I do not think the bands are primer dimers or nonspecific because they have a fixed length and intensity regardless of the sample and because the pcr cycles are only 29 and the annealing time is short. The attached photo shows a pcr of a trait about 600 bp long. Because the amplified is very strong, I always run also a 1:10 or 1:5 dilution before sending the sample for sequencing. Whenever I run dilutions to check them, the amplified at the correct height seems to disappear and 300 bp high bands appear (they are circled in red). I repeat that the correct length of the bands should be about 600 bp.