In the past 40 years, the national average for corn yield has increased by nearly 80 bu. per acre. Roughly half of that increase is due to changes in cultural practices such as better weed control, precision planters, increased plant populations, and taking the less productive fields out of production. The other half of this increase is due to genetic improvements through breeding and biotechnology. Can we expect a similar trend to continue? Have we reached a yield ceiling in corn? Based on information shared with me, genetic improvement for grain yields will continue and some exciting yield potentials are ahead of us. These improvements will result from the development of new biotech traits and the use of a breeding tool known as “Marker Assisted Breeding”.
Marker Assisted Breeding is a concept employed by corn breeders to help them develop new improved hybrids. Before this tool was available, breeders constructed breeding populations and developed thousands of new inbred parents from those populations in their nurseries. They then made “test crosses” with these new inbred parents by crossing them with other, unrelated inbred parents. This process creates thousands of new hybrids that the breeder evaluates. This process is repeated year after year. Typically, only a couple of these hybrids are good enough to be commercial hybrids but it takes years of testing to identify which ones. This whole process is a combination of art, science, and a bit of luck. Marker Assisted Breeding is a science-based tool that has all but replaced the art portion of this process and has greatly reduced the luck component. It is based on a concept known as indirect selection. The idea of indirect selection is to select for a trait that is easy to identify and select for, yet it is highly correlated to giving you the results you want. As an example of indirect selection, if you select for plants that can produce more kernels per plant, you indirectly select for plants that have more yield per acre. There are many components of grain yield in corn but it typically comes down to some combination of more grain weight per plant and /or more plants per acre. The corn plant has roughly 35-40,000 genes. It is estimated that about 250 of these genes directly influence grain yield. About 20 clusters of these genes work together to have the most impact. These are genes that breeders would like to easily manipulate and select-for (indirectly) during the process of developing new hybrids.
Let’s back up. Genes have one function in living organisms. They are a set of instructions to produce proteins. We cannot select for specific genes by looking at them under a microscope. We can, however, select for their presence or absence by doing an analysis for the proteins they produce. Since several yield genes are working together in clusters, it is almost impossible to do a protein analysis for those genes per se but if there was a gene next to that gene cluster that was easily traced due to the specific protein it produces, that gene would be an excellent marker. Another thing to remember is that genes are passed from generation to generation in groups so that genes that are close to each other travel together to the next generation. The closer that gene is to the cluster of yield genes, the more effective it is as a genetic maker for grain yield. Selecting for plants that are identified to have a marker or set of markers that the breeder knows are next to clusters of genes that he is looking for, gives that breeder a high probability of successfully selecting the most desirable plants.
To make all this work, extensive studies are done to identify the makers that are near the gene clusters of interest. To complicate things, different genetic families may have different clusters that influence yield. Therefore, different makers are necessary. The end result, however, is that breeders have a very effective tool to use in their selection process. It makes them more effective and efficient in selecting material that goes into their breeding pools, as well as selecting potential new inbred parents during the inbreeding stages.
I believe that genetic gain for grain yield in corn will continue not at the rate we have seen in the past, but will be even greater. This will mean new hybrids will continue to come out at a fast rate. Hybrid life spans of 3 – 4 years will be common because they will be replaced by newer hybrids that are 5 – 10 bu./acre better. I also believe future hybrids will be even more stable across environments. This will be due to additional biotech traits as well as identifying markers that are linked to genes that have a major effect on stability and plant efficiency. Yes, corn yields will continue to rise. It will be exciting to see just how much.
April 2012 Earfull Newsletter
Written by Dr. Rick Batty