Effect of diet and CSN1S1 genotype on nutritional, productive and metabolic responses of milking Girgentana goats.

From a group of milking goats genotyped using specific PRC protocols at DNA level, 12 goats, averaging 37.2±3.5 kg of live weight, were selected for having the same genotype at CSN1S2, CSN2 and CSN3 loci and differing for the CSN1S1 genotype (G): 6 goats were homozygous for a strong allele (AA) and 6 heterozygous for a weak alleles (AF). Goats of each genotype were allocated homogeneously, based on days in milking (DIM, 50 or 120 days), to 3 sub-groups and fed ad libitum in individual pen with 3 diets, in a 3 x 3 Latin square design with 3 periods (P) comprised of 14 days for adaptation and 7 days for data and samples collection. The diets (D) were sulla (Hedysarum coronarium L.) fresh forage (SFF), sulla fresh forage plus 800 g/d of barley meal (SFB), mixed hay plus 800 g/d of barley meal (MHB), with 130, 95 and 85 protein/energy ratio (P/E, g protein/Mcal net energy), respectively. During experimental period, milk production and feed intake were measured every day, and milk quality was detected three times. Blood samples were collected at the end of pre-experimental and experimental periods. Plasma content of NEFA, glucose, insulin, fT3 and fT4 was detected; also same plasma markers of oxidative stress were measured as Reactive Oxygen Metabolites (ROMs), biological antioxidant potential (BAP) and -tocopherol. Data were analysed by a mixed model with DIM, P, G, D and GxD as fixed effects, and goat as random effect. Data of pre-experimental period were used as covariates. There was no significant effect of G and interaction GxD on dry matter (DM) and nutrients intake, efficiency of dietary protein utilization for milk casein synthesis (EPU), glucose, NEFA, insulin, ROMs, BAP and tocopherol content. With regard to D effect, DM intake was lower with MHB than SFF and SFB (1655 vs. 1820, 1807 g/d; P<0.01), whereas protein intake increased passing from MHB to SFB and SFF (203 vs. 290 vs. 321 g/d, P<0.01). NE intake was higher in SFB (3.0 vs. 2.3, 2.4 Mcal/d for SFB, MHB, SFF; P<0.01), and ingested NDF was higher in SFF than in MHB and SFB (632 vs. 539, 483 g/d; P<0.01). The diet greatly affected milk yield, which increased from SFF to MHB and SFB (1353 vs. 1423 vs.1664 g/d; P<0.01). The significant effect of GxD on milk yield (P<0.01) was linked to the superiority induced by SFB diet in AA than in AF goats (1720 vs. 1608 g/d; P<0.05). Milk composition was affected by D equally in both genotypes. Barley supplement contributed to reduce fat (36 vs. 32 vs. 30 g/kg in SFF, SFB, MHB; P<0.01) and urea in milk (35 vs, 32, 31 mg/dl in SFF, SFB, MHB; P<0.01), whereas the fresh forage increased the casein content (27, 27 vs. 26 g/kg in SFF, SFB, MHB; P<0.01). The EPU was the highest in MHB group, due to lower protein intake, whereas it was favoured by energy supplement (191 vs. 164 vs. 115 g casein/kg protein intake in MHB, SFB, SFF; P<0.01). The milk of AA goats showed longer coagulation time (r: 15 vs. 14 min; P<0.05) and higher curd firmness (a30: 36 vs. 29 mm; P<0.01) than AF milk. The D affected NEFA, ROMs and BAP. SFF showed the higher NEFA than other diets (0.39 vs. 0.23, 0.21 mmol/l in SFF, MHD, SFB; P<0.01). The BAP increased in groups that utilized fresh forage compared to group fed mixed hay (7.68 vs. 8.01, 7.99 ln mol/l in MHB, SFF, SFB; P<0.05). The ROMs level was lower in SFF goats (3.90 ln U.Carr) compared to MHB and SFB. A significant effect of G (P<0.05) was detected on fT4, which was higher by 15% in the AF than in the AA (1.02 vs. 0.88 ng/dl; P<0.05). A slight increase of fT3 was detected in AA compared to AF (3.58 vs. 3.36 pg/ml; P=0.10). Interaction GxD (P<0.05) was found for fT3, being higher in AA goats than AF goats (3.92 vs. 3.03 pg/ml) but only when fed the MHB diet.