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Inoculum is the single most important source of variation in in vitro rumen fermentation systems used to predict forage quality. This investigation was carried out to study the effects of donor animal diet and donor animal species on the activity of the inoculum (whole rumen fluid) used in the two stage in vitro fermentation technique for estimating forage digestibility. Two wethers and two steers were fed either wheat straw (Triticum aestivum L.) smooth bromegrass (Bromus inermis Leyss.) alfalfa (Medicago sativa L.) hay (hay) or barley (Hordeum vulgare L.) and dehydrated alfalfa (1:l) pellets (barleydehy) in a double switchback arrangement with three periods of 28 days. Nineteen forages including alfalfa, smooth bromegrass, crested wheatgrass [Agropyron desertorurm (Fisch ex Link) Schult.] and oat (Avena sativa L.), wheat and barley straws were digested in vitro and in vivo by steers. Each substrate was digested in duplicate during three successive periods with inoculum from sheep and cattle consuming each of the three diets. Donor diet had a significant effect on in vitro dry matter (IVDMD) and acid‐detergent fiber digestibility (IVADFD) with inoculum from sheep and cattle. Values were lowest (P<0.001) with inoculum from animals fed straw and were highest when hay was fed. In vitro digestibilities were higher (P<0.001) with sheep inoculum than with cattle inoculum on straw and barley‐dehy diets; values were highest (P<0.001) with cattle inoculum on the hay diet. Estimates of in vivo digestibility using IVADFD were not significant. Linear regression equations, correlations, and standard errors showed a significant relationship (P<0.01) between in vivo dry matter digestibility and IVDMD for inoculum from both sheep and cattle fed either straw, hay, or barley‐dehy. The best fit of cattle in vivo dry matter digestibility was to IVDMD using inoculum from cattle fed the pelleted barley‐dehy diet (r=0.88; Sy.x, ± 2 2.49; P<0.01).

SrEecv, G. M., Csnrs'rrxssN, Devro A., CocHneN

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