The detection of the genetic modification in silage obtained from insect-resistant Bt maize by means of the polymerase chain reaction (PCR) is described. The detectability of the transgene was shown to be dependent on the length of the genomic target sequence chosen for amplication by the PCR. By amplifying a Bt-maize-specific DNA sequence of 211 bp the genetic modification was detected up to 7 months after ensilage. The effect of maize DNA degradation in the course of the ensilage on the detectability of target sequences was demonstrated in model experiments.