Several short-term studies (1-3 months) have reported that supplementation of cow diets with oilseeds modifies milk fatty acid (FA) composition. As the durations of these experiments were short, putative long-term effects on milk FA composition and dairy performances remain poorly known. The aim of this study is to quantify the effects of supplementation with different oilseeds rich in either c9-18:1 or 18:3ω3 FA during two consecutive lactations on milk yield and composition.

Sixty Holstein cows during a 1^st year, and 37 during a 2^nd one, received high-forage diets (60-70% of dry matter intake (DMI)). During indoor period, cows were fed grass silage and hay and during outdoor period cows were grazing pasture. Basal diet was supplemented or not (control diet (C)) with either different forms of rapeseeds, whole seeds (WR), extruded seeds (ER) or a fat rich meal (FRM), or with extruded linseeds (EL). The oilseed supplements provided 2.4-3.2% oil in the DMI.

Compared to C diet, FRM supplementation increased milk yield (+2.9kg/day; P<0.05) during indoor period and decreased milk protein content whatever the period (-1.8g/kg; P<0.09). During indoor period, oilseed supplements decreased milk 12:0+14:0+16:0 (-12.7g/100g FA; P<0.01) and increased milk c9-18:1 (+6.6, P<0.01). ER, FRM and EL supplements increased milk c9,t11-CLA (+0.32, P<0.01) as well as total trans FA (+3.4, P<0.01), whereas only EL increased 18:3ω3 (+0.72, P<0.01). The effects observed during grazing period (except for milk c9,t11-CLA) were weaker than those during the 2 indoor periods.

Oilseed supplementation effects were dependent on type and form of oilseeds. Effects on milk FA observed during 2 consecutive lactations were comparable to those observed during short-term studies.