ABSTRACT:Two concentrates, one protein-rich and one based on cereals, were combined with two silages with a crude protein content of 17 and 13% of dry matter (DM), respectively to give four different diets for dairy cows. Milk content of caseins (α S1 -, α S2 -, β-, and κ-casein) and whey proteins (α-lactalbumin (α-LA) and β-lactoglobulin (β-LG)) and the fatty acid profile of milk were analyzed before the start and on four occasions during the experiment. Milk analyses showed that diet had no influence on the protein profile of the milk. However, a significant increase of α-linolenic acid, 13 and 39%, was obtained on the high protein concentrate feed and on the silage higher in crude protein, respectively. Cows on the protein-rich concentrate diet increased the proportion of conjugated linoleic acid by 53%. Linoleic acid was not affected by the diet.Keywords: milk proteins; feed protein; clover; casein; CLA; α-linolenic acid; capillary electrophoresis The feeding regime for dairy cows differs between conventional and organic production. In organic production a higher percentage of roughage is required. In the rules for organic production within the European Union, roughage should represent at least 60% of the cow's diet with the exception of the first three months where 50% roughage is accepted (European Commission, 2008). Dairy producers also need to supply the cows with organically produced feed to fulfil the rules for organic production (European Commission, 2008). However, the amount of organically produced feed is limited on the market, resulting in increased interest in feed self-sufficiency on organic dairy farms. In addition, if organic dairy producers want to feed protein supplements, the supplements must also be organically produced, which increases the price of these products. Furthermore, supply of organic protein supplements on the market is limited, as is the choice of suitable protein feeds that can be organically produced on the dairy farm. It is therefore of great interest to investigate alternative feeding regimes without the use of protein concentrate supplements to evaluate effects on production parameters such as milk composition.High quality roughage combined with a concentrate consisting of only cereals could be an interesting feeding regime for organic dairy producers. This alternative feeding strategy is economically attractive since cereals can often be grown on the dairy farm. It is therefore important to evaluate the possibility of replacing protein concentrates to some extent with high-quality silage containing a high proportion of legumes such as clover (Rosati and Aumaitre, 2004). Earlier studies have shown that clover has a positive influence not only on feed intake, but also on milk yield (Bertilsson et al., 2002;Bertilsson and Murphy, 2003). It is well known that the fatty acid (FA) profile of milk can 305Czech J. Anim. Sci., 58, 2013 (7): 304-312 Original Paper be influenced by changes in the feed (Palmquist et al., 1993;Chilliard and Ferlay, 2004), while the total protein c...