Effect of the αs1-casein genotype and its interaction with diet degradability on milk production, milk quality, metabolic and endocrinal response of Girgentana goats

• We studied interaction between diet degradability and genotype at CSN1S1 locus in lactating goats.
• We evaluated productive, metabolic and hormonal response of goats at different αs1-casein genotype.
• Little effect of diet degradability and interaction with genotype on production and quality.
• Higher milk yield, casein percentage and lower urea in goats with strong alleles at CSN1S1 locus.
• Higher tyroid hormones in goats with strong alleles.

A study was carried out to evaluate if diets at different starch and protein degradability could influence productive, metabolic and hormonal response of goats at different αs1-casein genotype. Nineteen Girgentana goats at mid lactation were selected on the basis of their genotype at αs1-casein locus: nine goats homozygous for strong (AA) alleles and ten goats homozygous for weak alleles (FF). The goats were used in a 2 × 2 factorial arrangement of treatments, with two genotypes (AA, FF) and two diets at different degradability (H diet: RDP, 60.5% of crude protein; starch degradability, 78,4%; L diet: RDP, 50.9% of crude protein; starch degradability, 66.8%). The dry matter intake was not affected by αs1-genotype (respectively AA and FF goats: 2107 and 2086 g/d; P = 0.160), whereas it resulted significantly higher when goats were fed with the L diet (2130 g/d vs 2063 g/d; P < 0.001). Milk yield was much higher in AA compared to FF goats (1401 g/d vs 1076 g/d; P = 0.03). As expected, milk protein and casein resulted significantly higher and urea resulted significantly lower in AA goats (protein, 3.72% vs 3.20%; P < 0.001; casein, 3.11% vs 2.56%, P < 0.001; urea 56.6 mg/dl vs 69.2 mg/dl, P < 0.001) whereas no differences were found, between genotypes, in milk fat and lactose. No diet effect was detected on milk production and composition. The αs1 genotype has strongly influenced free triiodothyronine (fT3) levels, which were higher in AA goats (1.64 vs 1.37 ln + 1 pg/ml; P = 0.009). No other differences were evident between genotypes or diets. In conclusion, the hypothesis that diets at different degradability could modify the productive response of goats at different CSN1S1 genotype has not been demonstrated. It has not been clarified the metabolic and endocrinal mechanisms involved in the manifestation of the differences found in relation to the studied polymorphism.