Rapeseed or linseed supplements in grass-based diets: effects on milk fatty acid composition of Holstein cows over two consecutive lactations

Persistency of changes in milk fatty acid (FA) composition to 4 different oilseed supplements rich in cis-9 18:1 or 18:3n-3 was determined over 2 consecutive lactations in 58 and 35 Holstein cows during the first and second years, respectively. During the initial 5 wk of the study, all experimental 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 (FRM), or whole unprocessed rapeseeds (WR). Oilseeds were offered to provide between 2.5 to 3.0% of additional oil in diet dry matter. During indoor periods, cows received a mixture (3:1, wt/wt) of grass silage and grass hay, whereas cows were at pasture during outdoor periods. Over the entire study, oilseed supplements decreased the concentration of milk FA synthesized de novo and increased 18:0 and cis-9 18:1 content, with a ranking of treatment responses (highest to lowest) of FRM, EL, ER, and WR. Irrespective of period, both EL and FRM increased total milk trans FA content, whereas WR resulted in lower concentrations in milk from grazing cows. Relative to rapeseed, EL resulted in higher increases in milk cis-12,cis-15,trans-12 to -16 18:1, nonconjugated trans 18:2 (especially ∆11,15), and 18:3n-3. In contrast, rapeseed supplements resulted in a greater enrichment of cis-11 18:1, trans-4 to -9 18:1, and cis 20:1 than EL. Changes in milk FA composition to oilseeds were of greater magnitude during indoor than outdoor periods, where oilseed supplements often decreased cis-9,trans-11 conjugated linoleic acid content. During the second indoor period, both EL and ER resulted in higher total trans FA content, trans-10 18:1 in particular, than during the first indoor period, consistent with an interaction between dietary starch content and oilseed supplement. Overall, the extent of changes in milk FA composition were related to the nature (rapeseed or linseed) and form of oilseed (extruded, cold-pressed fat-rich meal or whole unprocessed), and their interactions with the composition of the basal diet (grass silage and hay or pasture; or dietary starch content). Milk FA responses were stable within each period and repeatable over both outdoor feeding periods, with extent of changes being comparable to reports from relatively short-term (1- to 3-mo) studies.