Management strategies reduce nitrate risk while grazing in double crop foragesWritten by Emilee Gibb
“Grazing cover crops that contain moderate to moderately high nitrate concentration is not without risk, but these cover crops can be grazed successfully,” said University of Nebraska-Lincoln Beef Systems Specialist Mary Drewnoski during a presentation of managing nitrate risk in double crop forages.
Many species that producers select as cover crops may be nitrate accumulators, meaning that special care must be taken while grazing cattle.
Warm season species such as corn, millet, sorghums, Sudan grass, sorghum/Sudan crosses and sunflowers, as well as cool season species such as brassicas and small cereal grasses including oats, cereal rye and wheat, are nitrate accumulators
“There is a risk of high nitrates in many cover crop systems. It is always a good idea to test cover crops,” said Drewnoski.
Forages with zero to 1,100 parts per million (ppm) of nitrate-nitrogen are considered safe. Forages with a ppm of 1,100 to 2,100 are considered moderate risk, 2,100-3,400 ppm are considered high risk, and a ppm greater than 3,400 are considered severe risk.
Drewnoski cautioned producers that laboratories do vary in how they report nitrate levels. The two most common forms that tests will report in are nitrate-nitrogen and nitrate ion.
The ability to utilize nitrate-containing feeds depends on the concentration level, she said.
“There are some management practices that we can use to reduce risk when grazing moderate to moderately high nitrate cover crops,” continued Drewnoski. “For any samples that are severe risk, or greater than 3,400 ppm nitrate-nitrogen, the safest alternative is not to use them.”
Regardless of whether a producer is grazing high nitrate forages or not, Drewnoski strongly suggested making sure that cattle are full before putting them out on pastures.
“This is a good management practice regardless of the nitrate level because this slows down the intake of those cover crops initially to allow the rumen to get adjusted,” she said.
The key factor of filling cattle prior to transitioning them is controlling the rate of nitrate intake.
The conversion of nitrate to nitrite occurs much more rapidly than the conversion of nitrite to ammonia, explained Drewnoski.
“Nitrite is the toxic compound that can enter the blood, bind to hemoglobin and make it incapable of transporting oxygen, thus causing the animal to suffocate,” she said. “If we can have a slow intake of nitrate, we can then keep the flow of nitrite to ammonia at the same level as the flow of nitrate to nitrite.”
It is also advisable to feed higher nitrate feeds to lower-risk cattle. Open cows are the lowest risk, followed by growing calves.
“Pregnant cows are at greatest risk because the fetus is particularly susceptible to low oxygen, and abortion can result,” continued Drewnoski.
“Animals can safely adapt to higher nitrate levels, and this is because the bacteria that can utilize nitrite and convert it to ammonia can increase in number as nitrite availability increases in the rumen,” commented Drewnoski.
To adapt cattle, it is critical for producers to maintain a slow but steady increase in nitrite so bacterial numbers can increase at the same rate and not leave excess nitrite to enter the bloodstream, she explained.
“We suggest adapting cattle by starting grazing the lowest nitrate fields and then work up to the highest,” said Drewnoski.
Producers are also advised to graze animals lightly on higher nitrate forages to allow animals to selectively graze plant parts.
“Leaves are lower in nitrate concentration and cattle naturally select leaves,” she said. “Forcing the cattle to eat the stem, especially the lower stem, will increase nitrate intake.”
The final management strategy that Drewnoski suggested was supplementing cattle with grain while transitioning them to high nitrate forages.
She explained that grain supplementation is particularly useful when feeding high-nitrate lower-quality feeds.
“It is useful because we can supply extra energy to the rumen microbes to convert that nitrate to bacterial protein and minimize nitrate concentration,” she said.
Alternatively, producers may not see a benefit from supplementing grain while feeding higher quality forages.
“This may not be a useful strategy with high quality forages such as brassicas because the rumen available energy is already quite elevated,” she concluded.