Phenolic compounds
Gossypol: Gossypol is a toxic compound found in the cotton plant. It is concentrated in the cottonseed but can also be found in other parts of the cotton plant such as hulls, leaves, and stems. Gossypol exists in two forms: free and bound. The free form is toxic, whereas gossypol that binds to proteins is in the “bound” or non-toxic form. The horse is relatively resistant, but, in general, monogastrics are usually readily poisoned. Dogs and swine are affected by gossypol and rabbits and guinea pigs are also quite sensitive. Ruminants are not usually very susceptible except young calves before the rumen is fully functional. The pigment glands may be removed from the seed by flotation. Alternatively the gossypol may be inactivated by heating but this procedure results in binding of the gossypol to lysine which reduces the nutritive value of the protein present. Feeding studies using purified derivatives (i.e. gossypol acetate) have reported a negative effect of free gossypol on fish growth and feed efficiency. Reduced lysine availability has been reported with cottonseed protein due to the ability of gossypol to bind with the reactive epsilon amino group of lysine during heat/feed processing.
Adult cattle can tolerate much larger amounts of free gossypol but toxicity has been reported with levels of 800ppm fed over a long period of time. The problem with gossypol is the toxic effect seems to be cumulative. The longer they are on a ration that contains much gossypol, the more likely they are to have toxicity problems. In Layers, excessive gossypol has been shown to affect weight gain, feed intake and efficiency, mortality, egg production, weight, quality, and hatchability. A few studies for detoxification of gossypol using soaking and boiling of cotton seeds and seed cakes have been reported. But the use of cotton seeds in animals other than ruminants is yet not possible. There is need to develop an efficient detoxification method in order to make it possible to use cotton seeds/cakes in non-ruminant feeding.
Tannins: Tannins are polyphenolic substances with various molecular weights and a variable complexity. These are chemically not well-defined substances but rather a group of substances with the ability to bind proteins in aqueous solution. Tannins are tentatively classified into two classes: hydrolysable and condensed tannins and are considered to have both adverse and beneficial effects depending on their concentration and nature besides other factors such as animal species, physiological state of the animal and composition of the diet. Although research on tannins has a long history, considerable additional research must be carried out in order to fully exploit benefits of incorporating tree leaves and agro-industrial by-products in livestock feed. Tannins are regarded as inhibitory to the growth of microorganisms by reducing the availability and digestibility of the micronutrients and minerals, impeding cell wall function, and interfering with the catalytic activity of extracellular enzymes.Rumen bacterial isolates which are tannin tolerant and may possess these defensive mechanisms against tannin include Streptococcus gallolyticus, strains closely related to S. bovis, Clostridium spp., Eubacterium spp., Proteobacteria, Klebseilla spp and Butyrivibrio fibrisolvens. Even though the population of tannin tolerant microorganisms may increase in ruminants fed tannin rich diets, these changes do not appear to compensate for tannin induced reduction in digestion of nutrients. It is not surprising therefore that the strategy of inoculating tannin tolerant bacteria into ruminants fed tannin rich diets has not improved the productivity of these ruminants. Although tannins may reduce the abundance of predominant microbial populations in the gut, severalstudies have shown that the effect on ruminal metabolism was insufficient to alter the efficiency of microbial protein synthesis. While it is indisputable that forage tannins reduce the digestibility of the macro- and micronutrients, additional studies are required to determine the impact of tannins on efficiency of microbial protein synthesis in the rumen under varying feeding conditions.Plethora of studies aimed at detoxification (by inactivation or removal) of tannins present in forage, tropical seeds and agro-industrial byproducts have been conducted at different places. In principle, these approaches can be applied to any feed resource rich in tannins. The three different types of approaches have been implied viz. physical, chemical and microbial. Characterization of enzymes responsible for degradation of condensed tannins in white-rot fungi needs to be attempted. These white-rot fungi degrade another polyphenol, lignin. This approach though in its infancy, may also find a place in industry in the future. Further studies are needed to exploit the full potential of this approach. The technologies for detanninification of tree leaves on a small-scale industrial level do not seem to be economically viable because large quantities of leaves are seldom available in one place (unlike agro-industrial by- products) and the cost of collecting them is high. However, the detanninification technologies presently available may well set the stage for utilisation of agro-forestry by-products including tree pods.
Although tannins in forage legumes have generally been classified as antinutritional, it is possible that these plants could be employed advantageously to improve animal production. Consequently, attempts have been made to reduce ruminal degradation of proteins in several range species including Atriplex nummularia and concentrate feeds (rapeseed meal, soybean meal, etc.) by mixing with tannin-containing acacia foliage, thus increasing ruminal supply of protein. Mixing of tamarind seed husk containing tannins with concentrate feed has also been shown to have a beneficial effect on productivity. The change in excretion pattern to higher amounts in faeces may be considered beneficial for the environment, and could also result in higher crop yield if the faeces may used as a manure. These findings warrant further investigations on other tannins and protein sources.
The related polymer, condensed tannin, built from flavanol units, has both anti- nutritional protein-binding effects and, perversely, the potential to improve protein utilization in ruminants. Condensed tannins are closely related to the anthocyanins responsible for many flower colours and their synthesis share a number of biosynthetic enzymes. Some attempts have been made to manipulate tannin biogenesis, often with flower colour as a marker. Selection of various forage legumes for high tannin has shown greater amounts of protein nitrogen surviving breakdown in the rumen and passing to the small intestine of grazing ruminants. However, in the longer term a more important application of tannin manipulation may be to reduce levels in the many tree legumes whose leaf protein content could add substantially to ruminant production in many parts of the world where protein-rich supplements are not readily available.
Sinapine: Sinapine is the choline ester of 3,5-dimethyl -4 hydroxycinnamic acid (sinapic acid). It is one of the best characterised phenolic compounds in plants. Presence of phenolic compounds in the rapeseed meal may contribute to certain undesirable properties including dark colour, bitter taste, and astringency. In addition to the use of various breeding techniques in order to reduce the sinapine contents, several processing methods have been tested to achieve this goal post harvest. Treatment of seeds with methanol-ammonia or gaseous ammonia has been shown to remove up to 94% of sinapine from mustard meal. This treatment has been reported to have beneficial effects in terms of, among others, reducing the fishy odour in eggs of brown egg layer chickens. Although these treatments lowered the progoitrin, soluble tannins, and sinapine contents, these effects were not sufficient to prevent inhibition of trimethylamine oxidation and subsequent rise in egg trimethylamine levels beyond tainting threshold.
Isoflavonoids and flavonoids: Isoflavonoids are common phenolic constituents in pasture legumes. Isoflavonoids are estrogenic and may cause reproductive problems in livestock grazing legume pastures. Formononetin is an isoflavones that occurs in subterranean clover, Trifolium subterrneum, and red clover, T. pretense. Although formononetin is not estrogenic, it is metabolized in the rumen to equol, a potent estrogen. The flavonoids which are abundant in many forage plants and the most common are apigenin, luteolin, quercetin, kaempferol and myrcetin. The products of flavonoids degradation in the rumen include acetate, butyrate, di- and mono hydroxyphenolics and phloroglucinol. Flavonoids are often present in small concentrations in animal and human diets, and there is little evidence of any detrimental effects. However recently it has been observed that two fodder shrub legumes, tagasaste and Acacia angustussima contain high concentrations of the flavones apigenin and luteolin, and both these plant exhibit anti-nutritional qualities. The interaction between these compounds and ruminal microorganisms and the nutritional consequences for the animal has been largely overlooked and thus requires closer examination.