Conservative Breeding Method
Backcrossing is considered to be a conservative breeding method because selection is primarily based on the improvement of a single trait. Improvements in other traits such as yield are not expected. Therefore the lines coming out of a backcrossing program have gone through a 'lag period' in which the plant breeder has not imposed selection for yield. These lines would be expected to experience a yield lag. The degree of yield lag depends on the progress made by breeders on the genetic improvement of yield in their other comparable elite lines during that same period. This progress in yield improvement has been 1-2% per year on average over the past several decades in the major crops.
Once a transgene has been backcrossed into an elite inbred background, it is no longer necessary for it to undergo backcrossing again. It can be used in the same manner as non-transgenic lines in breeding programs being mated to other lines and undergoing selection for improved qualities including yield. Thus, over time yield lag is no longer an issue in a particular transgenic event.