Bromatological composition in leaves of guayule grown in a Mediterranean environment

Guayule is a perennial shrub endemic to northern Mexico, which has received attention as an alternative source of natural rubber. As the guayule leaves represent a byproduct, their exploitation might contribute to improve the sustainability of whole rubber chain. In semiarid regions where guayule grows wild its leaves are grazed by sheep, goats and rabbits, even if information regarding cromatological traits are still missing. Native shrubs are essential dietary sources for ruminants in Southern Europe to alleviate feed shortages. It is important to investigate if guayule leaves might be an opportunity for feeding livestock in Mediterranean area. Within a research aimed at evaluating the guayule performances, our objective was to investigate the bromatological composition of guayule leaves in view of their exploitation as a fodder resource. Guayule was grown on experimental plots in Southern Sardinia, Italy. In late summer, leaf samples were hand plucked, to simulate grazing, from undisturbed three-year-old plants. Leaves were weighted, oven dried at 65 °C, then ground to 1 mm screen. Total N was determined using Kjeldahl method and crude protein (CP) was calculated by multiplying the N content by 6.25. Neutral and acid detergent fibres (NDF and ADF) and acid detergent lignin (ADL) were determined by using the procedure of Van Soest et al. (1991) and ether extract using (EE) Soxhlet extraction. Total digestible nutrients (TDN), digestible dry matter (DDM), dry matter intake (DMI), relative feed value (RFV) and net energy for lactation (NE l ) of leaves were estimated according to the equations adapted from Lithourgidis et al. (2006). Green leaves represented up to 15% dry of shoot biomass. Leaf DM content was in the range 100 - 130 g kg -1 dry matter (DM). CP concentration of leaves ranged from 137 to 175 g kg -1 DM. NDF did not exceed 374 g kg -1 DM and ash content ranged from 180 to 230 g kg -1 DM. On average, the values of TDN, DDM, DMI, RFV and NE l in leaves were 774 and 775 g kg -1 DM, 33.4 g kg -1 of body weight, 193% and 1.84 Mcal kg -1 , respectively. As regard Mediterranean conditions of Sardinia, CP of standing hay (i.e., ungrazed dry residuals in natural and improved pastures) decreased to 4% in summer, whereas the CP values in guayule leaves reached 17.5%. It is worth of note that leaf RFV exceeded 151 value, which is indicative of a prime forage. The potential of guayule to maintain green leaves at the end of summer coupled with its peculiar chemical composition can be exploited for compensating seasonal fodder shortages associated to drought season and/or the delayed establishment of annual forage crops. Our results give clues that guayule has the potential to supply high quality green forage for complementing conventional forage resources of Mediterranean farming systems. Results also encourage further investigations dealing with the chemical characterization of protein fractions, palatability and bioactive compounds in guayule leaves.