Abstract :
Eighteen Girgentana lactating goats, nine homozygous for strong alleles (AA) and nine
homozygous for weak alleles (FF) at s1-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. s1-CN, -CN and -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) s2-CN and s1-CN yield: s1-CN
was higher in AA than FF goat milk (15.5 vs 2.4 g/d), while s2-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 -CN and -
CN yield. As concerning individual casein concentration, s1-CN was higher for AA than
FF goat (12.4 vs 1.5 g/kg milk), whereas s2-CN and -CN were higher in FF than AA milk
(4.3 vs 1.4; 15.6 vs 12.9 g/kg, respectively); also -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 s1-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 s1-
CN biosynthesis; moreover, the lower presence of s1-CN in FF goat milk may be partially
counterbalanced by the other caseins
