Effect of energy and protein levels on feedlot performance and carcass characteristics of Mehraban ram lambs.

Fifty four Mehraban ram lambs (6-to 8-month old, initial live weight 35.4 +/- 4.2 kg) were assigned to a completely randomized design consisting of 9 groups and were fed for 70 days with diets containing three levels of energy (2.3, 2.5 and 2.7 Mcal metabolizable energy per kg dry matter) and three levels of protein (10.5, 12.5 and 14.5 percentage in dry matter). Either energy or protein levels alone significantly affected most of the parameters of lamb performance, but their interaction effect was significant only for feed conversion ratio, cold carcass weight, tail weight, flap weight and back fat (subcutaneous fat) depth. The lowest level of energy (2.3 Mcal ME per kg DM) resulted in a significant decrease in lamb performance as compared with other energy levels. Increasing energy concentration of the diet resulted in significant increases in fat percentage, but significantly decreased the moisture and protein content of the Longissimus dorsi muscle. Increased dietary protein level increased the daily DMI and ADG and at the same time improved the FCR. Hot and cold carcass weights increased significantly with increasing dietary CP levels, but dressing percentage was similar amongst the dietary protein densities. Dietary CP levels had no significant effect on the chemical composition of the Longissimus dorsi muscle. At the lowest energy level (2.3 Mcal ME per kg DM), dietary protein level had a significant effect on FCR (Table 4); with the diet containing 10.5% protein having the highest FCR. At the medium and low energy levels the lowest level of dietary protein concentration resulted in smaller carcasses. The highest level of protein along with the medium energy concentration resulted in smaller tail weights. Flap weight was significantly smaller at low energy concentration along with medium and low protein level. The lowest back fat depth was found in lambs fed on the low energy diet containing medium to high levels of protein.