Effects of extruded linseed or alfalfa protein concentrate in interaction with two levels of concentrates on milk production and composition in dairy cows

Improving ω3 fatty acid (FA) transfer from feed to milk and decreasing milk saturated FA (SFA) without increasing trans mono-unsaturated FA are desirable goals. Our objective was to study the effects of supplying two α-linolenic acid (ALA)-rich feedstuffs in interaction with proportion of concentrate in the diet on milk FA composition. The two ALA sources were extruded linseed (LIN, 700 g/d) and alfalfa protein concentrate (APC, 2 kg/d), supplying 115 and 49 g/d ALA, respectively, per cow per day. Two groups of 12 cows paired according to milk yield, milk fat and milk protein contents, lactation rank (primiparous or multiparous), lactation stage, DMI, milk proportions of saturated and polyunsaturated FA were fed a corn silage-based diet with 30% (C30) or 65% (C65) cereal-based concentrate and received the 2 ALA sources in a reversal design during 5-wk periods. The cows averaged 117±14 DIM at the beginning of the experiment. Data were analyzed according to a split-plot design using the Proc mixed procedure. There was no significant interaction between the concentrate level and ALA source for most measured parameters. Treatment C65 decreased milk fat content (−1.15%), milk fat yield (−301 g/d), and proportion of SFA, especially C16:0, C18:0, C4:0, C6:0 and C8:0, and increased trans-C18:1, especially t9, t10 and t12. Treatment C65 significantly increased milk yield (+3.7 kg), milk protein content (+0.16%) and milk protein yield (+141 g/d), decreased casein-to-protein ratio (−2.6 percentage units). Compared to LIN, APC increased milk fat content (0.33%), milk fat yield (76 g/d), proportion of SFA, especially C4 to C12, C14:0 and C16:0, and decreased cis and all trans C18:1 isomers. APC tended to decrease milk yield (0.9 kg/d, P=0.061) although DMI was not affected, and did not affect protein yield. APC increased both total protein and total casein contents without impact on casein-to-protein ratio and milk protein yield. The transfer rate of C18:3 from feed to milk was much higher in APC treatment than in LIN treatment (15.3% vs 4.7%). This trial shows that adding ALA-rich feedstuffs to the diet to enrich milk can have very marked effects on milk FA composition but also on milk protein composition. These effects are stronger with high concentrate diets, resulting in acidogenic risk. The rate of transfer of ALA from feed to milk also varies according to lipid source. Higher transfer rates lead to lower modifications of milk composition.