Goat alpha-s1 casein genotype interacts with the effect of extruded linseed feeding on milk fat yield, fatty acid composition and post-milking lipolysis

This study compares two groups of 23 and 24 goats with different CSN1S1 genotypes, matched for lactation number and stage. During the first experimental period (4 weeks) half of the goats of each genotype received the control diet (CT) and the second half received a similar diet containing extruded linseed (EL diet). During the second experimental period, the two groups of goats were switched from the CT to the EL diet and from the EL to the CT diet, respectively. Results confirm the lower milk protein (−4.8 g/kg) and fat (−6.5 g/kg) concentrations in the low genotype group, whereas milk yield was slightly higher (+0.49 kg/d) and lactose concentration was unchanged. Between the genotype groups, there were differences in milk fat proportions of at least 25 FAs, particularly the C7 to C13 saturated FAs and C16:0, C18:0, C18:2n-6 and cis9,trans11-C18:2 acids. In addition, delta-9 desaturation ratios were higher in the low CSN1S1 group for C10:0, C14:0, C18:0, trans11-C18:1 and trans13-C18:1. This strongly suggests an effect on mammary delta-9 desaturase activity, especially for C18:0 and trans11-C18:1. It is likely that higher desaturation ratios in the low genotype, as well as observed changes in other FAs, could compensate for the lower synthesis of medium-chain FAs (C7:0–C13:0), thus contributing to maintenance of the melting point of milk fat in this genotype. The effect of the EL diet on milk FA proportions was highly significant for almost all FAs, along with increased milk fat content and decreased post-milking free FA concentration. Numerous interactions (significant but of limited quantitative extent) between the CSN1S1 genotype and EL feeding were shown on milk composition, including 26 FA proportions or delta-9 desaturation ratios, and post-milking free FA concentration.