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The aim of the study is to establish the parameters of the relationship „Yield-ET by swaths” in a grass mixture of English ryegrass (Lolium perenne L.) and Red fescue (Festuca rubra L.). Data from an experiment with the application of a regulated water deficit, conducted at the Forestry University - Sofia, were used. The output data included the relative values of yield and evapotranspiration (ET) for each of the sub-crops separately. Two types of formulas were used: for the relationship „Yield-seasonal ET” and for the relationship „Yield-ET by phenophases”. From the first group, the two-power equation is recommended, according to which the first three swaths have greater requirements for ET, which can form some yield when providing at least 40-50% of the maximum ET. The fourth and fifth swaths show smaller requirements, but the differences compared to the first three swaths are in the range of 10-30%. Of the second group of equations, Steward’s linear formula, modified for use in swath crops, is recommended. It presents the relationship most realistically with R=0.983 and minimal difference between different swaths in terms of their sensitivity. [ABSTRACT FROM AUTHOR]