Preference of goats (Capra hircus L.) for tanniniferous browse species available in semi‐arid areas in Ethiopia

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Trees and shrubs commonly known as browse significantly contribute to ruminant nutrition in arid and semi‐arid areas of the world. Browse species make up a large proportion of goat diets under traditional farming systems in Ethiopia. Average crude protein (CP) and neutral detergent fibre (NDF) content of nine browse species were 145 and 334 g/kg DM, respectively, making them nutritionally superior to five tropical grasses with 76 g/kg DM CP and 754 g/kg DM NDF, both harvested at the vegetative stage from semi‐arid Ethiopia (Yayneshet et al., 2009). Browse species are also recognized for possessing plant secondary metabolites (PSM) such as polyphenols which may account for 50% of the organic matter (Reed, 1986). Over the past four decades, condensed tannins have received much attention for their beneficial roles in ruminant nutrition. In vitro and in vivo anthelmintic properties in sheep (Brunet et al., 2007), in vitro (Bezabih et al., 2014) and in vivo methane reduction in goats (Puchala et al., 2005) and dairy cows (Huyen et al., 2016a) are the widely reported positive attributes of certain tannin‐containing tropical and temperate forages. On the other hand, condensed tannin concentrations of more than 5% DM have been reported to reduce voluntary feed intake, digestibility and lower productivity of grazing ruminants (Min et al., 2003). However, this threshold level was based on temperate forages and did not account for the type of ruminant species nor the dietary niche or experience with tannin‐containing forages of the animal.
Tannin concentrations and structural characteristics vary widely among species (Abdulrazak et al., 2000) and tannin sources (Huyen et al., 2016b). The wide variation in characteristics among browse species provides choices for browsing herbivores such as goats. Preference involves the interaction between sensory input (taste, smell, sight) and post‐ingestive feedback (Provenza, 1996). The latter is non‐cognitive and influenced by early‐life experience. Herbivores routinely exposed to tanniniferous diets usually adapt in terms of behaviour and physiology while satisfying their nutrient requirements. A common adaptation to tannin ingestion in domestic and wild herbivores is increased secretion of proline‐rich proteins in saliva to form a complex with tannin (Shimada, 2006). Intrinsically, the saliva of goats is high in proline‐rich proteins regardless of previous exposure to tannins (Ventura‐Cordero et al., 2015). Another mechanism of herbivores is diet diversification, which also explains the trade‐off between ingesting nutritious feed and minimizing detrimental effects of PSM ingestion (Alonso‐Díaz et al., 2010). Chemical and physical characteristics of feed, physiological stage and previous dietary experience determine preference in goats (Morand‐Fehr, 2003). Preference studies can be used as a tool to evaluate tropical tannin‐containing forages (Alonso‐Díaz et al., 2010), and the use of a tannin binding agent, polyethylene glycol (PEG), enables the investigation of feeding behaviour of goats browsing on tannin‐containing woody species (Decandia et al., 2008). Landau et al. (2002) reported an increased intake of lentisk (Pistacia lentiscus L.), a tannin‐rich forage, by naturally browsing Mediterranean goats having free access to PEG (4000) flakes compared to their control counterparts that did not receive PEG. In another study, supplementation of 50 g/day of PEG (4000) to Sarda goats browsing in lentisk (P. lentiscus L.) dominated Mediterranean scrubland increased the proportion of lentisk in the diet compare to the PEG‐unsupplemented group (Decandia et al., 2000).
Although the nutritional value of some browse species in the Tigray Region of northern Ethiopia has been studied (Yayneshet et al., 2009; Melaku et al., 2010), information on the preference of these browse species by goats is scarce. Knowledge of the browse species preference of goats is important to develop strategies for effective utilization of browse species growing in sub‐Saharan Africa.
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