– Dan Lima, OSU ANR Extension Educator, Belmont County
There is a family of compounds engineered by plants collectively referred to as phenolic compounds because they contain what is known as a phenol group:
This phenol group is the base for over 10,000 individual compounds engineered by plants used: to deter herbivory, as pathogen defense, as structural support (lignin), as photo protection, and even to attract pollinators. This group of compounds can also be referred to as secondary metabolites because even though they are important, most of these compounds are not vital for plant survival. Another group of secondary metabolites produced by plants and some fungi are alkaloids. Some examples of plant alkaloids are: caffeine, codeine, and nicotine. There are over 15,000 examples of these molecules. All of these compounds represent a wide array of plant survival tactics. Specific compounds such as: tannins, cyanogenic glycosides, and alkaloids have negative effects on livestock that can lead to the animal getting sick or possibly dying; I will focus on those specifically.
Tannins are a polymer molecule, meaning that they create long chains. This group of phenolic compounds is thought to play a major role in plant defense, much like lignin. If you have applied herbicide to a mature plant that is highly lignified such as mature grasses or iron weed with seed head set, the herbicide will not be as effective on that plant due to the high lignin content in the plant leaves and stems. Tannins work in a similar fashion in that they are primarily used by the plant to deter feeding from herbivores such as livestock. Tannins are a general toxin that will reduce the survivorship of mammals that feed heavily on the compounds. Deer, cattle, and horses for instance will avoid grazing on high tannin levels in certain plants and fruits. Unripe fruits as an example will accumulate high levels of tannins to avoid being eaten until the seeds are ready for animal dispersal. Tannins will also help the plant defend against fungal and bacterial pathogens. Grasses do not typically contain high levels of tannins with the exception of sorghum. Tannins can be found in legumes like birdsfoot trefoil and in smaller concentrations in white clover. One of the positive consequences of tannins in grazing forages is that it can reduce the potential for bloat in livestock. This could be a side-effect from how tannins and proteins will interact and create complexes. These tannin-protein complexes make the protein unavailable to the animal and consequently lower the nutritional value of the forage. (Taiz and Zeiger, 2002)
Alkaloids get their name because these molecules typically have a positive charge with an alkaline pH and are very water soluble. These molecules are thought to be utilized for plant defense and to prevent herbivory. Unlike tannins, many alkaloids are not unpalatable to livestock and in some instances seem to be preferred. Alkaloids will typically interfere with animal nervous systems and are toxic to not only livestock but humans also. Coniine is the alkaloid in poison hemlock that is fatal to livestock as well as people. Reed canary grass is an example of a high alkaloid grass. This could be one reason why some livestock find it less desirable to graze. An interesting thing about alkaloids is that these compounds can be produced by fungal symbionts and give the plant an advantage in an environment. An example of this is tall fescue with its fungal endophyte. Endophyte infected tall fescue will be more resistant to disease and have much better persistence; however animals that graze the alkaloids produced in these grasses could show signs of severe heat stress and reduced blood flow to peripheries like the ears and hooves. This could lead to infections in the hooves and prevent gain in these animals. There are novel endophyte varieties that have been shown to give the grasses the disease resistance without the toxic alkaloid production. As we learn more about these compounds, we can develop plant varieties that suit our needs better.
Cyanogenic glycosides are compounds that when broken down release cyanide (HCN). Cyanide will bind to metalloproteins found in animal mitochondria and will cause animal death by halting cellular respiration. This decomposition happens as a result of herbivory or simply by the wilting of broken branches. The cyanogenic glycoside molecule is found in plant leaves and is stable until the plant leaf is damaged, resulting in a release of an enzyme that will alter the molecule and once oxidized will release cyanide gas. These compounds are mainly found in plant species of the rose family. A member of this family is the black cherry tree. The reason why wilted cherry leaves are toxic to livestock is because upon decomposition or wounding, the leaves will release the cyanide gas and kill the animal.
There are a lot of studies on how different phytochemicals, especially secondary metabolites, interact individually with the animal or as a mixture. One example was how the tannins in birdsfoot trefoil prevent bloat by binding to proteins in the legume. Some researchers have shown that cattle grazing legumes high in tannins could also tolerate high alkaloid producing grasses better than their counterparts (Lyman 2008). There is much research in the biochemistry of phytochemicals and animals, including ourselves. The antioxidant property of anthocyanin (a phenolic compound) found in red wine is what comes to mind. What seems to work well for many farmers is having a variety of things in a pasture field so that no one harmful compound by itself will accumulate to very high levels. Research in this area is ongoing and new breakthroughs will surely come from this type of research.