New development of tannin assay enables more use of tree foliage as animal feeds

Feeding Mulberry tree leaves to cattle Tree and shrub leaves form a major part of livestock diets in hilly, sub-tropical and tropical regions. However, these trees and shrubs also contain tannins, which are astringent, bitter-tasting plant polyphenols.  High concentrations of tannins reduce feed intake, decrease the nutritive value of plants, and can also be toxic to animals. Therefore, the ability to measure tannin levels in tree foliage is vital for exploring the use of this plant material as animal feed.

Goats grazing on Atriplex browses which are high in tannins A Co-ordinated Research Project entitled “The use of nuclear and related techniques to develop simple tannin assays for predicting and improving the safety and efficiency of feeding ruminants on tanniniferous tree foliage” was initiated in 1999 (involving 11 countries). The aims of the project were to standardise and refine the methods used for quantifying tannins in feedstuffs, and then to analyse the relationship between the tannin contents of tree foliages and its nutritive value  and to study the possibility of ‘inactivating’ tannin before feeding to animals.

Assay development and use

Tannins are complex molecules of plant polyphenols with molecular weights ranging from 500 to over 20,000 Da and are generally divided into hydrolysable tannins and condensed tannins. Both types are difficult to measure.

Two simple methods for measuring tannins in foliage have been developed in this project: a spectrophotometric method for measuring hydrolysable tannins and a protein precipitation capacity method for measuring activity of tannins. The study of biological effects of hydrolysable tannins and their ecological significance has been neglected until now, largely because a simple and reliable assay for measuring hydrolysable tannins has not been available. In this project, many browse species were found to be rich in hydrolysable tannins, which can be toxic at high levels, highlighting the need to measure both condensed and hydrolysable tannins. These new methods will provide impetus to the researchers trying to quantify and ascertain the biological significance of hydrolysable tannins. This project has also led to the simplification of one of the key and most sensitive methods existing for determining the protein precipitation capacity of tannins, which will be very beneficial to a large number of laboratories in developing countries.

The methods developed and simplified in this project are robust, fast and inexpensive, and provide a useful tool for screening and identifying the suitability of tree foliages as animal feeds. These advances will improve greatly the capacity to explore native plant resources in developing countries for non-conventional food resources for livestock production.

Correlating tannin assays to biological value

The question of which tannin assay(s) best predicts the nutritive value of tannin-rich feeds when fed to livestock has challenged livestock scientists for many years. The results from this project have partially answered this question. There were significant correlations between the tannin concentrations and the in vivo apparent digestibility of protein (IVADP), indicating that there is potential to use these methods to evaluate the nutritive value of tannin-containing feed resources. In addition, the measurements of total phenols and total tannins were closely correlated to in vitro indicators of rumen function and to IVADP, which suggests that these assays can provide information on the biological activity of tannins. These two assays are simple, quick and do not require expensive equipment. This makes them practical to use in developing countries.

Approaches for better utilization of browses

It is possible to ‘inactivate’ tannins by treating the materials with polyethylene glycol (PEG) prior to feeding, which enhances the nutrient availability from tannin-rich feeds. However, the high cost of PEG limits its use in practice. If a cheaper alternative to PEG could be identified, it would increase the range of plants available as feed resources.

A number of approaches for detannifying browse species were examined during this project, and one promising approach was using alkali. Wood ash and urea, both cheap source of alkali, were effective in reducing the tannin content and could be used in the field. However, the effect of alkali treatment was found to be variable in this study and requires more research to identify the conditions under which it will be effective.

Addition of small amount of tannin-rich Acacia leaves in the diet has been shown to enhance the utilisation of good quality dietary protein by protecting them against degradation in the rumen and making them available to the animal post ruminally.

Over 40 new feed resources were evaluated for tannin types and biological activity. The information generated has lead to the identification of several protein-rich and tannin-low feed resources for use in animal diets.