Legume-cereal mixtures are a valuable source of protein for animals. Mixtures of lupine with oat are recommended for cultivation in weaker soils. This paper presents the result of the study from 2009-2011 aimed to estimate the effect of composition and harvest time of mixtures of blue lupine with oat on the content and yield of total protein in green matter. Two factors were studied in the experiment: I) mixture composition: blue lupine – pure sowing, oat – pure sowing, blue lupine 75% + oat 25%, blue lupine 50% + oat 50%, blue lupine 25% + oat 75%, II) harvest time: flowering phase of blue lupine, the flat green pod phase of blue lupine. During harvest of mixtures, mean samples of fresh matter were collected from each plot, in order to perform chemical analyses. The content of dry matter and total protein in the collected plant material was determined and the total protein yield was calculated. The obtained results of the study allow us to state that the dry matter yield of mixtures of blue lupine with oat harvested in the phase of flat green pod was significantly higher than the dry matter yield of mixtures of blue lupine with oat harvested in the flowering phase of blue lupine. Mixtures harvested in the flowering phase of blue lupine contained more total protein than mixtures harvested in the phase of flat green pod of blue lupine. Green forage from blue lupine harvested in the flowering phase was characterized by the highest content of total protein. Total protein yield from mixtures of blue lupine with oat obtained in the flat green pod phase of blue lupine was significantly higher than the total protein yield obtained from mixtures harvested in the flowering phase of blue lupine. The highest total protein yield was provided by a mixture of blue lupine with oat with 50% of both components harvested in the phase of flat green pod of blue lupine. Mixture of blue lupine with oat with 50% of both components harvested in the flat green pod phase of blue lupine provided the highest dry matter yield with the highest amount of total protein.

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