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The objective of this experiment was to compare the effects of dietary supplementation with cooking grease recovered from rinse-trap water lines (rinse-trap grease; RTG) versus conventional supplemental fats (tallow; TL, and yellow grease; YG) on 84-d growth performance, dietary energy, and carcass traits of feedlot lambs. Forty-eight Pelibuey × Katahdin lambs (27.7 ± 3.4 kg) were assigned in a randomized complete block design to evaluate: 1) basal diet without supplemental fat (Control); 2) 4% TL; 3) 4% YG, and 4) 4% RTG. Supplemental fats replaced maize in the control diet. Rinse-trap grease contained greater moisture (16.5 vs 0.92%) and impurities (3.6 vs 0.56%), and less total fatty acid (64.90 vs 89.60%) than that of conventional fats (TL and YG). Daily weight gain and gain efficiency were similar for control and RTG supplemental lambs, whereas ADG and gain efficiency were greater for lambs fed conventional fats than control or RTG-supplemented lambs. Both dietary net energy (NE) as well as ratio of observed-to-expected dietary NE were 4% greater for lambs supplemented with conventional fats vs RTG. Supplemental fat increased fat deposition but did not affect any other carcass measures or non-fat visceral mass. Estimated NE value for RTG was 57% of the average NE value (6.11 Mcal/kg) of tested conventional fats. Supplementation with RTG does not affect diet acceptability, and accordingly, is a suitable energy source for feedlot lambs. However, due to its lower total fatty acid content, its energy value is much lower than conventional supplemental fats. [ABSTRACT FROM AUTHOR]