Posted: Thursday, November 29, 2012 12:00 PM
Experts warn that too much nitrate could accumulate, causing illness in cattle
With this year's drought in major feed and cattle production areas, livestock producers need to know the risks of feeding drought-impacted forages.
This year's drought-stressed forages carry a high risk for nitrate toxicity to cattle, said Rachel Endecott, extension beef cattle specialist with Montana State University, during a webinar last week hosted by the Agriculture and Applied Economics Association.
Plants that accumulate nitrates include small grain forage crops -- such as wheat, barley, millet and oats -- corn, sorghum and sudangrass. The highest concentration of nitrates is going to be in the stems or stalks of the plants, especially in the lower section.
"Leaves have less nitrate, often very little, and grain generally has none," she said.
Nitrate uptake is normal, and under normal growing conditions, it is converted to nitrite, which is then converted to ammonia in the leaves to be used in protein synthesis for the plant to grow, she said.
However, when leaves are negatively impacted by drought, nitrate accumulation occurs in the stem. Eating those stems can lead to nitrate toxicity in cattle, she said.
The nitrate conversion pathway in ruminants is exactly the same in plants. If cattle are eating forage with high nitrate concentrations, the conversion from nitrite to ammonia in the animal is overwhelmed.
"The problem with nitrite is that (nitrite) ion competes with oxygen for red blood cells," she said.
When the nitrite comes in contact with those red blood cells, it converts hemoglobin into methemoglobin, which is incapable of transporting oxygen, she said.
From a chronic standpoint, animals that are exposed to a moderate level of nitrate for a period of time will exhibit reduced appetite and milk production, rough hair, lack of vigor, weight loss or no weight gain, and abortion, she said.
From an acute nitrate toxicity standpoint, animals will exhibit an accelerated pulse rate, labored breathing, muscle tremors, weakness and a staggering gate, cyanosis (mouth tissue turning blue) and eventually death, she said.
"My recommendation is always test suspect forage. If concentrations are high, you have several options," she said.
High-nitrate forages can be diluted with other, low-nitrate forages. Or a livestock producer can find a feedlot full of steers that might feed it in small amounts. Producers should also avoid feeding high-nitrate forages to susceptible animals, such as pregnant cows.
"In the worst-case scenario, hay is probably going to need to be destroyed," she said.
She warns producers there are no visual signs of high nitrate concentrations in forage and recommends testing it before feeding.
"You can have some lovely looking hay ... that's hotter than a pistol," she said.
Corn grown in this year's drought also carries a high risk of nitrate concentration in the stalks.
The highest nitrate accumulation in corn takes place during periods of heavy uptake. So if the drought occurred during or immediately after pollination, it has a potential for high nitrates. If the drought occurred before pollination, the corn is probably OK, she said.
A lot of producers might have ensiled drought-stressed corn this year. Nitrates actually dissipate during fermentation, but livestock producers should wait at least three weeks to feed silage to allow fermentation to progress, she said.
Ensiled, drought-stressed corn generally provides 90 to 100 percent of the feeding value of normal silage. That said, there is less fermentation and less nitrate breakdown if corn is ensiled at less than 55 percent moisture, and producers should consider testing it in the pit, she said.
Producers who have cattle grazing corn stocks should also be cautious. They should not turn cattle in hungry and should pull cattle off as soon as they've eaten most of the leaves and husks.
They might also consider fencing off pivot corners and edges where plants were severely stressed and dosing cattle with a nitrate-utilizing bacteria to help the rumen adapt to higher nitrate levels, she said.