The poster “Titanium dioxide as digestibility marker – Between and within cow variation in faecal concentrations” is the result of a production trial conducted at AU Foulum, Aarhus University, Denmark, as part of WP5 in the SmartCow project. Twelve lactating dairy cows were used in a 4 x 4 balanced Latin square with 4 diets, with 60:40 forage:concentrate ratio as dry matter (DM), and 4 periods of 21 days each. Cows were fed 4 diets, 2×2 factorial arranged, including low vs high grass silage digestibility and 18 vs 42% of diet DM from maize silage. Eight subsamples (2 per day over 4 days) of faeces were collected from individual cows in each period. Individual fecal subsamples were analyzed for titanium dioxide (TiO2) and nitrogen (N). The between-cow variation was calculated from a linear mixed model, and it accounted for 2.20 and 2.06% for TiO2 and N faecal concentration, respectively. Within-cow variation of fecal concentration of titanium dioxide (standard deviation within cow and period) was not affected by the diets, supporting that TiO2 is a suitable digestibility marker for diets with different grass-silage digestibilities and different inclusion rates of maize silage. No effect of day of sampling was observed, suggesting that the sampling days can be reduced to less than 4. You can find the full abstract at page 610 (poster 16, session 72) in Book of Abstracts of the 72nd Annual Meeting of the European Federation of Animal Science, and the poster on the SmartCow website. As European Union recently have decided to phase out the use of TiO2 (colouring agent E171 in food), researchers in ruminant nutrition are in urgent need of finding alternatives to TiO2 as digestibility marker. Hopefully, the work done so far on TiO2 can be used as a standard to find and test new markers. If a potentially suitable internal marker is identified, using the samples from experiment such as this one will be a way to facilitate a direct comparison with TiO2.