Dilute High-Nitrate Feeds to Avoid Problems
Occasionally a rancher gets stuck with high-nitrate feeds that were harvested after frost. These can often be safely fed — if diluted with other forages, says Barry Yaremcio, beef and forage specialist in Alberta, Canada.
Annuals, perennials and legumes can all develop high levels of nitrates when stressed or lightly frozen. Nitrates convert to nitrites after being eaten, and this compound ties up hemoglobin, the oxygen-carrying part of the blood. Affected animals show respiratory distress and may suffocate. Other symptoms include drooling, nervousness, convulsions, diarrhea and abortion.
"When introducing high-nitrate feeds to cows, do it gradually. After about six days the rumen bacteria will adjust and can detoxify a greater amount of nitrates per day. The safety margin increases," he says.
As bacteria are changing, the number of red blood cells in the body also increases, but this may take 14 to 21 days. "The animals do adapt and become able to handle higher levels of nitrate," he says.
The old rule of thumb was that any feed higher than 0.5% nitrate was toxic. "This is based on a 1964 study in which sodium nitrate was infused into the jugular vein of cows. At half a percent, animals quickly went into serious respiratory distress.
This is not the same as what occurs with nitrate in forages, however. It may take 36 hours to 72 hours to digest forages, resulting in a more gradual release of nitrogen into the blood. If cattle are allowed adequate time to adjust to eating about one-fourth of the ration as high-nitrate feed, and amounts are gradually increased over a 2-week period, you can probably get up to 0.75% or even 0.9% nitrates without problems. The adjustment period is crucial," he explains.
Pregnant cows are likely to abort if fed high-nitrate feeds in late pregnancy. The fetus requires greater amounts of oxygen as it grows larger.
"Spontaneous abortions will occur in pregnant cows when fed high-nitrate feed during the last 30 days. As calving approaches, maximum nitrate concentration in the ration should be reduced back to the 0.5% limit for the last month of pregnancy," Yaremcio says.
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