Fava beans belong to the Fabaceae family along with other well-known legumes like soybeans, peas, beans, alfalfa and peanuts. Obviously, all of these legumes have played a significant role in human nutrition, with references to fava beans dating their use at least to 6,000 B.C. and perhaps even before. Fava beans (Latin name, Vicia faba) are known as broad beans (human staple) or horse beans (animal feed), the latter being of a smaller size. Their origin is lost in time, but they are considered native in many places of the world like northern Africa, South America and southern Asia.
There are few places in the world that do not cultivate fava beans today, either for human consumption (as a culinary item or even as a snack) or for animal feed. The reason behind such widespread cultivation is the adaptability of cultivars to diverse growing conditions, although water availability largely dictates yields. For example, in dry climates, the yield of dry beans can be below 1,000 kg/ha, whereas with sufficient water (and other suitable conditions), yields are known to exceed 6,000 kg/ha. This compares very suitably with an average tonnage of 3,500 kg/ha in dry soybeans (again, such values vary widely according to growing conditions).
The need for an alternative to soybean meal
In present times, an effort to rationalize globalization, or even reverse it, is underway in many countries, including some of those considered as the most wealthy ones worldwide. Protein is the second most expensive nutrient in animal feeds, following energy that is being sourced through a variety of cereals either locally or through a more expansive network of exporters. In contrast, soybean meal has become the dominant protein source in animal feeds globally, and to make matters even more complicated, there are only a few countries where soybeans can grow to such levels as to guarantee a surplus for exports.
Thus, soybean meal export trade has become a commodity that is the privilege of a handful of exporting countries. In other words, the majority of countries depend on soybean meal imports to feed their animals. Thus, there are many projects undergoing in which local protein sources are being reexamined as a means of reducing dependency on protein imports. Such is the case of fava beans, and it merits a closer look as it is bound to make an impact on the way we feed animals in the near future.
Nutrient profile of fava beans used as animal feed
There are two major types of fava beans: those from white-flower varieties and those from colored-flower varieties. Their chemical composition and nutritive value is about the same with 25 to 27 percent crude protein, 7 to 8 percent crude fiber, 1 to 2 percent crude fat, and about 10 and 13 MJ/Kg metabolizable energy for poultry and pigs respectively. Compared to soybean meal, the major difference is in the amount of crude protein (44 to 48 percent in soybean meal), whereas the rest of the nutrients and energy concentration are rather similar. Even the amino acid profile is quite similar with soybean meal having 6.1 grams lysine per kg protein, whereas fava beans contain slightly more at 6.5 percent. Digestibility values do not differ much either.
In practical terms, it is the absolute lower value in crude protein concentration that places fava beans in disadvantage to soybean meal. Here it merits mentioning that we have been comparing a whole fruit (legume), that of fava beans, to a byproduct of soybean oil extraction, that of soybeans. Whole soybeans contain less protein but more oil/energy than soybean meal — but it is soybean meal that is being used extensively as protein feed. Such possibility does not exist with fava beans, as their oil content is negligible. Thus, by replacing soybean meal with fava beans, the concentration of the latter will have to be higher — roughly 1.7 kg of fava beans are needed to replace 1 kg of soybean meal (on a protein alone basis, which is not correct in terms of modern feed formulation, but it serves to illustrate the point of this discussion). And, here is where the problems begin.
Antinutritional factors in fava beans
Most if not all legumes contain a plethora of antinutritional factors. These compounds have a wide array of functions within the plant, but when consumed by animals they inhibit the normal use of nutrients in legumes and other ingredients. They are not toxins but rather impediments to normal digestion and metabolism. For example, a well-known antinutritional factor is that of trypsin inhibitor activity (TIA) compound. This factor inhibits the action of trypsin, an enzyme secreted during protein digestion. As such, the organism either suffers from lower protein digestibility or it is forced to secrete higher amounts, leading again to a net protein loss as enzymes are proteins too.
In the case of soybean meal, the majority — but not all — of antinutritional factors are denatured and thus neutralized to a great degree by the process of heat treatment. This is why raw soybeans are not being fed to monogastric species. Of course, soybeans have other issues too, but this is not relevant to our discussion. In contrast, fava beans are not being cooked, and even when they can be treated by some form of heating (for example, extrusion), this has never been sufficient enough to destroy the majority of antinutritional factors. As such, this problem limits the usage of fava beans, although some recent efforts have demonstrated that partially dehulling can create a form of a fava bean meal that contains relatively few antinutritional factors — but this remains a most recent development that requires much evidence to support practical application.
The colored-flower varieties of fava beans contain more tannins, which are bitter compounds and are not destroyed easily. Tannins (usually about 0.3 to 0.5 percent in fava beans) reduce feed intake per se, and depress digestibility of protein and energy. Not all tannins are necessarily bad, and perhaps a minor amount can even be beneficial under certain conditions, but again, this requires further investigation. For the moment, white-flower varieties of fava beans are the ones recommended for replacing higher levels of soybean meal.
Favism and vicine?
Favism is an inborn error of metabolism that affects about 1 percent of the global human population, and more than 10 percent in areas that have historically suffered from malaria. It is characterized by a glucose-6-phosphate dehydrogenase deficiency (G6PDD). This enzyme controls the production of a most powerful internal antioxidant: glutathione peroxidase. Following a specific trigger, symptoms like jaundice (yellow skin) and dark urine develop, followed by hemolytic anemia that can lead to death. In essence, red blood cells simply die because they cannot cope with the production of oxidative free radicals as their G6PDD enzyme is inhibited leading to oxidative “asphyxia.” Such problems can be triggered by many causes, one of them being fava beans, hence the name of this genetic anomaly. It is speculated that similar genetic anomaly exists in animals, again those that have historically been exposed to malaria. At the moment, the spread of this problem in farm animals is unknown, but it might be worth exploring it as modern poultry and pig genetics are controlled by a handful of genetics companies.
In animals consuming raw fava beans, increased mortality is to be expected among populations with genetic predisposition to favism.
Vicine, from which fava beans take their Latin name, is an alkaloid glycoside. It is the causative factor of favism and is considered a toxin. It is found in relatively high levels in fava beans, less in peas and negligible amounts in soybeans. There are also other compounds in fava beans with similar toxicity divicine, covicine and isouramil. There is no known antidote to these toxins, although symptoms in humans can be treated with some success. The only solution for those who have the genetic deficiency problem of favism is not to consume fava beans that have not been cooked thoroughly (cooking destroys these toxins). In animals consuming raw fava beans, increased mortality is to be expected among populations with genetic predisposition to favism. It is not possible to cook fava beans sufficiently for feeding to animals, not only because of the energy cost, but also because such heating will result in destroying the natural and useful protein for which fava beans are needed.
Considering that colored-flower varieties of fava beans are the least desirable for animals — in terms of antinutritional factors, it would be advisable not to feed them raw at all in any monogastric species. If cooked, then 5 percent would be the maximum level in diets for sensitive animals, raised to 7.5 percent for older/mature animals. In contrast, these levels can be doubled when fava beans are from white-flower varieties known to contain very low levels of tannins. Again, the exact performance of each variety must be well established before feeding to any animal — and luckily such variety/cultivar specific trials are propagating.
When mortality spikes due to feeding fava beans, this is an indication of favism. In such case, the only solution is to discontinue feeding fava beans. Again, this is something that could potentialy be the target of genetic exclusion from breeding companies, especially if fava beans become a common ingredient. We might see favism-free genetics being advertised in the not-so-distant future. Until then, trial and error remains the only way to find out if an existing genetics source is susceptible to favism.