Quick and uniform emergence is one of the factors determining height and stability of the yield of cereals, especially of their spring forms. Thus, it is relevant to learn about the effect of fractionation of the seed material of spring wheat and spring rye. Over the years 2004-2009 two series of microplot experiments were conducted, whose aim was to determine and compare the effect of seed grain fraction on growth and yield of spring rye cv. Bojko and spring wheat cv. Monsun, as well as evaluation of the response of spring rye to seed grain size depending on the sowing date. The experimental factors for spring rye included: A – sowing date (optimum and delayed), B – seed grain size (fractions: >2.8 mm, 2.5-2.8 mm, 2.2-2.5 mm). The factor for spring wheat was seed grain size with levels analogical as for spring rye. The value of the yield components and the yield of grain and straw of spring rye, as well as of spring wheat from the fraction >2.8 mm were generally higher than from the smallest grain 2.2-2.5 mm. Large plump seed grain had a greater significance for the yield of spring wheat than that of spring rye. The use of a grain smaller than 2,8 mm for sowing caused a greater reduction in its ear density and yields of grain and straw compared with spring rye. An unfavorable effect of using a small grain for sowing, fraction 2.2-2.5 mm, on 1000 grain weight of spring wheat manifested itself especially with the delayed sowing date. Delay in sowing rye resulted in a decrease in plant density after emergence, grain number per ear, grain weight per ear, 1000 grain weight, and in the yields of grain and straw.

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