Effect of dietary energy and protein levels on fatty acid composition of intramuscular fat in double-muscled Belgian Blue bulls

Seventy six Belgian Blue (BB) bulls, with double-muscled conformation, were randomly allocated to six dietary treatments in a 2 × 3 factorial experiment. The treatments included low (N(L)=127 g CP/kg DM), medium (N(M)=153 g CP/kg DM) and high (N(H)=172 g CP/kg DM) levels of dietary protein in combination with low (E(L)=7.38 MJ ME/kg DM) and high (E(H)=8.03 MJ ME/kg DM) levels of dietary energy. Fatty acid composition was determined on total lipid samples of the M. longissimus thoracis of all animals and on the separated triacylglycerol and polar lipid fatty acid fractions of the medium-protein group. Dietary energy and protein levels influenced the carcass characteristics of BB bulls but the effects were small. Carcass fat cover score, carcass fat content and intramuscular fat content were slightly but significantly higher in the animals on the high versus the low energy diets. The overall intramuscular fat content was very low (<1%) and the overall polyunsaturated:saturated fatty acid ratio (0.39) high compared to normal figures for beef. The high versus the low dietary energy level increased the monounsaturated (P<0.001) and decreased the polyunsaturated (P<0.001) fatty acid proportion with no change in the saturated fatty acid proportion. This may have been due in part to the addition of beef tallow to increase the energy level of the diet. Concomitant shifts in the individual fatty acids included increased proportions of C14:0 (P<0.001), C16:0 (P<0.03), C16:1 (P<0.01), C18:1 (P<0.001) and decreased proportions of C18:2 (P<0.001) and C20:4 (P<0.001). Increasing dietary protein levels had inconsistent effects on the fatty acid profiles. The proportion of polar lipid fatty acids in the total fatty acids was 0.34 and 0.25 for the E(L) and E(H) group, respectively, in the Nm treatment group. Changes in fatty acid proportions of the triacylglycerol fatty acid fraction were similar to those seen in the total lipid fatty acids when related to dietary energy level, but no significant changes in the polar lipid fatty acid proportions were observed. Significant linear relationships were found between measures of carcass and muscle fatness and fatty acid proportions. With increasing muscle total fatty acid content, saturated and monounsaturated fatty acid proportions increased (r=0.38 and r=0.55, respectively, P<0.01) and the polyunsaturated fatty acid proportion decreased (r=-0.73, P<0.01). The diet had some effects but the relatively unsaturated intramuscular fatty acid composition was mainly related to the low total fat content of the BB animals.