Two different corn cultivars were compared with their genetically modified counterparts containing the gene coding for the Cry1A(b) protein of Bacillus thuringiensis (Bt). There were no analytical differences between the conventional and transgenic genotype kernels, whereas stovers from Bt+ plants had higher sugar (148.3 g kg-1 versus 115.9 g kg-1; P < 0.01) and lower NDF (592.7 g kg-1 versus 631.5 g kg-1; P < 0.05) contents than Bt- maize. A comparison of the amino acid profiles showed higher phenylalanine content in kernels from the Bt+ plants (49.1 g kg-1 vs 47.8 g kg-1; P < 0.05) which was, however, not reflected in the protein content. The initial dry matter rumen degradability of the isogenic kernels was higher than that of Bt+ varieties (569.5 g kg-1 vs 543.7 g kg-1; P < 0.05), whereas the lower fibre content increased the dry matter (548.6 g kg-1 vs 526.6 g kg-1; P < 0.01) and protein (695.6 g kg-1 vs 647.9 g kg-1; P < 0.01) degradability after 24 h of incubation in stovers from Bt+ plants. The NDF degradability was higher in Bt- corn varieties because of the higher proportion of hemicellulose in the total fibre.