Weed & Pest
Nitrate levels can reach toxic levels in weeds, forage during drought years
Most ranchers realize annual forages can develop nitrate problems when drought occurs, but according to a Montana extension beef cattle specialist, nitrate toxicity can also occur in some common weeds.
Rachel Endecott presented “Feeding Risks from Drought Impacted Feeds and Forages” during a recent Ag in Uncertain Times webinar. Endecott said nitrate toxicity from drought can occur in wheat, barley, millet, oats, corn, sorghum and sudangrass. It can also occur in weeds like redroot pigweed, common lambsquarters, kochia, wild sunflower, Russian thistle, witchgrass, Canadian thistle and black nightshade, which all have tendencies to accumulate nitrates, she said.
“With weeds, nitrates tend to peak at the pre-bud to bud stages,” she explained. “The nitrate concentration will decrease as the weeds mature.”
In annual forages, Endecott said nitrates tend to accumulate the most in the stem or stalk, with the highest amount being in the lower third of the stalk. The leaves often have very little nitrates, and the grains have none.
“Nitrate uptake is a normal part of plant metabolism,” Endecott continued. “Nitrate is converted to nitrites, which is then converted to ammonia for protein synthesis for the plant to grow.”
Drought conditions favor nitrate accumulation. Since the conversion to ammonia occurs in the leaves, if the leaves are negatively impacted by drought, nitrate begins to accumulate in the stem, she explained.
The nitrate conversion pathway is exactly the same in the rumen as it is in plants, Endecott continued.
“What happens when there are high nitrate concentrations is it overwhelms the conversion pathway from nitrite to ammonia. The nitrite ion competes with oxygen for red blood cells, and the nitrite converts hemoglobin to met-hemoglobin. The met-hemoglobin is incapable of oxygen transport, so the animal develops a nitrate toxicity,” she explained.
If an animal is suffering from nitrate toxicity, symptoms can vary, but Endecott said producers should be on the look out for symptoms in chronic animals like reduced appetite, reduced milk production, rough hair and unthrifty appearance, weight loss or no weight gain, and abortion.
Animals suffering from acute nitrate poisoning can have an accelerated pulse rate, labored breathing, muscle tremors, weakness or staggering gait and cyanosis. They may die without quick treatment.
The best prevention for nitrate toxicity is testing forages.
“There are no visual clues that a hay is high in nitrates,” she stated. “It may look really good, but it may also be high in nitrates.”
When harvesting hay, nitrates will accumulate more in the hay overnight because the plant can’t photosynthesize in the dark.
“There will be a reduction in nitrates from morning to afternoon in grain hay as the plant photosynthesizes,” she said. “But, if it’s a hot sample, the drop in nitrates won’t be enough to get the hay into a safe range.”
If the forage test shows the sample is high, Endecott said producers have some options.
“It can be diluted with other, low nitrate forages,” she said. “Producers can also avoid feeding it to more susceptible animals, like those that are pregnant, and find a feedlot full of steers.”
“If the levels are too high to be adjusted to a safe level, a producer may have no option but to have a marshmallow roast and destroy the hay,” she added.
Giving a bacteria bolus
If producers plan to utilize high nitrate feeds, they may want to consider dosing the cattle with a nitrate-utilizing bacteria bolus seven to 10 days before turnout. The bolus will help the rumen adapt to higher nitrates in feed. Endecott said the bolus is slow releasing and provides the nitrates with utilizing bacteria.
“You will be in good shape if you keep them exposed to nitrate forage,” she explained to one producer. “However, if you take them off and put them back on, you will need to rebolus.”
For cattle grazing cornfield residue, Endecott urged ranchers to fence out corners and edges of a drought-stressed irrigated cornfield where the potential may exist for more nitrate problems. She also discouraged producers from leaving cattle on cornfields too long because of feed shortages, which forces the cattle to consume the stalks.
“Generally, cattle prefer grazing the leaves and husks, which are low in nitrates, but they will eat the drought-stressed stalks if they are forced, too,” she said.