Many cattle producers are now planting brassicas as forages and cover crops. These are great annual forages, but they can include a couple of “gotchas” that cattlemen should be aware of.
Lots of farmers and ranchers are planting the new hybrid varieties of forage brassicas and other plants in the cabbage family. These forages represent one of the most stunning breakthroughs in forage breeding – interspecies hybridizations between plants such as rape, kale, turnips, swedes, cabbage and even radishes. With their high levels of protein and energy, these forage brassicas show fast emergence and rapid regrowth, and they can give consistently high yields with multiple grazings, even in the face of water stress and cold stress.
But … but … we also should be aware that brassicas have a dark side – a Mr. Hyde side of their biochemical personality, if you will. Livestock nutritionists, who tend to get animated about things like this, count at least five potential nutritional problems with brassicas. If I was thinking about planting brassicas, I would really want to know about these potential problems and their risks before I buy seed. Wouldn’t you? So let’s talk about them. There’ll be lots of chemistry here, along with some jargon, so take your time.
Potential problem 1: Iodine
Brassicas belong to the cabbage family, aka the crucifers, along with other plants such as radish, horseradish, watercress and the mustards. All crucifers contain compounds called glucosinolates. These compounds cause the mouth-tingling “hot” flavor associated with the brassicas. But glucosinolates can also cause severe problems with iodine metabolism in animals. Recall that iodine is used by the thyroid gland to make the hormone thyroxine, which controls critical physiological functions like basal metabolic rate.
Once glucosinolates enter the digestive tract, they are transformed into compounds such as thiocyanates, isothiocyanates and goitrin. The thiocyanates and isothiocyanates interfere with the thyroid’s ability to absorb iodine from the blood. Goitrin, which is more prevalent in rapeseed meal, prevents the thyroid from incorporating iodine into thyroxine molecules and also from releasing thyroxine into the blood. In either case, animals can suffer from the classic symptoms of iodine deficiency: goiter, stillbirths, weak calves and lambs, poor thermoregulation, reduced growth, etc. Because one of the symptoms is goiter, reference books often refer to these compounds as goitrogenic glycosides. In any case, we can overcome the problems of thiocyanate and isothiocyanate by increasing the iodine levels in the diet. Also, goitrin is a problem that we can reduce through plant breeding.
Potential problem 2: Erucic acid
This is a doozy, but it really involves rapeseed rather than the leafy rape forage. Rapeseed contains quite a lot of fat (oil) that is commercially extracted, leaving rapeseed meal as the residue. Rapeseed oil is highly valued in many industries; it is used in diesel fuels, rubber additives, commercial waxes, the manufacture of nylon and as a lubricant in high-speed engines. One important component of rapeseed oil is a 22-carbon fatty acid called erucic acid, which can comprise as much as 25% to 55% of the oil. Unfortunately, erucic acid is not good for livestock. It causes poor performance, kidney degeneration, enlargement of the adrenal glands and increased death losses. It also causes severe heart problems in laboratory rats.
Rape forage is not a problem because the leaves are low in fat, but rapeseed meal can still contain residual fat (at approximately 7% of the dry matter), which can include some erucic acid. This problem has been known for a long time. Even the older textbooks caution against feeding large amounts of rapeseed meal to livestock.
Erucic acid also poses a real issue in human nutrition, since rapeseed oil can potentially be used in items such as margarine and salad oil. To solve this problem, and because rape is widely grown in Canada, Canadian plant breeders conducted extensive selection trials, and by the 1970s they had developed some rape varieties containing low levels of both erucic acid and glucosinolates. Those folks, however, recognized that the name “rape” presented a marketing nightmare since its nutritional problems were so well known. A sales jingle that proudly proclaims, “New, improved rape: It won’t kill people” is not exactly effective publicity. So researchers came up with a new name for these improved varieties: Canola – Canadian Oil Low Acid.
Potential problem 3: Sulfur
That’s strange; what could be the problem with a standard required nutrient? Well, the potential problem is polioencephalomalacia (PEM, usually called “polio” among livestock producers – not to be confused with the unrelated human disease caused by a virus). PEM shows up as neurological symptoms, including incoordination and convulsions, often with the animal’s head thrown back and “stargazing,” as well as blindness. We see this periodically in feedlot lambs and calves fed high-grain diets, but also with high-energy, low-fiber forages that contain high levels of sulfur. The forage brassicas fit this latter description quite nicely. Brassica plants are well known for their appetite for extra sulfur, and sulfur levels in the leaves are often higher than 0.5%, which is quite high.
In the past, PEM has been typically attributed to a thiamine deficiency and treated with thiamine injections, but researchers have found that PEM is also associated with the formation of hydrogen sulfide (H2S) in the rumen and the subsequent inhalation of this gas by the animal. Although we don’t yet understand the biochemical mechanism linking sulfur with PEM, we do occasionally see PEM in animals grazing a field of high-quality brassica forage. The practical response to this problem is to move them off that field, at least for a little while.
Potential problem 4: Acute bovine pulmonary emphysema (ABPE)
Also called “fog fever” in England, this is a problem primarily of cows moved abruptly from dry, poor-quality forage to lush, high-quality pasture. For example, in the Western states, we can see this syndrome after moving animals from dry summer rangeland to irrigated fields of succulent forage brassicas. But this is not just a problem of brassicas. ABPE can occur when animals are moved to many types of lush, high-quality forages, including young, highly fertilized grass pastures. Within 10 days, animals start showing symptoms including labored breathing, frothing at the mouth and grunting. The metabolic problem is caused by a compound called 3-methylindole produced under these conditions by rumen microbes when they ferment the amino acid tryptophan. The 3-methylindole causes lung damage. Sadly, ABPE has a high mortality rate and no good treatment. Producers try to prevent ABPE by manipulating the rumen environment, such as offering hay in the lush field or using a mineral mix or supplement that contains an ionophore.
Potential problem 5: Anemia
Properly called brassica anemia, symptoms can appear after a few weeks of grazing brassicas. Animals suffer from scours and jaundice, and their growth slows. The culprit is a relatively rare sulfur-containing amino acid called S-methylcysteine sulfoxide (SMCO). Although by itself SMCO is not a toxin, rumen microbes convert SMCO into the toxic compound dimethyl disulfide, a substance that destroys the membranes of red blood cells and causes severe hemolytic anemia. Hemoglobin levels in the blood fall, and the hemoglobin escaping from damaged cells infiltrates other tissues, causing jaundice and red urine.
Brassica anemia is associated with older plants or flowering plants. This is only a problem in ruminants, especially cattle, because the activating reactions occur in the rumen, upstream from the rest of the digestive tract. Horses, pigs, rabbits and elephants are not affected, nor are humans (in case you like eating cabbage), because in these animals SMCO is absorbed from the intestine before it can be converted into the destructive dimethyl disulfide. There is no direct treatment for this anemia, except to move the animals off the pasture onto non-brassica fields, where they will recover in three to four weeks.
The bottom line
Whew! Five metabolic problems. This is not a good report card.
What is the bottom line? With so many nutritional risks, you’d think folks would avoid brassicas at all costs. But they don’t. Thousands of farmers use these forages without incident because they don’t graze them for too long, or their brassica varieties are low in toxic compounds, or they feed sufficient iodine, or … well, there’s a lot we don’t know about these plants. We really can’t predict exactly if or when these nutritional problems will occur, but the good news is that we have some ways to cope with these problems.
So would I plant brassicas? You bet I would. Their advantages far outweigh the risks, and brassicas are still one of our best tools for grazing animals. As for the nutritional problems – well, I would just keep them in mind, watch my livestock, manage carefully and cross those toxicity bridges if I come to them.
