Yield Drag and Yield Lag
Yield drag and yield lag are 'old ideas' in plant breeding that are now being used in comparing genetically engineered crops to non-genetically engineered crops.
Yield drag represents a negative effect on grain yield associated with crop plants that have a specific gene or a specific trait.
Yield lag, however, is a relative reduction in yield observed in some hybrids or varieties compared with the yield observed in the most recently produced hybrids or varieties.
The next few examples will help explain the concepts of yield lag and yield drag.
Soybean breeders have known for decades that as they selected for higher protein levels in the seed there was a 'drag' on yield. With continued breeding efforts, they have overcome this drag to a certain extent, but developing high yielding high protein lines remains a constant challenge.
Genetically engineered crops are not necessarily more prone to yield drag or yield lag than non-genetically engineered crops. However, genetically engineered crops are unique in that an additional gene is being placed into the chromosome. The transgene may insert in the middle of a gene that already occupies that position. This renders the resident gene inactive and unable to produce the protein it encoded. If this gene is critical to yield, a drag yield potential will result. Another potential cause of yield drag is that genetically engineered plants are also being directed to make a brand new protein, sometimes in large quantities. The pool of protein building blocks (amino acids) is limited in a plant and thus the new protein could be made at the expense of the proteins that are normally produced. This could result in a shortage of other proteins and also a drag in the yield potential.
Wheat breeders often use the backcrossing method to 'move' into good wheat lines disease or insect resistance genes from low yielding wheat lines or even wild relatives of wheat. The several years it takes to develop these lines puts them behind lines that have had their yield potential improved by crossing good lines with other good lines. Thus their comparative yield may lag behind lines that do not have the pest resistance genes.
The extent of this yield lag depends on the time it takes to obtain a transgenic elite line through backcrossing. That is why companies and universities are continually improving methods to transform and regenerate elite lines for each crop and expedite the backcrossing method.