Aim of this study was to evaluate the effects of grazing on Trifolium subterraneum and Lolium multiflorum, as pure or associated crops, on the chemical composition and on the fatty acid profile of the intramuscular lipids of the meat of lambs. Forty Comisana male lambs, on average weighing 13.75±1.90 kg, were divided into four homogenous groups of ten and called, in relation to the diet: group T those grazing on T. subterraneum; Group L on L. multiflorum; Group TL on adjacent monocultures of T. subterraneum and L. multiflorum (66.6 and 33.3% of surface, respectively); Group LT on adjacent monocultures of T. subterraneum and L. multiflorum (33.3 and 66.6% of surface, respectively). Every 10 days, samples of forage species ingested by grazing lambs were collected and analysed. At 90 days of age, with an average live weight of 25.44, 23.44, 24.69 and 24.75 kg for T, L, TL and LT group, respectively, all lambs were slaughtered and a sample of Longissimus dorsi muscle for each animal was collected to study the chemical and acidic composition. No significant differences among the groups were observed for the growth performance and for the chemical composition of the meat. As regards the fatty acid classes, significant differences (P < 0.05) were observed for the monounsaturated fatty acids, which were lower in the group T (35.46%) than those of the groups L (38.24%), TL (38.63%) and LT (38.59%), whereas, significant higher values for the group T were observed for the polyunsaturated fatty acids of the n-3 (4.49%) and n-6 (8.26%) series than those of the n-6 series for group L (6.79%; P < 0.05) and than those of both series for group LT (n-3 = 3.64%; P < 0.05 and n-6 = 6.43%; P < 0.05). The fatty acids that have significantly determined the modifications of the acidic classes were: oleic acid, which showed significant (P < 0.05) lower values in the group T (26.70%) than the levels observed in the groups L (30.33%), TL (30.39%) and LT (30.63%) and the linoleic, linolenic and rumenic acids which were significantly (P < 0.05) higher in the groups T (linoleic = 5.13%; linolenic = 1.97%; rumenic = 0.46%) and TL (linoleic = 4.75%; linolenic = 1.82%; rumenic = 0.41%) than those of the groups L (linoleic = 4.10%; linolenic = 1.52%; rumenic = 0.26%) and LT (linoleic = 3.95%; linolenic = 1.42%; rumenic = 0.33%). These differences could be due to the different dynamic activity of the cellulolytic bacteria in the rumen, related to the different levels of fibrous fractions of the diets. No significant difference was observed for saturated fatty acid, unsaturated/saturated fatty acids ratio and Atherogenic and Thrombogenic indices among the groups, whereas, PUFA/SFA ratio showed significant (P < 0.05) higher value in group T than that in the group LT. T. subterraneum monoculture grazed as monoculture (T) and in mixture with L. multiflorum (66/33, TL) increased the linoleic, linolenic and rumenic acids improving the dietetic nutritional characteristics of the lamb meat.
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