Simple SummaryTheCSN1S1gene encodes for one of the primary milk proteins in goats. Its polymorphism strongly affects αs1-casein levels and plays a crucial role in determining both milk quality and quantity. The study aimed to evaluate whether a different energy intake level (70%, 100% and 150% of the total requirements indicated by INRA) could unveil any interactions with the genotype at the αs1-casein gene with respect to milk yield and casein profile. The results demonstrated that the higher energy input, along with the presence of the strong allele at theCSN1S1locus, improved milk production and casein concentrations, highlighting the existence of an interaction between αs1-casein polymorphisms and diet on the dairy performance of goats.AbstractA total of twenty-seven Rossa Mediterranea lactating goats, consisting of nine homozygous for strong alleles (AA), twelve heterozygous (AF) and six homozygous for weak alleles (FF) at theCSN1S1locus, were used to evaluate the effect of genotype, diet and genotype × diet interaction on goat milk traits and casein profile. The goats were used in a 3 × 3 factorial arrangement of treatments, with three genotypes (AA,AFandFF) and three different energy intake levels: high (H), medium (M) and low (L). The diets supplied a complete pelleted feed containing 65% of alfalfa hay, respectively, at 150%, 100% and 70% of the total energy requirements. Milk yield was significantly affected by the genotype and diet: Lower levels were found inFFgoats than inAAandAFgenotypes (673.7 vs. 934.5 and 879.8 d/g, respectively;p =0.002) as well as in goats fed with the L diet (651.5 vs. 1041 and 852.9 g/d for H and M diet, respectively,p <0.001). The genotype influenced the casein profile. Specifically,AAgoat milk exhibited higher concentrations of total casein and αs1-casein compared toAFandFFgenotypes (for total casein and αs1-casein, respectively: 24.9 vs. 20.4 and 19.8 g/kg,p =0.001; 7.2 vs. 3.7 and 0.7 g/kg,p <0.001), while theFFgenotype showed higher values for αs2-casein concentrations compared to homozygousAAand heterozygousAFgoats (3.1 vs. 2.4 and 2.5 g/kg, respectively,p <0.001). A significant genotype x diet interaction occurred for αs2-casein levels (g/kg) (p =0.034) and αs1-casein yields (p =0.027): The αs2-casein level was not affected by the diet inAAgoats, whereas it increased with energy intake inAFandFFgenotypes. Conversely, the αs1-casein yield gradually increased with energy intake inAAandAFgroups, whereas the diet inFFgoats did not modify it. The results demonstrated that high energy input, as well as the strong allele at theCSN1S1locus, enhanced milk production and casein concentrations. Furthermore, they confirmed the existence of an interaction between αs1-casein polymorphism and diets, influencing the milk casein composition and yield.