weather interactions.
“Nitrate accumulation is caused by plant stress,” Selk continues. “This time of year that’s heat and drought. The plant’s metabolism slows but it continues to take up nitrogen. The problem is that it’s not growing fast enough to use that nitrogen so it accumulates as nitrates in the plant.”
The nitrate is converted to nitrite. As the nitrite is absorbed into the blood, it forms methemoglobin, which interferes with the blood’s ability to transport oxygen.
“That’s why a cow with nitrate toxicity acts like she can’t catch her breath,” Selk says. “There’s little or no oxygen in her blood — her blood will actually be a chocolate brown. She may also have a dark discoloration around her eyes and other symptoms.”
The veteran animal scientist strongly favors testing before cutting the vulnerable forages.
“With nitrates, if you cut the hay and levels turn out to be high, your options become very limited,” he explains. “You may be able to dilute the amount cattle consume by adding or mixing other forages or feedstuffs, particularly if you start slowly and let the cattle adapt.
“If you test ahead of cutting, though, you can leave it in the field and, hopefully, you’ll get some rain and normal plant metabolism will return nitrates to normal levels. You’ll lose some quality but you won’t be baling up toxic hay.”
Another suggestion is to raise the cutter bar when harvesting, Selk says: “That’s going to reduce the tonnage, of course, but the greatest concentration of nitrates is in the lower portion of the stem. Yes, total hay will be reduced but do you really want extra tons of toxic material?”
The absolute worst-case scenario, he says, is to kick out high-nitrate hay for the first time to hungry cows.
The important management action, however, is to know if nitrates are present and at what level. Field-testing can determine the presence of nitrates but not the quantity. For that, samples need to be submitted to a university or commercial forage-testing laboratory. In both cases, Selk suggests producers contact their county ag extension educator for assistance.
Knowing the level of toxicity clarifies cattlemen’s options. Most labs, like OSU’s Soil, Water and Forage Analytical Laboratory (soiltesting.okstate.edu), present results in actual nitrate concentration in parts per million (ppm). Some, however, use different data expressions so producers need to be clear on how results are reported.
From the OSU lab, a result in the 5,000-10,000 ppm range is what Selk calls a “gray area” because while there have been reported cases of abortion in pregnant cows consuming forage of that level, normally little or no production loss is seen, particularly among yearlings and open cows.
“Above 10,000 ppm there is reported data of mortality,” Selk notes. “Not all of them are going to die but there is potential for lethality. That’s really the cutoff point — if you have that much I’d be thinking of ways to dilute the diet.”
Those scales are fairly conservative for good reason, he adds: “There is tremendous variability in the cow’s ability to tolerate the toxicity.”
Health vulnerability at those levels relates to the total diet consisting of high-nitrate forage.
“Years ago, Gerald Horn at OSU, demonstrated that if you could get enough corn — about seven pounds per day — into a cow consuming high-nitrate forage, you could dilute the effect,” Selk points out. “There’s a double impact — the cow isn’t going to consume as much forage and the starch in the corn is going to help the microorganisms in the rumen convert the nitrates.”
Selk points out that it’s easy to confuse the issues of nitrate toxicity with prussic acid toxicity but while there are similarities, there are important differences.
Prussic acid poisoning, like nitrate toxicity, causes the animal to be oxygen starved the mechanism is almost opposite. The cow has super-oxygenated blood — cherry red — caused by the cyanide-containing compound, but it’s not dispensed to the tissues.
It can also be triggered by drought, as well as frost, and is most associated with sorghum-type summer annuals. Millet, however, does not have prussic acid issues so some cattlemen may jump to the errant conclusion that it is nitrate-safe as well.
Unlike nitrate toxicity, prussic acid levels decrease as the hay is cured in the windrow before baling.
Nitrate levels, Selk emphasizes remain constant once the forage is cut.
“That’s why it’s so important to test for nitrates before haying any susceptible forage,” he concludes. “That will give you more management options and might just avoid major herd health issues.” £
OSU fact sheets PSS2903 (nitrates) and PSS2904 (prussic acid) offer a more comprehensive look at the issues.