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Two rumen cannulated sheep were grazed on semi-arid rangeland in a yearlong study to assess the seasonal changes in dietary composition and rumen fermentation pattern. Average CP, ADF and lignin content of the pasture during monsoon, winter and summer were 9.43, 4.50 and 3.80; 28.73, 51.38 and 53.84 and 4.60, 11.52 and 16.59% respectively. The pH values of the rumen liquor in monsoon, winter and summer season were 6.86, 6.90 and 7.11 respectively. Ammonia nitrogen (mg %) significantly (P<0.05) declined from 22.86 in monsoon to 5.28 in winter and 3.32 in summer. Total nitrogen (%) of rumen liquor in monsoon and winter was almost similar (2.1) and significantly (P<0.05) increased in summer (5.7). Total volatile fatty acids (TVFA) concentration (m mol/litre) increased from 71.45 in monsoon to 112.1 in summer. It is concluded that season has profound effect on dietary composition and rumen fermentation pattern of grazing sheep in semi-arid region.