Micromineral status of free-ranging Zebu cattle and their salivary response to dietary tannins

Objectives: The decrease of available feed resources of free-ranging cattle in eroded areas coincides with the emergence of plant species that use antinutritional factors like tannins as an evolutionary strategy to avoid them being grazed. Therefore, the occurrence of tannins in the vegetation can be considered as an indicator of relative overgrazing. Zebu cattle react to dietary tannins through salivary secretion of proline-rich proteins, hence salivary composition might be an estimator of differences in tannin pressure and thus overgrazing status among regions. Methods: Blood samples were obtained by venipuncture and saliva samples with a sponge, from one bull (2-4y) from each of 10 ranging zebu herds in each of 6 regions in the Gilgel Gibe catchment in Southeast. From each region, the main consumed plants were collected and their tannin concentrations were quantified. Plasma samples were analysed for microminerals (induction-coupled plasma spectrometry). Saliva samples were cleared from feed particles and bacteria by 0.3 µm filtration, freeze-dried, and analysed for total amino acid concentrations (after hydrolysis). Results: Average tannin concentrations per region ranged between 0 and 3.9% on dry matter basis. The ratio of proline to the sum of total amino acids in the saliva was significantly higher in tannin-rich regions. High tannin levels were associated with reduced plasma Cu, whereas Fe was not affected. The ratio of salivary arginine to ornithine was higher in tannin-rich regions. Conclusions: The data indicate that zebu cattle show increased secretion of proline-rich proteins in their saliva when more tannins are present in their ranging area, suggesting that salivary amino acid profile can be a useful tool to estimate the availability of feed resources for ranging cattle. The mineral binding capacity of tannins was probably responsible for the reduced Cu status, whereas the Fe status was not affected, likely due to its ubiquity in the region. The salivary arginine to ornithine ratio, as a measure of urea synthesis and hence ruminal protein breakdown, points to lower rumen degradable protein availability in high tannin regions.