The growth and carcass performance of sheep reared under integration in the mature oil palm plantation and feedlot system were studied using a systems approach. Simulation models of the growth and carcass of the Dorset x Siamese Long Tail (DSLT) and Dorset x Malin (DMalin) sheep were described. The study indicated that within 120 days of the experimental period, the DSLT and DMalin sheep reared in the feedlot system grew at the rate of 103.2 grams per day while the grazing sheep in the manure oil palm canopy grew only 43.3 grams per day (P>0.01). Although the sheep in the feedlot systems, the fattening sheep had a higher MEI (metabolisable energy intake) per metabolic weight than the grazing sheep (0.71 vs. 0.42 MJ ME/kg LW (0.75)/day; P<0.01).