The aim of the study was to examine CLA (cis 9 trans 11) content in milk depending on rations used, number of lactation, its phase and the productivity of cows as well as fat content in milk. The studies were carried out on 429 cows fed pasture during summer and 41 cows fed TMR. It was found that the CLA content in milk is determined by feeding as well as by other, unconnected factors. From the consumer’s point of view, the most beneficial influence had a ration in which the only juicy roughage was green pasture forage while the concentrate constituted an addition of about 10% of dry matter. Among winter rations based on conserved fodders, the one with a predominance of wilted grass silage was observed to have a more beneficial influence. However, feeding with the TMR system resulted in the smallest CLA content. It was found that the CLA content was the highest in milk from older cows in their 4th lactation and in the period beyond the 120th day of lactation. Moreover, the highest content of CLA was observed in cows with a moderate level of milk production (below 6.000 kg). These findings justify creating ecological farms keeping cows. The analysis of minimal and maximal values of CLA content in the milk of investigated cows reveals big individual differences, so that further research should be carried out in order to explain the existing divergences. Milk collected from family farms dominating in Poland - where summer feeding is based on green forages and pasture thanks to its healthful properties (measured by the CLA content) may constitute a serious competition to milk produced in other EU countries, where the TMR feeding system prevails and farmers increasingly abandon grazing their cows on pasture.
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