Effect of diet at different energy levels on milk casein composition of Girgentana goats differing in CSN1S1 genotype

Eighteen Girgentana lactating goats, nine homozygous for strong alleles (AA) and nine homozygous for weak alleles (FF) at αs1-casein (CSN1S1) locus, were used to evaluate the effect of genotype, diet and genotype × diet (G × D) interaction on the composition of goat milk caseins. Goats were used in a 2 × 2 factorial arrangement of treatments, with two genotypes (AA, FF) and two diets at different energy levels (high-energy diet (D65) and low-energy diet (D100)). The experiment consisted of two simultaneous 2 × 2 Latin squares for the two genotypes, with one square for each level of energy. Capillary electrophoresis was used for the determination of relative casein (CN) composition. αs1-CN, κ-CN and β-CN yield was significantly higher with D65 than D100 (10.2 vs 7.2; 3.8 vs 2.6; 18.6 vs 13.6 g/d, respectively). Genotype significantly affected (P < 0.05) αs2-CN and αs1-CN yield: αs1-CN was higher in AA than FF goat milk (15.5 vs 2.4 g/d), while αs2-CN was higher in FF than AA goat milk (4.7 vs 2.8 g/d); no genotype effect (P > 0.05) was reported for κ-CN and β-CN yield. As concerning individual casein concentration, αs1-CN was higher for AA than FF goat (12.4 vs 1.5 g/kg milk), whereas αs2-CN and β-CN were higher in FF than AA milk (4.3 vs 1.4; 15.6 vs 12.9 g/kg, respectively); also κ-CN tended to be higher in FF goats. Diet did not significantly influence concentration of individual caseins. A significant G × D interaction was found only for αs1-CN concentration, which decreased (−10%) when AA goats shifted from D100 to D65. In conclusion, high energy input consistently improved total casein yield beside genotype. The higher casein yield of AA goats mainly depends on αs1-CN biosynthesis; moreover, the lower presence of αs1-CN in FF goat milk may be partially counterbalanced by the other caseins.