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This study was conducted to evaluate the effect of harvesting season on chemical composition, in vitro gas production and tannin biological activity of Prosopis juliflora pod. P. juliflora pods were collected during dry (May/June) and wet (September/October) seasons from three districts (Asayta, Dubti and Amibara) of Afar region, Ethiopia. Dry matter, ash, neutral detergent fiber, acid detergent fiber, acid detergent lignin, total phenols, total
tannin and condensed tannin content were higher (P<0.05) for pods collected during dry season while crude protein content was lower (P<0.05) for dry season pods. Gas production at 24h incubation, gas production from immediately fermentable fraction (a), gas production from water insoluble but potentially fermentable fraction (b), potential gas production (a+b), the rate constant of gas production (c) were higher (P<0.05) for pods collected during wet season whereas, lag time (L) of P. juliflora pod were higher (P<0.05) during the dry season. Lower (P<0.05) organic matter digestibility, metabolizable energy and short chain fatty acids contents were recorded at 24h of incubation of pods collected during dry season than those of wet season. The tannin activity was higher (P<0.05) for pods collected in the dry season (7.76%) than in the wet season (4.16 %). The overall results of this study revealed a decline in crude protein, organic matter digestibility, short chain fatty acid and metabolizable energy of P. juliflora
pods during the dry season. Therefore, it is recommended to use different strategies such as supplementation of pod by mixing with better quality feeds or application of physical and chemical treatments to reduce the impacts of dry season on nutritional quality of P. juliflora pod.