America's two leading field crops yield more grain today than they did 75 years ago. But while corn has taken giant leaps forward in average bushels per acre, soybean yields have advanced in baby steps.
Between 1930 and 2003, average corn yields jumped nearly sevenfold, from 20.5 bu./acre to 142.2 bu./acre. In that same period, average soybean yields not quite tripled, from 13 bu./acre to 33.4 bu./acre. National soybean yields have hovered around 40 bu./acre for about a decade.
Why the widening yield gap between corn and soybeans? There are many reasons, said Purdue University agronomists Jeff Volenec and Scott Jackson. Among them: genetic differences between the two crops and greater attention paid to corn research.
"We're looking at about a 0.4 bu./acre/year average increase for soybeans. For corn it's 1.5 bu./acre/year," said Volenec, a professor of crop physiology. "Will soybeans equal the annual increase in corn yields in the near future? No. Can we improve on the 0.4 bu./acre/year? Yes."
Collaborative research by crop geneticists, physiologists, agronomists and breeders could boost soybean yield potential, Volenec said. However, researchers aren't likely to increase average soybean yields more than a few tenths of a bushel in the next 10 years, and may never be able to place the oilseed on a similar yield growth track as corn, he said.
The challenge for researchers continues to be cracking the unique genetic makeup of the soybean plant.
Soybeans have about half the DNA of corn, yet soybeans are much more difficult to cross than corn genes. Corn is more efficient at turning sunlight into energy – a process known as photosynthesis – than soybeans. And although soybean plants generally handle environmental stresses better than corn, soybeans tend to lose more grain – or fail to make it altogether – than their kernel-producing field mate.
"Developmentally, these two plants are very distinct," Volenec said. "Corn is what we call a determinate plant. It grows, develops, sets an ear and produces a tassel – but does it only once. That means that if field conditions are good during the important phases of development, you get high yields.
“Soybean, on the other hand, is an indeterminate plant. It continuously produces more pods and more flowers during development. If stress hits early in development soybeans can make up any reproductive losses, such as aborted flowers and aborted pods, later in its plant development. Because of that, soybean yields year in and year out are more stable than what you find with corn."
More stable, yet well short of potential, Volenec added.
"If you look at the number of flowers and the number of small pods that actually abort from an individual soybean plant, it is about two-thirds of its reproductive potential," he said. "So if that plant has a capacity to produce a hundred pods it loses, roughly, 66, as small flowers and young pods that never develop seeds – something we can harvest. What you eventually recover is about 33 pods from that plant.
"There's a tremendous opportunity to increase yield capacity, if we could understand pod and flower abortion."
Jackson, an assistant professor specializing in soybean genomics, said breeding programs historically have placed more emphasis on corn, and for good reason.
"Doing breeding work in corn is easier than it is in soybeans," Jackson said. "First, making controlled genetic crosses between two corn plants is much more simple than crossing soybeans. In soybeans this is very difficult because you have to, by hand, take off the male parts of the plant – what we call emasculate. In a corn plant all you have to do is put pollen on the silks of the plant and bag it up so that no other pollen gets in. It's a controlled pollination.
"Second, there's the issue of the product from the cross. If you cross two plants in soybean you get one flower and, maybe, three or four seeds. If you do a cross in corn you get an entire ear, or hundreds of seeds. Genetically, it's easier to look for what you want in 200 seeds than it is in three seeds. That's a major advantage in doing genetics in corn."
As such, there are about 600 public/private breeders concentrating on corn and only 160 focusing on soybeans, Jackson said. Between 1998 and 2003, corn research funded by a science-based federal agency received $106 million while soybean research received just $15 million, he said.
Future soybean research should address the pod abortion puzzle and other yield-restricting problems, Volenec and Jackson said. Those issues include:
Researchers who tackle those issues will need to be careful not to remake soybeans in corn's image, Volenec said.
"Could we convert soybeans into a corn-type synthetic mechanism? Possibly," he said. "It would take thousands of genes to do that and it may not be practical in the foreseeable future.
"At the same time, if we change the composition of soybean seeds to have less oil and protein, we no longer have soybeans. And the reason we grow soybeans is for oil and protein."