Tagasaste contains antinutritional factors including polyphenols, tannins and alkaloids.
Tannins
Tannin content is variable and depends on the origin of the plant, the cultivar, the preservation process (raw, wilted or dried), the age of regrowth, etc. (Borens et al., 1990; Kumara Mahipala et al., 2009b; Assefa et al., 2012; Assefa et al., 2008b; Chinea et al., 1998). Tannin levels found in different experiments are summarized in the table below:
Table 1: Levels of condensed and hydrolysable tannins.
Country | Total tannins | Condensed tannins | Hydrolysable tannins | Results | Reference
New Zealand | No tannins found in the leaves | - | - | - | Borens et al., 1990
Australia | 8.9 g/kg DM | - | - | - | Kumara Mahipala et al., 2009b
Canary Islands | - | 0.1 to 0.45 g/kg DM | - | - | Chinea et al., 1998
Ethiopia | - | 7 to 32 g/kg DM | 16 to 197 g/kg DM | HT decreased with the cutting age of regrowth. All but one accession had CT lower than 26 g/kg DM. | Assefa et al., 2008b
In Ethiopia, hydrolysable and condensed tannins were measured on 65 accessions of leaves at different ages of regrowth and treatments (fresh, wilted or dried) (Assefa et al., 2008b). Despite its great variability, the condensed tannins content was below the threshold of 50 g/kg DM which is considered to have a negative effect on voluntary intake and nitrogen digestibility in ruminants (Barry et al., 1999). In vitro DM degradability was tested with or without polyethylene glycol, which is known to alleviate the tannins' effects. No differences were found, suggesting that the level of tannins had no effect on degradability (Kumara Mahipala et al., 2009b). Moreover no bloat occured when animals grazed tagasaste (Cook et al., 2005).
Alkaloids
In the Canary Islands, alkaloids were present in the leaves of several morphological types, ranging from 0.6 to 11.3 g/kg DM (Muzquiz et al., 1996; Ventura et al., 2000). In Ethiopia, lower levels from 0.024 to 0.053 g/kg DM were found in the leaves, although higher levels were observed in branches, barks and stems with 0.20, 0.24 and 0.14 g/kg DM, respectively (Assefa et al., 2008b). The alkaloid content also varied with the type and accession (Muzquiz et al., 1996; Ventura et al., 2000). Sparteine is the main alkaloid and represents about 90% of alkaloids in most of the analyzed types (Muzquiz et al., 1996; Ventura et al., 2000).