Feed intake, milk composition and cheese-making properties in Girgentana grazing goats with different genotype at as1-casein and k-casein

Abstract

Milk ability for cheese manufacturing depends on both animals utilization of dietary nutrients and genetic polymorphism of caseins. It is well known that strong alleles associated with high content of as1-casein increase cheese-making properties of goat milk, whereas there is little information about effect of genetic variants of κ-casein. The aim of this experiment was to investigate the effect of some composite as1-casein and κ-casein genotypes n changing feeding behavior at pasture and milk yield and properties of Girgentana goats. Forty goats were genotyped at as1, as2, b and κ-casein loci using specific PCR protocols at DNA level and IEF technique at milk protein level. Twelve goats, differing only for as1-casein and κ-casein genotype and averaging 136±5 days in milk and 38±6 kg of live weight, were selected and divided equally into 3 groups according to their genotypes: WA, with weak alleles (FF) for as1-casein and AIEF for κ-casein; SA, with strong alleles (AA) for s1-casein and AIEF for κ-casein; SB, with strong alleles (AA) for as1-casein and BIEF for κ-casein. No goats with weak alleles for as1-casein and BIEF for κ-casein were found. Over a 5 weeks period in spring, goats were allowed to graze daily an Italian ryegrass and berseem clover mixture, and supplied with 500g/d of barley. Measurements, sampling and analyses of individual milk and forage selected by goats were performed weekly. Forage intake of goats at pasture was assessed by n-alkane technique. Data were analysed by MIXED procedure of SAS 9.1.3, using a model with genotype as fixed effect and goat as random effect. The genotype did not influence the goats intake of DM (1180,1137, 1250 g/d DM for WA, SA, SB), net energy, crude protein and NDF, and milk yield (994, 1104, 1130 g/d for WA, SA, SD). Milk casein increased passing from WA to SA and SB genotypes (2.66, 2.93, 3.33%; P<0.01), similarly to casein/fat ratio (0.66, 0.74, 0.79%; P<0.05). the genotype associated with high as1-casein was responsible of a reduction in whey protein (0.71, 0.49, 0.55% for WA, SA, SB; P<0.01) and an increase in the casein N/NT ratio (71, 79, 80 for WA, SA, SB; P<0.001). The values of casein N/N intake ratio (109, 143, 158 g/kg for WA, SA, SB; P<0.001) indicate an effect of strong alleles of as1-casein in improving the efficiency of dietary N utilization for milk casein synthesis. Milk from WA genotype showed higher pH (6.70, 6.63, 6.58; P<0.05) and lower titratable acidity (3.15, 3.55, 3.76 °SH/50ml; P<0.05) than SB milk, with intermediate values for SA genotype. Clotting time (r) (10.7, 10.5, 9.3 min for WA, SA, SB; P<0.05) and curd firming time (k20) (1.56, 1.56, 1.24 min for WA, SA, SB; P<0.001) were lower in milk from goats with BIEF for κ-casein than in milk from goats with AIEF, regardless of as1-casein alleles. Curd firmness (a30) was improved by strong alleles than weak alleles of as1-casein, and more by BIEF than AIEF for κ-casein (41.8, 51.1, 59.8 mm for WA, SA, SB; P<0.001). These first results suggest that the strong alleles of as1-casein could act at metabolic level by increasing the efficiency of dietary nitrogen utilization and, consequently, the milk casein synthesis, and evidence the additional role of BIEF alleles of κ-casein in increasing casein content and improving coagulation properties of goats milk, but require further investigation to be confirmed.