A total of 700 day old female broiler chicks (Ross 308) were distributed into seven groupsin acompletely randomized designwith five replicates, and 20 chicks in each replicate. Dietary treatments, timeof the switch from an unsaturated sunflower oil (SFO) to a saturated tallow (T)dietary fat source, were: 1) SFO for the entire growth period (6 weeks), 2) SFO for 5 weeks followed byT for last week, 3) SFO for first 4 weeks followed byT for last 2 weeks, 4) SFO for first 3 weeks followed byT for last 3 weeks, 5) SFO for first 2 weeks followed byT for last 4 weeks, 6) SFO for first 1 week followed byT for last 5 weeks and 7) T for the entire growth period. Chicks had free access to water and experimental diets during the experimental periods.All diets werecalculated to beisocaloric and isonitrogenous. Body weight and feed intake of chicks fed different diets were recorded at 21 and 42 days. At the end of experiment (on 42 day), two birds were randomly selected from each pen and slaughtered and then breast, leg and thigh muscles and abdominal fatwere sampled to determine fatty acid profile. The SAS general linear models (GLM) procedure was used for the statistical analysis of data.There was statistically significant differences between dietary treatments for body weight, feed intake and feed conversion ratio during the period from 22 to 42 d and the entire growth period, with a higher values of body weight for chicks fed diets supplemented with SFOthroughout the entire growth period (P<0.05). The lower feed conversion ratio was observed in chicks fed T-based diet for the entire growth period. Chicks with more access to T-based diet during the feeding program had significantly higher amounts of saturated and monounsaturated fatty acids in their carcass compared to those fed SFO based diets (P<0.05). Shifting fromSFO to T diet just for a few weeks time was capable in altering the profileof fatty acids in tissues toward a moresaturatedpattern. تفاصيل المقالة