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Independent digestion trials with 5 forages were conducted to compare n-alkane with indigestible acid-detergent fiber (IADF) as internal markers to predict in vivo dry matter digestibility (digestibility). Forages were mixed grasses from subirrigated meadow (meadow), meadow regrowth (regrowth), native range (range), mature mixed grass hay from meadow, and alfalfa (Medicago sativa L.) hay. Meadow, regrowth, and range diets were immature grasses harvested 0.5 hours before feeding. Feces from the meadow hay and alfalfa hay trials were divided to compare freeze drying and oven drying (60 degrees C). All diets were subjected to in vitro fermentation for 0, 48, or 96 hours. N-alkane was separated from samples by 4.5-hour saponification with alcoholic KOH followed by extraction with n-hexane. Indigestible ADF was measured by 96-hour in vitro fermentation followed by ADF extraction. Digestibility estimated by markers was compared with in vivo digestibilities. N-alkane based digestibilities were lower (P < 0.01) than in vivo digestibility for all diets. N-alkanes provided higher estimates of digestibilities than IADF for meadow (P < 0.01), regrowth (P = 0.06), and alfalfa hay (P = 0.06), and lower digestibility for meadow hay (P = 0.02). Digestibilities calculated using n-alkanes for range tended to be higher (P = 0.14) than IADF values. Freeze drying increased (P < 0.01) the amount of n-alkane extracted from alfalfa hay, but did not affect (P = 0.1) the amount extracted from meadow hay. N-alkane disappeared (P < 0.001) from residue collected after 48 hours of in vitro fermentation, but no additional disappearance (P = 0.78) was evident at 96 hours. Neither marker was completely recoverable, although recovery of n-alkane was higher than indigestible ADF for 4 of the 5 forages tested.