If you need another reason to keep weeds out of your alfalfa, here's one: forages having toxic concentrations of nitrates found in several weeds do ugly things, all of a sudden, to livestock that ingest them.
John Tegzes, a veterinary toxicology specialist at the University of California, Davis, says common weeds such as pigweed, nightshades, lambsquarters, and goosefoot can raise nitrate concentrations to a potentially toxic level.
He told the recent California Alfalfa and Forage Symposium in Modesto that oat hay, sudangrass, and sorghum have been linked to nitrate toxicosis, while alfalfa contaminated with pigweed or goosefoot may contain potentially toxic nitrate concentrations.
Nitrates accumulate in plant tissue, more in the stalks than the leaves, and concentrations remain the same after the crop is dried.
Tegzes said several conditions enhance uptake of nitrates by plants. Heavy fertilization, application of herbicides (particularly 2,4-D), drought, low temperatures and frost, and even cloudy weather may increase nitrate levels, particularly in late summer or early fall in California.
Nitrate occurs in water too, and Tegzes warned that the cumulative amounts of nitrate from all sources are telling in the amount consumed by livestock. Although algae growing in a water supply will tend to decrease nitrate, it may cause problems of its own for the animals.
Nitrite, the harmful form, is converted from nitrate in forage by microorganisms of ruminants. This action produces methemoglobin, which does not react with oxygen and causes telltale, dark-brown blood in the animals.
Since their blood is starved for oxygen, these animals appear to have difficulty breathing, along with symptoms of tremors, diarrhea, rapid heartbeat, and convulsions. Symptoms may occur 30 minutes to four hours after feeding, and death can occur within 24 hours. Pregnant stock, outwardly healthy, can abort fetuses three to five days after heavy exposure. Poor growth, poor feed efficiency, reproductive problems, and decreased milk production are other clinical signs.
“This is very difficult to diagnose and treat, since it happens so quickly,” Tegzes said. “These animals accumulate nitrate over a period of time, and it may not be very obvious. Then all of a sudden they are dead.”
The veterinarian said the various symptoms make visual examination of animals inconclusive, but when blood is drawn, its dark color reveals the true cause.
Losses to nitrate toxicity were high during the past year, especially during the late summer and early fall, he said. Potential toxicity occurs when nitrate is 0.5 percent and greater, and acute toxicity is likely if nitrate exceeds 1 percent. Tegzes recommended that feed for pregnant ruminants not exceed 0.2 percent nitrate, or 2000 parts per million.
The limit for nitrates in water consumed by livestock is 1,500 ppm. For reference, it's 45 ppm for humans. Once nitrates exceed 300 ppm in water, palatability is affected, and horses will not drink water that is in the 200 to 300 ppm range, although cattle will.
So, what to do to avoid all this? “First, examine forage for evidence of harmful weeds,” he said, adding that you can also analyze the forage on-farm with convenient color tests to show nitrate concentrations in samples of the forage.
It is also important to cut hay late in the day on a sunny day. This helps to convert nitrate in the hay to protein and other nitrogen sources.
Another means is delaying harvesting, since the more mature the plants, the less nitrate they have. The highest concentrations occur just before flowering.
“Waiting at least a week for harvesting after a rain that ends a drought period will help decrease the nitrate. If you absolutely have to use feed that is high in nitrate, you can ensile it. Ensiling decreased nitrate concentration by 30 percent to 50 percent in one to two months. But test the silage after that one- to two-month period to make sure it no longer has toxic levels.”
Feeding of grains or diluting rations with feeds known to contain no nitrates will also help reduce the nitrate level.
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