The grass may be greener on the other side of the fence, but that’s not always a good thing.
As the rains continue through the summer, ranchers need to be aware of problems–like acute bovine pulmonary edema and emphysema–that can arise from switching between dry, fall pastures and lush, green pastures.
Dr. Tim McAllister, Research Scientist, Ruminant Nutrition and Microbiology, Lethbridge Research Centre, Alberta, says there are actually two closely-related diseases; one occurs in feedlot cattle and the other in pastured cattle. The disease in pastured animals has been called fog fever, lung fever, bovine asthma, acute alveolar emphysema and atypical interstitial pneumonia. Affected cattle are often called “lungers” or “panters.”
“The disease that occurs in pastured animals is probably connected to tryptophan metabolism, and the formation of 3-methylindole,” McAllister says.
The lush, rapidly-growing forage contains an amino acid called tryptophan—a normal component found in protein.
“In the feedlot, by contrast, the problem is not necessarily related to the protein level in the feed. There are other complicated factors going on, in terms of immunity and responses in the lung that lead to the condition,” McAllister says.
Grazing animals are generally affected when they go out on lush pasture after being in dry conditions. Production of 3-methylindole in the rumen is due to rapid metabolism of tryptophan by rumen bacteria. “This compound is converted into another toxin that causes the lung damage when it gets into the lungs,” he says. Toxins from the gut are absorbed into the bloodstream and carried to the lungs, where the reaction takes place.
“It causes edema in the lungs, and this fluid interferes with the breathing process and the exchange of oxygen,” explains McAllister. Symptoms appear suddenly and the most obvious sign is labored breathing, often with grunting sounds. The animal may try to breathe through its mouth and may froth at the mouth. It may stand with head and neck stretched forward, mouth open, breathing rapidly and shallowly. One way to tell the difference between a cow with this condition and a cow with infectious pneumonia is that there is no fever and they are not dull. Affected cattle may seem bright and perky and may still try to eat or drink.
“The affected animal experiences respiratory distress (and that’s the only sign of problems). Even slight exertion makes it harder to get enough oxygen. If you try to move them and push them too hard, they may have a heart attack,” he says.
The heart rate rises because of the shortage of oxygen and may be as high as 150 beats per minute if the animal tries to move.
According to the Beef Cattle Handbook article on Acute Bovine Pulmonary Edema and Emphysema in Beef Cattle, clinical signs usually occur one to 14 days after an abrupt change to lush green pasture. Death may follow within two to four days of first appearance of clinical signs. At necropsy the lungs are two to three times heavier than normal lung tissue because the air pockets and airways are filled with frothy fluid.
“If animals on green pasture start to have problems and are exhibiting clinical signs of respiratory stress, rounding them up could increase stress even move, and they may die,” says McAllister. “You might be able to take your time and try to gradually move them off the pasture, or you might be able to confine them to the area they are in, with portable electric fence, and feed them hay. The important thing is to remove the animals that are not yet exhibiting clinical signs, and get them off the lush pasture,” he explains.
“Once they are showing clinical symptoms, however, there is little that can be done to reverse the condition. Emergency slaughter is the only option. Taking the lush pasture away from animals that are already having trouble breathing will not make the condition better,” he says.
Treatment is a challenge, and not very effective. “There isn’t much we can do in terms of treatment. Most of the time these animals are emergency slaughtered. Things like antihistamines and steroids have been tried, but once they have this condition, the damage is already done. You have to try to avoid it, in advance, before they actually show symptoms.” Prevention is the best treatment.
“People have used anti-inflammatories and preventing the production of prostaglandins—because prostaglandins play a role in the conversion of the 3-methylinole into its toxic metabolite. With treatment we sometimes can prolong their lives; the animals will survive longer and make it to an emergency slaughter facility, but the condition cannot be reversed,” he says.
Some that are only mildly affected will live, but the lung damage is permanent. They still have respiratory problems, and audible wheezing as they breathe. “This is from earlier damage, and these animals might be more susceptible to secondary infections caused by bacteria or viruses,” says McAllister.
To prevent this problem in the first place, the animals should be given time to adapt to lush green feed. “If you are bringing them in off very dry rangeland or any other dry, low-protein forage, you can gradually adjust them to a higher protein diet with a high quality hay before you turn them out in the green pasture. If it’s a grass-based pasture you can probably let them into it for only a short time the first day, and bring them back in for hay feeding, gradually increasing their time on the pasture over several days. This might not work on alfalfa because it would increase the likelihood of bloat, however. Another thing you can do is let the pasture become more mature before you turn them in to it. This will lower the protein level in the pasture and reduce the risk,” he says.
“There is no cure for this condition. At some point the animal experienced these toxins, and what you are seeing is clinical symptoms as the result of damage that has already occurred,” says McAllister.
Ranchers should closely monitor pasture conditions and observe grazing animals, and be prepared to move them to better pasture before they exhaust their current food supply and are forced to eat poor quality feed. According to the Beef Cattle Handbook, cattle that have been allowed to remain on dry, overgrazed rangeland are prime candidates for emphysema because they are hungry when they go into the new pasture and overeat on the lush forage.
Research has demonstrated that after two or three weeks of poor quality forage (with crude protein less than 6.5 percent and acid detergent fiber greater than 50 percent) conditions in the rumen become optimal for elevated 3-Methylindole production. The goal of preventative management is to provide a gradual adaptation to the lush forage.
Besides feeding hay, limiting access to the new pasture for the first few days will aid the transition. This might mean daily moving the cows onto and off the pasture. An example is given, with the suggestion of allowing cows that are full of hay to graze for an hour on the lush pasture. On the second day, and subsequent days, you can increase the grazing time by one hour, for the first seven days. This transitional period gives the rumen microbes time to adapt to the new forage, and reduces 3-Methylindole production. Feeding the cows restricted quantities of swathed green forage in addition to hay would also enable the rumen to adapt to the lush green feed.
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