Seasonal dynamics of nutritional quality of California chaparral species

This study determined the seasonal dynamics in the chemical composition and in vitro digestion properties in leaf and stem of 11 chaparral species collected monthly during fall, spring, and summer at two sites in California, USA: (i) Hopland (Adenostoma fasciculatum, Arctostaphylos canescens, Arctostaphylos glandulosa, Arctostaphylos stanfordiana, Quercus durata, Ceanothus cuneatus, Eriodyction californicum, and Baccharis pilularis) and (ii) Sierra (Quercus wislizenii, Quercus douglasii, Ceanothus cuneatus, and Heteromeles arbutifolia). Among the browse species studied, stems of A. canescens collected in fall showed the lowest concentration of crude protein (CP), while C. cuneatus (in both sites) and B. pilularis leaves in spring showed the highest. Neutral detergent fibre (aNDF) and acid detergent fibre (ADF) contents were lowest in spring for leaves of E. californicum and C. cuneatus, respectively and highest for fall stems of C. cuneatus (Sierra). All species contained condensed tannins (CT) except B. pilularis and E. californicum. Soluble CT concentration was lowest in fall leaves of A. fasciculatum and highest in leaves and stems of A. canescens throughout the seasons. Condensed tannins from C. cuneatus (Sierra and Hopland) had the lowest ability to precipitate protein (avg = 0.02 mg BSA/mg SCT), whereas Quercus spp. had the greatest (avg = 0.16 mg BSA/mg SCT). In vitro fermentation study using bovine rumen fluid revealed that leaves of E. californicum in spring and stems of A. fasciculatum in fall had the highest and lowest values of in vitro organic matter digestibility (IVOMD), metabolizable energy (ME), and gas production, respectively. Fermentation rate (γ) varied from 0.03 to 0.096 ml/h in leaves and from 0.03 to 0.09 ml/h in stems. Compared to the leaves, stems of all species had the lowest concentration of CP, IVOMD, ME and the highest concentration of fibre aNDF and ADF. Among all chemical compounds and fermentation parameters examined, only CP content from most species showed a clear tendency to decline from spring to fall. Linear regression showed that aNDF and ADF contents, as well as CT reactivity are the main constraints for the gas production of chaparral browse plants by ruminants. All these data suggest that predominant California chaparral species do not represent a high quality feeding source for ruminants used to reduce fuel load in chaparral. However, plant's nutrient content and digestibility attributes serve as a guide to the feeding value of forage species, but they are not sufficient to determine its degree of utilization by ruminants. Accurate assessment of the feeding value of these browse species requires measurement of animal response to free choice selection studies.