Lafayette, Indiana
July 15, 2004
Source:
AgAnswers - An Ohio State Extension and Purdue Extension
partnership
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
bushels per acre to 142.2 bushels per acre. In that same period,
average soybean yields not quite tripled, from 13 bushels per
acre to 33.4 bushels per acre. National soybean yields have
hovered around 40 bushels per 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-bushel-per-acre-per-year average increase
for soybeans. For corn it's 1.5 bushels per acre per 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-bushel-per-acre-per-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:
• Leaf canopy
— If science can find a way to manipulate the canopy to increase
light availability to the soybean plant, higher yields are
possible, Volenec said. The same was found to be true with corn
in research conducted about 40 years ago. Leaves on today's corn
hybrids grow upright, where they used to grow out and down.
• Genetic
variation — The available soybean germplasm, which is used to
produce new soybean varieties, is less diverse than corn and
many other field crops, Jackson said. Because genetic variation
is limited, introducing disease- and pest-resistant genes into
new soybean cultivars occurs less often.
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." |