In light of the interesting discussion on corn yields, and to
provide more substance to my cursory post and report on the 1998 Tri-Society
annual meeting session on post-Green revolution yield prospects, I e-mailed
Don Duvick, formerly of Pioneer, who participated in the session. Don's
response below is interesting and useful.
When the Duvick-Cassman paper is available, I will send a note to
Sanet. It addresses these issues in much more detail.
Nov. 6, 1998 e-mail to C. Benbrook from Don Duvick regarding Sanet
discussion re corn yields --
I would guess the discussion concerned my data comparing Pioneer
one OPV) that were introduced during the 1930 - 1990 years. The data showed
that when planted at modern (high) planting rates, the newer the hybrid the
higher the yield (with the OPV, the best on hand in 1930, at the bottom). But
when all entries were planted at about one plant per square meter, the yield
curve was flat. At old-fashioned (lower) rates of planting the trend was
upward, but at a slower rate than when planting rates were high (modern).
I'll attach a graph which explains this clearly.
The moral is that for 60 years and more, breeders have been selecting
genotypes that can withstand crowding (and all of the stresses it imposes) and
still make an acceptable ear on every plant. Today's hybrids are planted
about three times as thick as in 1930 and yield about three times as much per
acre. Also, all other measurements I made showed that the trend is to improve
stress tolerance in many ways (e.g., stronger roots and stalks, less premature
death), or to avoid excessive energy use (for instance, by making smaller
tassels and less pollen).
This trend occurred in hybrids but it could just as easily have been done with
OPVs (in fact, it has been done by other breeders) with the same results. It
has been repeated (without conscious planning) by breeders in all parts of the
world (with hybrids because that is what they were breeding).
I now include three of the top OPVs for Iowa (from the 1920s and 1930s) in my
trials and find that although they differ from each other in yielding ability,
all three of them are on the bottom for yield and also for stress resistance
at normal planting rates, but are equal in yield to hybrids at the super-low
rate (1 plant/square meter).
The paper by Cassman and me also showed data from another kind of "no stress
environment, much different from the super-low planting rate. These data were
from irrigated contest fields in Nebraska (for example), with lots of
fertilizer and water. But the hybrids were planted at very high densities --
sometimes even higher than the highest density in my trials. Thus, plants
were under stress for sunlight and probably other things that aren't easily
noted. But these tests resembled mine in that over a period of about 20-25
years, yields have been flat, even though new hybrids are entered every year.
At this very high yield level (about 300 bu./acre) there seems to be no
improvement in yielding ability as new hybrids are introduced.
So, when yields per plant are very high (my super-low density) or yields per
acre are very high (Nebraska irrigated) there is no genetic gain in yielding
ability over the years. (OPVs were not entered in the Nebraska irrigated
trials, so one can only guess what they might have done, but I think they
would have done very poorly, for they can't stand crowding, even with plenty
of fertilizer and water.)
It appears that breeders have improved yield potential for the conditions
under which they were testing (on-farm conditions with non-excessive amounts
of fertilizer, less than perfect watering, and high density planting) but they
have not improved yielding ability for conditions that did not exist in their
trials. This is not very surprising I suppose, although sometimes correlated
responses can occur in absence of direct selection.
This is a very long response to your question. It still is not clear to me
what kind of comparisons were proposed in the mail exchanges. Perhaps the
idea was to compare OPVs and hybrids in well-tended fields with modern
planting rates and fertilizer levels. I don't know if this would be a
meaningful test, for OPVs bred for the 1920s would not know what to do with
modern densities. There are a few synthetic OPVs (as at ISU) that have been
carefully selected for modern growing conditions, but they have had much less
breeding work than current commercial hybrids, so that would not be a fair
test of OPV versus hybrids either.
Of course, a breeder such as Arnel Hallauer would say that no matter how good
the OPV, by definition an OPV is a mixture of single cross hybrids, and
therefore the highest yielding single cross hybrid in the OPV will always
outyield the OPV. Modern single cross hybrids are the equivalent of the best
single cross in an OPV. And double cross hybrids were the equivalent of a
cross between the two best single crosses in an OPV. (I should say, also,
that any breeder automatically includes stability of yield in the definition
of high yield; one-shot winners are not high yielders on average, and the
average — year in and year out, and from one field to another — is what counts
This is a very long response to your question, but maybe it can get a
conversation started. Good to be in touch.
Charles Benbrook 208-263-5236 (voice)
Benbrook Consulting Services 208-263-7342 (fax)
5085 Upper Pack River Road firstname.lastname@example.org [e-mail]
Sandpoint, Idaho 83864 http://www.pmac.net
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