Effects of nutritional level on lactating response of Girgentana goats with different CSN1S1 genotype

Goat polymorphism at αs1-casein loci (CSN1S1) influences milk yield and composition. Milk of goats with strong alleles associated to high αs1-casein shows higher fat and casein, longer coagulation time, firmer curds and variation in fatty acids than milk from goats with weak alleles linked to low αs1-casein. Since these milk properties are also affected by nutrition, the aim of this study was to investigate the impact of nutritional level on milk production traits of Girgentana goats with different CSN1S1 genotype. From a group of goats genotyped using PRC protocols at DNA level, 12 goats having the same genotype at αs2, β and κ- casein loci and differing for CSN1S1 genotype were selected: 6 were homozygous for strong alleles (AA) and 6 heterozygous of strong and weak alleles (AF). Within each genotype, goats were divided into 3 sub-groups based on days of milk (50 or 120 d), housed individually and fed ad libitum with 3 diets in a 3x3 Latin square design with periods of 21 d (14 d for adaptation, 7 d for measuring and sampling). The diets with different nutritional levels were as follows: fresh sulla (Hedysarum coronarium L.) (FS); fresh sulla and 800 g/d of barley (FB); sulla hay and 800 g/d of barley (HB). The milk yield increased passing from FS to HB and FB (1352, 1423, 1664 g/d; P<0.01). The effect of genotype on milk yield was linked to the nutritional level, since the milk from FB was higher by 350 g/d in AA and 200 g/d in AF goats. In both genotypes, barley supplement reduced milk fat (3.6, 3.2, 3.0% in FS, FB, HB; P<0.01) and urea (35, 32, 31 mg/dL in FS, FB, HB; P<0.01), and fresh sulla increased casein content (2.7, 2.7, 2.6% in FS, FB, HB; P<0.01). The AA genotype did not increase the casein percentage, but AA milk showed longer coagulation time (r: 16 vs 13 min; P<0.01) and higher curd firmness (a30: 35 vs 29 mm; P