Phenolic profile of shrub live oak and its relation to goat diets in central Arizona.

Abstract

The occurrence and concentration of substances like glycosides, alkaloids, certain amino acids, and, especially, phenolics in vegetation has been seen as a defense against environmental perturbations. Whether this phenomenon is associated with the disruptive effect of man, herbivore predation or the invasion of arthropods and microorganisms remains to be ascertained by ecologists. However, contemporary research shows that most so-called secondary metabolites in plants render them less sensitive to adverse factors in the natural environment. Likewise, herbivores that adapted to chemical defense in plants were rewarded with wider food source and might have been able to more efficiently exploit rapidly changing foraging environments. This study focuses on relationships between goats, their ruminal microbes and tannin-rich plants in a central Arizona chaparral vegetation type dominated by shrub live oak (Quercus turbinella). The study tested hypotheses involving goat dietary selection, estimated through microhistological analysis of feces; phenolic profile of shrub live oak, expressed as condensed and hydrolyzable tannins, protein binding capacity of oak leaves extract, expressed as bovine serum albumin (BSA) precipitation; and apparent dry matter digestibility of mature and immature oak leaves by goat ruminal microbes measured at 6, 12, 24 and 48 hours. Findings of this research indicated that goats prefer leaf and shoot ends of shrubs, mainly shrub live oak, despite the constant high concentration of tannins in that plant. Hydrolyzable tannins in oak are synthesized during summer/fall and late spring, and the synthesis of condensed tannins is mostly restricted to winter and early spring. Precipitation of BSA by extract of oak leaves, apparently associated with hydrolyzable tannins, did not discourage oak consumption by goats. The digestibility of younger and older oak leaves was more influenced by time of incubation than level of phenolics in plant tissue. This research emphasizes the adaptability of goats to ecological zones where presence of toxins and digestion reducing compounds in plants adversely affects survival and fitness of non-adapted herbivores.