Nitrate levels in forages can sometimes be a problem for cattle. Depending on growing conditions, certain plants may accumulate too much nitrate to be safely fed to cattle unless those high-nitrate feeds can be diluted by mixing with other forages.
Colby Elford, livestock and feed extension specialist, Ministry of Agriculture, Moose Jaw, Sask., says it is important to test nitrate levels in forages. When ruminant animals consume feed that is high in nitrates, the nitrates are quickly converted to nitrite in the rumen by the rumen microbes. The original form (nitrate) in the plant is not poisonous. It’s the nitrites that are toxic if they accumulate in the animal’s system. If you are feeding a high level of nitrates it eventually becomes toxic.
As long as we stay below those thresholds while feeding nitrate-containing forages, the nitrates will be converted to nitrite in the rumen and the microbes continue the process of converting nitrites to ammonia, and ammonia to amino acids to create protein. Problems occur when cattle consume too many nitrates. Rumen microbes convert these to nitrites quickly, but the conversion of nitrites to ammonia is slower, and excess nitrites accumulate in the animal’s body.
“If you know what the levels are in a feed, they can usually be dealt with by blending the high-nitrate feed in a ration,” notes Elford. “We encourage producers to go ahead and cut their feed for quality and then follow up with a full feed test that includes nitrates. This will enable them to make a feeding plan, after they know those numbers.”
Researcher Tim McAllister at Agriculture and Agri-Food Canada’s Lethbridge Research and Development Centre says there are a number of different triggers that lead to nitrate problems. “The primary one is early frost when plants are still growing, producing a lot of proteins and enzymes,” he says.
As the plant is growing, it’s capturing carbon dioxide from the air and utilizing photosynthesis to create the protein it needs for growth. “When we have early frost, this damages the plant cells, causing them to rupture releasing the proteins. Then plant enzymes and microbes break down those proteins into amino acids and remove the nitrogen from them in the form of ammonium, which can be converted into nitrate within the plant.”
The nitrite is the problem; it can be absorbed through the rumen wall or absorbed into the body via the small intestine. “When it enters the blood system it combines with the hemoglobin to form methemoglobin which causes the blue colour of blood that is characteristic of the inability of hemoglobin (in the red blood cells) to transport oxygen,” explains McAllilster.
This results in the “poisoning” we see in cattle. The body is literally starving for oxygen. “If you were to take a blood sample it would look blue rather than red, since the nitrite binds to the same place on the red blood cell that oxygen would normally bind to when hemoglobin transports it. Thus the animal is oxygen-deficient.”
“Often you won’t see any symptoms; you just find the dead animal,” says Elford. If you happen to see them before they die, they might appear weak, struggling to breathe, with increased heart rate. They may slobber and drool saliva. They are essentially suffocating.
There are a number of things that can increase this risk. “A high-protein forage is more likely to cause nitrate poisoning than a low-protein forage,” says McAllister. “Excessive amounts of nitrogen fertilizer on a crop or pasture (or cattle manure as fertilizer) will increase the protein levels in the plant and increase that risk as well. The degree of damage, as from frost, will also be a factor. Some plants are more susceptible to damage because they are not as cold-tolerant,” he explains.
Major issues with nitrates usually follow a frost but it can also happen with other stressors to the plant, like drought.
“The most common situation where we hear about nitrate poisoning is with greenfeed or oat hay or barley hay,” says McAllister. “You don’t hear about it with silage because it’s harvested earlier and not as apt to have frost damage. Also the proteins in the plants are converted during the ensiling process,” he says.
Some classes of livestock can handle higher nitrate levels than others. Pregnant cows are often the most vulnerable, with the first sign of nitrate poisoning being abortion, especially in early pregnancy.
“If nitrate levels in a certain forage are too high to be safe, there are several ways to deal with this. The best way in most cases is to dilute the nitrates by feeding other forage along with the high-nitrate feed. The solution is through dilution. It’s not like you have to throw it away; you can often mix it in the diet, starting with a small amount of the high-nitrate feed,” McAllister says.
By letting the animal adapt, gradually increasing the amount of high-nitrate feed in the diet mix, the microbes in the rumen can actually adapt and become more efficient at converting nitrate to nitrite and the nitrite to ammonia.
“The microbes’ ability to adapt is significant, to the point that you can get up to levels of nitrate that if you had fed that in the beginning it would have likely killed the animal,” he explains.
“If you don’t want to take that risk (of trying to adapt the animals to higher levels), you can simply keep feeding a mix of feeds to dilute the nitrates,” says McAllister.
“If you are going to have problems, it will usually be when you first introduce cattle to that forage, so you need to know the nitrate levels and manage accordingly,” says McAllister. Forage samples can be taken from the fields and pastures or core samples from bales, and sent to a lab that does the testing.
Elford encourages his clients to get a full forage analysis even if nitrates is their only concern. “If you are going to the trouble of taking samples and sending them off, you might as well check protein and energy levels,” he explains.
Phosphorus could be another concern with the higher grain content in an annual forage. “If that’s the case, we can tailor a mineral program to make a proper calcium-to-phosphorus ratio,” he explains.
It’s a balancing act, and these things can change from year to year if the growing conditions are different, even if it’s the same field and same kind of crop.
Get a good sample
It’s easy to take core samples from a bale, but harder to check a forage that will be grazed. “This involves taking plant samples and trying to get a representative sample from throughout the field. It won’t be as good, but will give a clue. We suggest taking samples from different parts of the field, making sure you get some from the hilltops as well as the bottoms, and the edges as well as the middle. Pool all of those together and then take a sub-sample from the large sample after you’ve mixed them. Or, you can take several samples from different areas of the field and send them all for testing.” Then you’d know if one part is worse than the others, or the dry edge is worse than the middle.
“On a large field it would be a good idea to check several areas. If you are talking about an entire quarter section there could be a lot of variation,” he says. It’s very inexpensive to do the tests, compared to losing animals or abortions.
“If you think you have an issue with sick or dead animals, have a veterinarian check them in case it is a disease or some other condition. It might not be nitrate poisoning. It would be good to figure it out before you have many losses, and all of that could be avoided if you’ve done a test ahead of time and can manage the feed appropriately,” says Elford.