Browse tree leaves in sheep and goats nutrition

Abstract

Tree and shrub leaves digestibility in herbivores may be restricted by negative effects on digestion by generally high levels of secondary compounds (Provenza, 1995; Salem, 2005; Salem et al., 2006), such as phenolics, saponins, alkaloids, lectins. High levels of secondary compounds will likely negate at least some negative effects, for example, depressing feed intake, impairing digestibility and/or having a toxic effect on rumen microorganisms (Mangan, 1988; Salem et al., 2006). Most tropical browse species contain substantial amounts of phenolic compounds, mainly tannins (Makkar and Becker, 1998; Salem, 2005) which can reduce their nutritional value, as most tannins bind to feed proteins thereby making them unavailable to ruminal microorganisms and post-ruminally. Thus, use of high tannin browse species as supplements to crop residue based diets may not increase the productivity of animals as ruminally available N frequently limits ruminal microbial growth and subsequent degradation of structural carbohydrates. Goats are effective browsers, having the ability to utilize woody species and low-quality forages better than cattle and sheep, and can adapt to harsh environments (Tisserand et al., 1991; Silanikove, 2000a, 2000b; Salem et al., 2004a). Attempts have been made to deactivate tannins, and other secondary compounds, in temperate and tropical forages. Polyethylene glycol (PEG), a non-nutritive synthetic polymer, has a high affinity for phenolic compounds, especially tannins, and thereby deactivates them by forming tannin-PEG complexes (Makkar et al., 1995b). Thus, PEG can prevent the formation of complex with tannins or liberate protein from tannin-protein complexes (Barry and Manley, 1986), and it has been used to mitigate adverse effects of secondary compounds on rumen fermentation, as well as improve performance (i.e., growth and milk yield) of ruminants browsing diets high in secondary compounds. For example, addition of PEG to browse and herbaceous legumes high in secondary compounds increased in vitro gas production and short chain fatty acid production, although microbial N production and efficiency of microbial protein synthesis decreased (Norton and Ahn, 1997; Getachew et al., 2001; Ammar et al., 2010a; Salem et al., 2010a). Degen et al. (1998, 2000), in a study to determine the nutritive value of four browse tree species in terms of nutrient and secondary compounds as well as to assess the capability of PEG added to the diet to mitigate adverse effects of secondary compounds on feed intake and nutrient digestibility in sheep and goats, used Acacia saligna (a tannin rich leguminous shrub species), and suggested that effects of PEG may persist for up to 14 days in sheep and goats after PEG feeding is terminated. Therefore the nutritional value in vivo y in vitro of some tree fodders in sheep and goats will be discussed in this chapter. © 2011 Nova Science Publishers, Inc.