A 1990-2000 strict field experiment on medium soil compared the effect of oat after various forecrops in crop rotation, with oat share of 25, 50 and 75%, on the weed infestation in the field and soil. It was shown that an increased frequency of oat in crop rotation did not differentiate the number, biomass and composition of weeds in the communities. In the four-course crop rotation with a 50% share in which oat was sown after two consecutive non-cereal crops (potato and persian clover) and after oat, the number of weeds was effectively limited, particularly of Chenopodium album and Thlaspi arvense. After 11 years there was recorded a clear increase in the number of weed seeds; the greatest in crop rotation in topsoil; the greatest in crop rotation in which oat accounted for 50% of all the crops (in fields after potato and pea), the lowest when oat was cultivated after oat.

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