 Milk fat composition varies with the season (summer vs. winter) but also with the milking time (AM vs. PM).  These observations could allow the diversification of dairy products by season and milking time Differences in milk composition between morning and evening milk are documented for major milk components. This study extended research to milk fat composition. Milk samples were collected between October 2007 and November 2011 in 491 Luxembourg farms and analyzed by MIR spectrometry. The milk contents of saturated (SFA) and unsaturated fatty acids (UFA) were predicted from the recorded MIR spectral data. As expected, the milk composition and, especially, the milk fat composition, were affected by AM / PM milking during the lactation. Therefore adjustments for morning / evening milking are required before using it jointly. Differences in composition could allow different uses of milk. V. M.-R. Arnould 1,2,*, N. Gengler 2,3, and H. Soyeurt 2,3 1 CONVIS s.c., Ettelbruck, Luxembourg; 2 University of Liège, Gembloux Agro Bio-Tech, Animal Science Unit, Animal Breeding and Genetics Group, Gembloux, Belgique; 3 Fond National pour la Recherche Scientifique (F.N.R.S.), Bruxelles, Belgique. * Financed by National Research Fund, Luxembourg (AFR PHD RE) The first author acknowledges the financial support of AFR/FNR (Aide à la Formation Recherche/Fonds National de la Recherche Luxembourg) PhD grant (AFR PHD RE). CONVIS s.c. and Luxembourg farms are acknowledged for providing data. Abstract Milk fat composition Contact: Objective To Analyse the milking-to-milking variability in milk and milk fat quality.  Data.  Holstein cows (n=32,339 cows from 491 herds) in first lactation (5 < DIM < 366).  Data set:  130,997 records in morning milking data (AM)  125,384 records in evening milking data (PM)  Fatty acids predicted from FOSS milk spectrum (Mid-Infared spectrometry): saturated (SAT) and unsaturated fatty acids (UFA) Material and method  There were no significant difference between AM and PM milk fat percentage  BUT: SAT was higher in the evening milking (fig. 2 and 3).  SFA/UFA was lower during the evening milking (fig. 3).  There were also some variations in milk fat composition according to the milking time and the season. Figure 3. Evolution of SAT (g/dl milk) during the first lactation and according to the milking time and the season. Figure 2. SAT (g/dl milk) and UFA (g/dl milk) contents during the first lactation and according to the milking time and the season. Figure 4. Evolution of SAT daily heritability values accros lactation and according to the milking time  Heritability is defined as proportion of observable differences between individuals that is due to genetic differences  Daily heritability values for SAT were: AM: 0.33 ± 0.01 PM: 0.38 ± 0.01 Conclusion NS  Model:  Fixed effects: Herd * date of test Classes of 15 days in milk Classes of age at calving  Random effects:Additive genetic effect Permanent environment Residual effects  Variances components estimated by REML (Restricted Maximum Likelihood )  The significance of differences was evaluated by Student t- test Figure 1. Difference in milk fat percentage according the milking time and the season during the first lactation. *** NS Milk fat Variability Among Morning And Evening Milk Compositions During The Lactation