Amino acid profile of the protein from whole saliva of goats and sheep and its interaction with tannic acid and tannins extracted from the fodder of tropical plants

The foraging response of goats and sheep to tannin-rich plants (TRP) could be influenced by their physiological adaptation to eat fodder from a specific ecological niche. The aims of this study were: (1) to describe the amino acid profile of the protein from whole saliva of adult goats and hair sheep browsing/grazing native vegetation, (2) to measure the interaction of whole saliva from goats and hair sheep with tannic acid, (3) to compare the tannin-binding capacity of lyophilized whole saliva from goats and sheep to precipitate tannins extracted from the fodder of tropical TRP (Acacia pennatula, Lysiloma latisiliquum, Leucaena leucocephala and Piscidia piscipula). Fresh whole saliva was obtained from 42 adult goats and 51 adult hair sheep. A second batch of saliva (obtained from 75 adult goats and 93 adult hair sheep) was lyophilized whole saliva. Protein from whole saliva in both animal species had a similar pattern of amino acid composition. Histidine and arginine were found, accounting for 30% and 24% of the amino acid content, respectively. Whole saliva from goats and sheep showed a linear dose response to increasing concentrations of tannic acid at each time point measured (P < 0.001). Saliva from goats was able to precipitate higher quantities of TRP extracts from A. pennatula and L. latisiliquum than from P. piscipula and L. leucocephala (P < 0.05). Saliva from sheep precipitated higher quantities of L. latisiliquum and P. piscipula extracts (P < 0.01). Also, salivary proteins from goats had higher capacity to precipitate tannins from A. pennatula than sheep, while salivary protein from sheep had higher capacity to precipitate tannins from P. piscipula than goats (P < 0.05). The results provide evidence of the differences between goats and sheep in the capacity of their saliva to interact with local sources of tannins, which could explain the different feeding behaviour of goats and sheep. The presence of histatins may help to explain the in vivo AH effects of TRP fodder.