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The aim was to determine effects of addition of polyethylene glycol (PEG) on in vitro gas production and microbial protein synthesis of the shrub species Acacia constricta, Acacia shaffneri, Calliandra eriophylla, Condalia lycioides, Cordia parvifolia, Larrea tridentata and Mimosa biuncifera, which are commonly consumed by grazing small ruminants in semi-arid regions of Mexico despite the abundance of tannins in their foliage. Three individual samples (a minimum of 10 plants of each) of each shrub species were collected from different areas and prepared for chemical and secondary metabolite analysis. In vitro evaluations for each sample of each shrub species were completed in three incubation runs in different weeks using calibrated glass syringes in a 7 x 2 factorial design (i.e., 7 shrub species x 2 treatments with or without PEG). In vitro gas production after 24 and 96 h, partitioning factor (PF), metabolizable energy (ME) content, efficiency of microbial protein synthesis (EMPS), and volatile fatty acids (VFA) were determined. A shrub species x PEG interaction (P<0.05) only occurred for some VFA, such as C3, C4, C4i and C5i, concentrations. Relative to control, addition of PEG increased (P<0.05) the concentrations of individual VFA in A. shaffneri, C. lycioides and C. parvifolia. Interactions between shrub and PEG occurred (P<0.001) for all gas production parameters (i.e., b, k, IVGP(24), IVGP(48)). The fractional rate of gas production and IVGP(24) (h) were highest in A. constricta and lowest (P<0.001) in A. shaffneri. L tridentata had the highest PF and C. lycioides the lowest. A. constricta had the highest ME content. Purine content and EMPS differed (shrubs x PEG; P<0.001) among shrubs. Microbial protein synthesis was highest in M. biuncifera and lowest in C. eriophylla, while total VFA were highest in C. eriophylla. Incorporation of PEG increased (P<0.001) in vitro fermentation parameters and ME content with L. tridentata and C. lycioides being the highest. PEG also promoted reductions in the EMPS and PF values. In vitro fermentation variables such as fractional rate of gas production. VFA concentrations and EMPS support the potential of A. constricta, C. parvifolia and M. biuncifera as protein and energy sources for small ruminants in semiarid regions of North Mexico. Increments in gas production in these shrub species confirmed the affinity of PEG to bind condensed tannins and reduce EMPS. (c) 2012 Elsevier B.V. All rights reserved.