Ensiling - a method for reducing tannin contents in red sorghum breeds

Autor: Hartmann, A.; Friedel, K.; Díaz Casas, R.F.; Ott, E.M. und Gabel, M.
In:

Proceedings of the 3rd International Conference on Agricultural Development and Sustainability, June 14 - 16, 2005, Santa Clara, Kuba

Jahr: 2005

Einordung:
Institut: Professur Ernährungsphysiologie und Tierernährung

Abstract:
Introduction
Sorghum is an unaspiring cereal and therefore especially suitable for the forage supply under subtropical and tropical conditions as to be found in countries like Cuba. Compared to white sorghum the red breeds are characterized by high tannin contents and high resistance to fungal decay and peck marks. Nevertheless, the high tannin contents of red sorghum affect the feed value negatively and limit its use in the feed of ruminant and especially monogastric animals.
Concerning the fodder preservation in Cuba, ensiling is the recommended method as the air humidity is relatively high throughout the year. A reduction of the tannin contents during the process of ensiling is possible and could be a promising way in order to improve the nutritional value of red sorghum.

Objectives
For this study two main objectives were summarized:
• Determination of the ensilability of red sorghum and examination of the influence of ensiling additives (molasses, lactic acid bacteria)
• Investigation of ensiling as a possible method for reducing the contents of condensed tannins

Material and Methods
Ensiling experiments with sorghum panicles of two red breeds (C.I.A.P.2E, SD51-2-2) with different tannin contents were carried out at the University of Santa Clara in Cuba. The sorghum panicles were harvested at five maturity stages (from early milk to soft dough) in order to document the changes in tannin and phenol contents during vegetation as well as the suitability of the panicles for ensiling.
For the ensiling experiments 600g of plant material were filled into plastic bags, compressed and the air was exhausted using a vacuum pump. The bags were incubated for 21 days at room temperature.
Four silage treatments were conducted:
- control (without additives)
- addition of 4% molasses
- addition of 3x105cfu/g FM of Lactobacillus rhamnosus (LBR) (isolated at C.I.A.P.)
- combined addition of molasses and Lb. rhamnosus.
Each treatment was carried out in triplicate. The fractions of condensed tannins (extraction with butanol-HCl; acc. to Porter et al. 1986) and total phenols (staining with folin-ciocalteus-solution; acc. to Julkunen-Tiito 1985) were determined before and after ensiling. As parameters for silage quality the pH, content of alcohols and the volatile fatty acids as well as an organoleptic evaluation (smell, structure, colour) were used.

Results
• Determination of condensed tannins and total phenols in sorghum panicles
Table 1 and table 2 show a continuous increase of condensed tannins as well as total phenols during vegetation. For SD51-2-2 higher contents of condensed tannins and total phenols compared to C.I.A.P.2E were observed at nearly all maturity stages.

Table 1: Content of condensed tannins (% of DM) in sorghum panicles related to different maturity stages (n=3)

Table 2: Content of total phenols (% of DM) in sorghum panicles related to different maturity stages (n=3)

During the ensiling process a reduction of the tannin and phenol contents took place in all silages among all treatments (see table 3 and table 4). Compared to the initial contents the tannins were reduced in average by 53% and 43% in the silages of C.I.A.P.2E95 harvested at early dough and soft dough. SD51-2-2 showed a decrease of 59% at early dough and 70% at soft dough.
As a result of the maturity stage the energy contents of the sorghum panicles were highest at the last two harvest dates. These are the most adequate panicles to feed in practice. Therefore table 3 and table 4 show only results for those maturity stages. However, the reduction rates at milk maturity stage varied between 50% and 69% though the initial contents were very low (see table 1).

Table 3: Influence of maturity stage and silage additives on the content of condensed tannins (% of DM) in sorghum panicle silages (n=3)

Table 4: Influence of maturity stage and silage additives on the content of total phenols (% of DM) in sorghum panicle silages (n=3)

Concerning the contents of total phenols a reduction of 28% (early dough) and 21% (soft dough) could be observed in C.I.A.P.2E95. In sorghum SD51-2-2 the contents were lowered by 30% (early dough) and 31% (soft dough).


• Examination of the quality of sorghum panicle silages
As it is shown in table 5 the pH values of the silages document a good ensilability of the used plant material. Both breeds at all treatments are below the dry material corresponding critical pH of 5.0 (C.I.A.P.2E95 early dough, critical pH of 4.75) described by WEISSBACH (1993). The combined use of lactic acid bacteria and molasses affected the acidification process favourably. PH values as low as 3.7 – 3.8 could be observed with this treatment.

Table 5: Influence of maturity stage and silage additives on the pH values of sorghum panicle silages (n=3)

The results presented in table 6 confirm the conclusion drawn above. The use of a combination of LBR and molasses could increase the production of lactic acid in the silages remarkably. In general the contents of lactic acids were higher at C.I.A.P.2E95.
The contents of acetic acid were generally low. At early dough and soft dough stage maximum values of 0.4% were observed. Butyric acid was not found in any of the silages. The contents of ethanol varied from 0.2% to 1.5% among the different breeds and treatments. The lowest values were determined in the LBR treated variant (0.2% to 0.3%).
Taking an organoleptic examination the silages were classified into categories “very good” and “good” according to the “DLG evaluation instruction” (GROß & RIEBE 1974).

Table 6: Influence of maturity stage and silage additives on the lactic acid contents (% of DM) of sorghum panicle silages (n=3)

Conclusion
The objective of this study was to investigate if ensiling could be a possible method to reduce antinutritive substances as condensed tannins and phenols in red sorghum breeds and to examine the ensilability of this feed.
The determination of condensed tannins and total phenols revealed an increase of both fractions during the ripening process of the sorghum panicles. During ensiling of the plant material the contents of condensed tannins were reduced for up to 70%. The contents of total phenols were lowered for up to 31%.
The pH values and the contents of lactic acid which were found in the silages prove a good ensilability of the sorghum panicles. This confirms the results of the organoleptic assessment. Adding a combination of lactic acid bacteria and molasses seemed to be advantageous for silage quality.
Thus, ensiling panicles of red sorghum breeds is a suitable method both to preserve this forage and to reduce its contents of condensed tannins and total phenols.


Literature
Groß, F. and Riebe, K. (1974): Gärfutter. Verlag Eugen Ulmer, Stuttgart
Julkunen-Tiito, R. (1985): Phenolic constituents of leaves of Northern Willows: methods of analysis of certain phenolics. Journal of the Science of Food and Agriculture 33, 213-217
Porter, L.J., R.Y. Wong, H.F. Benson, B.G. Chang, V.N. Viswanadhan, R.E. Gandour and W.L. Mattice (1986): Conformational analysis of flavan. Journal of Chemistry Research 86, 830
Weißbach, F. (1993): Grünfutter und Grünfutterkonservate. In: Jeroch, H.; Flachowsky, G. and Weißbach, F. (ed.): Futtermittelkunde. Gustav Fischer Verlag, Jena, Stuttgart

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