The experiment was conducted (a) to determine the relationship between in vitro digestible organic matter (IVDOM) and in vivo digestible organic matter (DOM); (b) to quantify the monthly variation in chemical composition and digestibility of medic pasture; and (c) to compare the chemical composition and digestibility of manually collected pasture samples with those collected in oesophageally fistulated (OF) sheep. The equation, in vivo DOM = 0,643 IVDOM + 24,5 (r= 0,91), was derived from the data and may be used to convert IVDOM to in vivo DOM contents. Significant (P ≤0,01) variation was found in the CP (OM basis) (from 11,7 ± 1,5% to 37,5 ± 0,7%), CF (OM basis) (from 12,7 ± 1,0% to 42,1 ± 2,2%), IVDOM (from 36,4 ± 3,9% to 77,6 ± 4,9%), and in vivo DOM contents (from 47,9 ± 2,6 to 74,4 ± 3,0%) between months. A typical seasonal pattern, related to the amount of rain received, was observed with significant (P ≤0,01) differences in CP, IVDOM and in vivo DOM contents between months. The CP content was found to be 15,4 ± 5,8% in the summer, 27,1 ± 8,1% in the autumn, 30,8 ± 3,7% in the winter, and 20,8 ± 7,3% in the spring. The corresponding IVDOM content was determined to be 50,4 ± 8,5% (summer), 61,3 ± 11,3% (autumn), 74,2 ± 4,0% (winter), and 64,3 ± 7,4% (spring). Sheep selected pasture samples with a higher percentage CP than those of manually collected samples during the wet season. Fistula sample values (f) may be predicted from manually collected samples (h) by the equations, CPf = – 0,76 + 1,5 CPh (r= 0,82), CFf = – 2,8 + 0,99 CFh (r= 0,74) and IVDOMF = 26,5 + 0,61 IVDOMH (r= 0,87).