1. The maturity stage of the grass affects milk fatty acids of cows grazing a mountain grassland Gabriele Tornambé 1, Bruno Martin 2, Philippe Pradel 3, Antonino Di Grigoli 1, Adriana Bonanno 1, Yves Chilliard 2, Anne Ferlay 2 1 Dipartimento di Scienze Entomologiche, Fitopatologiche, Microbiologiche agrarie e Zootecniche. Università degli Studi di Palermo, Italy 2 UR1213 Herbivores. INRA Centre de Clermont-Ferrand/Theix, France. 3 UE Monts d'Auvergne. INRA Centre de Clermont-Ferrand/Theix, France. Contex t The milk content of oleic acid (major isomer of monounsaturated fatty acids, MUFA) and polyunsaturated fatty acids (PUFA), including conjugated linoleic acid (CLA) and ω3 FA, all well known for their putative positive action on human health, increases when cows are grazing. Nevertheless, a considerable variability is often reported even for cows only fed pasture. This could be partly explained by the botanical composition and the growing stage of the grassland. Material and methods A diversified mountain grassland was continuously grazed according to a strip grazing system by 6 cows (5 Montbeliarde and 1 Tarentaise) fed only pasture. Two grazing periods: June (first growth) at a stoking rate of 75 m 2 cow -1 day -1 and October (regrowth) at a stoking rate of 150 m 2 cow -1 day -1. FA profile analysed twice a week both in June (9 dates) and October (2 dates) on samples of grass and individual milk by gas chromatography. Data analyzed by the MIXED procedure of SAS: sampling date (11 dates) as main effect and animal as random effect. Conclusions These results underline the marked variability of the milk FA composition observed throughout a grazing period even when cows are fed the same grassland. Milk shows the highest concentrations of CLA-c9t11 and C18:3n-3, having beneficial interest for human health, when cows are grazing vegetative grass at the early growth or regrowth stages. Results Grass 46 plant species identified: 14 poaceae (44% of total DM) and 32 dicotyledons (56% of total DM). During the first growth, content of C18:3n3 decreased (-4 g·100g -1 of total FA) while the C16:0, the C18:1c9, and the C18:2n6 content increased (respectively +1.0, +2.6 and +3.4 g·100g -1 of total FA). During the regrowth, FA composition was similar to that observed in the beginning of June. Milk During the first growth, saturated FA content increased while the PUFA content decreased, mainly due to the decrease in C18:3n3. The sum of the trans-FA in milk fat also decreased. Variability of milk FA composition during one month in our study was very high for C16:0, C18:1t11, CLA-c9t11, and C18:3n3, certainly because of the advanced mature stage of the grass and the decreased allowance of grass during this heat wave period. 23 rd General Meeting of the European Grassland Germany, Kiel August 29th - September 2 nd 2010 SFA (g·100g -1 total FA) 45 46 47 48 49 50 PUFA (g·100g -1 total FA) 2 3 4 5 CLA-c9t11 (g·100g -1 total FA) 0.5 1 1.5 2 C18:3n3 (g·100g -1 total FA) 0.5 0.7 0.9  trans (g·100g -1 total FA) 3 4 5 6 7 8 36101216192326303736101216192326303 Protein (g/kg) 5 10 15 20 25 30 35 40 Milk yield (kg) 7 Day Month JuneOctoberJuneOctober Objectives The aim of this experiment was to quantify the effect of the growing stage of a mountain grassland pasture on the grass and milk FA composition. Evolution of milk yield, protein and FA (adjusted means ± standard error)