Ecotypic variation in forage nutrient value of a dominant grassland species, Andropogon gerardii Vitman (big bluestem), was quantified across a longitudinal precipitation gradient of the US Great Plains. Ecotypic variation of A. gerardii has been documented across this gradient, but the extent to which forage nutrient value differs among ecotypes is poorly known. Seven indicators of forage nutrient value (neutral detergent fiber [NDF], acid detergent fiber [ADF], in‐vitro dry matter digestibility [IVDMD], crude protein [CP], crude fat [CF], ash content) and relative feed value [RFV] were examined in 12 populations representing four ecotypes corresponding with distinct climate regions: eastern Colorado, central Kansas, eastern Kansas and southern Illinois. Vegetative material of A. gerardii was collected from each population in July 2010. A greenhouse study tested the effect of watering regime on seedlings of the ecotypes from three of the precipitation regions grown under controlled conditions. Forage nutrient value indicators nitrogen and CP increased, and ADF decreased east to west, while IVDMD decreased across the gradient corresponding with less annual precipitation. The greenhouse experiment showed that sampling before and after water treatment affected forage nutrient value measurements, with the exception of NDF and CF. Nutrient value was most related to soil moisture and phenology, with smaller differences among ecotypes. Nutrient value of populations from the southern Illinois ecotype changed the least in response to variation in soil moisture. The southern Illinois ecotype will likely maintain forge nutrient value under variable precipitation projected to occur with climate change better than the ecotypes from more westerly parts of the range of A. gerardii.

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