Features Agronomy Soybeans
Have soybean yields reached a plateau?

There is a general, long-term trend of increasing soybean yields. In fact,
they have doubled in the last 63 years, at the same time as a huge expansion
occurred across southern Ontario. Closer inspection of the recent years, however,
reveals something else. Yields reached 41bu/ac in 1987 and have reached that
level five times since, but have never exceeded it.

The general trend seems to be upward but with an extreme decline in 2001, back
to 1940's levels, followed by two poor years and then a recovery to the previous
best levels. The 2001 year was marked by dry weather and an explosion of soybean
aphids. The following year had a cold, wet spring with delayed planting. In
2004, high rainfall in May delayed planting in many areas.

A detailed look at the weather records reveals that provincial average soybean
yields did not relate well to monthly temperatures, nor did they relate very
well to monthly precipitation. Researcher, Wally Wilhelm, in Nebraska, reported
the same thing, after investigating 16 years of yield and weather data in his
state. In his report, the only thing measured that related well was that high
temperatures from July 23 to August 12 decreased yields and accounted for 50
percent of the variability in yield from year to year.

Yamoah and his co-workers, also doing research in Nebraska, looked in more
detail at weather effects on soybean yields. Over a 12 year period, early May
temperatures above five degrees C with moist soil (to get germination), in combination
with May average temperatures (to get early growth), explained more than 80
percent of the variability in their corn and soybean yields.

Dick Cooper, working at the Ohio State University station at Wooster, tried
for years to grow 100bu/ac soybeans. He planted May 1, maintained high fertility,
irrigated when necessary and protected with fungicides. He reached the 100bu/ac
level twice, way back in 1977 and 1985, although his 1998 and 1999 yields were
in the 90s. Years with low temperatures like 1992 had top yields in the 60s.
One thing was consistent: he could not get those bin-busting yields without
planting early.

The picture seems to be that there are two major weather effects on soybean
yield. The crop needs to start fast in the spring and avoid stress in August.
Growers do not have control of the weather, but there are some ways to take
advantage of this information. One is: when the soil is ready for planting in
the spring, plant! It is like planting a solar panel that grows. The sooner
the solar panel is intercepting sunlight and turning it into photosynthate,
the higher the yield. When corn planting is done, do not wait before planting
soybeans. Over the 42 years, frosts low enough in May to kill soybeans have
only occurred four times at London.

The apparent stall in yield improvement in Ontario does not seem to be genetic.
While there was some yield drag while plant breeding companies put huge resources
into new varieties tolerant to glyphosate, that is now over and yielding ability
continues to improve. One easy way to measure genetic improvement is to look
at the performance of OAC Bayfield in the Ontario Oil and Protein Seed Crop
Committee trials. This variety was chosen because it has been reported in the
trials since 1985. In that time, its yield index has gone from 115 to 101. That
means in 1985 it yielded 15 percent more than the average of all the varieties
in the 2600CHU trials. Over the last three years, it yielded 101 percent of
the average yield of all the varieties in that trial. In other words, relative
to OAC Bayfield, the newer varieties have increased 14 percent in yield in 20
years. Strangely, those genetic advances have not translated well into average
provincial yields. In comparison, between 1990 and 2005, average Ontario corn
yields have increased by about 25 percent.

So what is happening? Yield potential is getting better but average yields
are not. Why? I have two opinions: One is that, for most of the last 42 years,
about half the yield gain came from genetic improvement and half came from agronomic
improvements, including flexible cutterbars, header height control, better planters
and better weed control.

Over the last several years, improvements in equipment that translate into
big increases in yield just have not happened. At the same time, yield-robbing
problems have proliferated, like soybean cyst nematode, soybean aphids, spider
mites, diseases like sudden death syndrome and more extreme weather. Altogether,
we have not made much progress.

Soybeans seem to be much less responsive to management than corn. In our own
trials, soybean yields have not responded to N fertilizer and responses to inoculation
with rhizobia only averaged 1.5bu/ac. Responses to applied P or K only occurred
at very low soil test levels. Growers in Ontario manage their corn a lot more
intensively than they do their soybeans and there is a simple reason. Other
than planting earlier and selecting the very best varieties, it is hard to identify
management procedures that would give big returns. One area now under investigation
is how to convert more flowers into pods. About 75 percent of soybean flowers
fail to form a pod and that represents a lot of yield potential.

In 2006, we had good early growth and little extreme heat in July and August.
If this analysis is right, the 2006 crop should set a new record.

Dr. Dave Hume is a professor emeritus in the Plant Agriculture
Department at the University of Guelph and a senior Agri-Coach for Agri-Trend
Agrology.