Rapeseed or linseed in grass-based diets: effects on conjugated linoleic and conjugated linolenic acid isomers in milk fat from Holstein cows over 2 consecutive lactations.

Changes in the distribution of conjugated linoleic (CLA) and conjugated linolenic (CLnA) acid isomers in milk from Holstein cows in response to 4 different oilseed supplements rich in either cis-9 18:1 or 18:3n-3 were determined over 2 consecutive lactations in 58 and 35 cows during the first and second years, respectively. For the first 5 wk of the first lactation, all cows were fed the same diet. Thereafter, cows received 1 of 5 treatments for 2 consecutive lactations, including the prepartum period. Treatments comprised the basal diet with no additional lipid, or supplements of extruded linseeds (EL), extruded rapeseeds (ER), cold-pressed fat-rich rapeseed meal, or whole unprocessed rapeseeds to provide 2.5 to 3.0% of additional oil in diet dry matter. During indoor periods, cows were housed and received a mixture (3:1, wt/wt) of grass silage and hay, whereas cows were at pasture during outdoor periods. Over the entire study, EL resulted in the enrichment of ∆11,13 CLA, ∆12,14 CLA, trans-9,trans-11 CLA, trans-13,trans-15 CLA, ∆9,11,15 CLnA, and cis-9,trans-11,trans-13 CLnA (identified for the first time in bovine milk fat) in milk fat, whereas ER and cold-pressed fat-rich rapeseed meal in particular, increased milk fat trans-7,cis-9 CLA concentration. With the exception of the first indoor period, whole unprocessed rapeseeds decreased cis-9,trans-11 CLA, trans-9,cis-11 CLA, and trans-10,trans-12 CLA abundance. During the second indoor period, EL increased milk trans-9,cis-11 CLA and trans-10,cis-12 CLA concentrations, but the increases in cis-9,trans-11 CLA, cis-12,trans-14 CLA, trans-11,cis-13 CLA, and cis-9,trans-11,cis-15 CLnA concentrations to EL and ER were lower for the second than first indoor period. In contrast to the indoor periods, EL and ER decreased milk cis-9,trans-11 CLA, trans-9,cis-11 CLA, and trans-10,cis-12 CLA concentrations at pasture. The extent of changes in the relative distribution and abundance of CLA and CLnA isomers in milk fat were related to the nature (rapeseed or linseed) and form of oilseed (extruded, cold-pressed fat-rich meal or whole unprocessed) supplement and their interactions with the composition of the basal diet (conserved grass or pasture and dietary starch content). Furthermore, milk fat CLA and CLnA responses to treatments were repeatable between both outdoor periods. Variations in milk fat content and yield measured during the entire study were significantly and inversely associated with milk trans-10 18:1, trans-10,cis-12 CLA, and in particular, trans-9,cis-11 CLA concentrations.