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Soybean lessons learned

Did the 2005 crop year indicate what global warming will bring in the future?

November 13, 2007  By Dave Hume

18aDid the 2005 crop year indicate what global warming will bring in the future?
Should soybean growers shift to later maturity varieties? Are there other changes
that growers should consider? Here are some of my opinions, coloured by 35 years
of field research experience: What management factors, if corrected, give the
biggest yield responses?

In my opinion: variety selection, fertility, planting date and absence of weed

Variety selection – There is a wealth
of information on varieties available at www.


Note two things: if you click on the graphs summarizing performance of varieties,
in each graph there are five to 12 bushel per acre differences between the poorest
and the best varieties. Look for varieties that are performing much better than
the ones you are now growing.

Second, note that in the cooler areas (up to 3300CHU), there are yield advantages
for the later maturing varieties within a set of trials. However, in the 3300CHU
to 3500CHU trials, there has been no advantage in going to later varieties.
What does this mean? If you are thinking about switching to later varieties
because you think global warming will cause future summers to be like 2005,
the results do not support making that switch in 3300CHU to 3500CHU areas. In
shorter season areas, if there is a top performer only two to three days later
and a lot better than the variety you are currently growing, then there's very
little risk in making a switch. However, do not jump into varieties that are
a week later. That is too much risk.

Fertility – In my experience, in Ontario
we virtually never get a yield response to adding fertilizer N. With N prices
so high, do not spend any money on N for soybeans. They can fix all of their
N requirements, regardless of yield level. If soybeans can photosynthesize well
enough to support really high yields, they also can fix enough N to support
those yield levels.

On farms without manure, most of the soil test results I am seeing are low
in K and, less frequently, some are also low in P. I have also had an opportunity
to look at minor element levels through these soil test results. We are very
fortunate in Ontario to have relatively few minor element problems. For example,
soil test results from most of Manitoba show very high pH levels (above 8),
which ties up iron (Fe) and causes iron chlorosis. Growers in most of Manitoba
and North Dakota have learned that they can only grow soybean varieties which
can tolerate low Fe levels. Here, in Ontario, that's not a problem. Similarly,
I have rarely seen problems with low sulphur, boron or copper. There are some
problems with low available magnesium in high pH (greater than 7.5) soils and
some analyses result in recommendations for zinc.

Planting date – There is a rapid shift
going on toward earlier planting. We have quickly gone away from the old maxim
that soybeans were planted in late May and the new norm is that soybeans are
planted in early May, when corn planting is finished, or even in late April.
Most of the experimental results across Ontario and the corn belt indicate two
things. One is that yield really drops off with planting dates after June 1
but that, for most dates in May, there is generally not a lot of yield response
to early planting. However, there is a growing body of evidence suggesting that
if all systems are go for high yields (more than 60bu/ac), that yields decrease
with later planting dates in May.

My interpretation is that, in years like 2005, when early growing conditions
are warm and favourable, early planting lets the soybean crop flower early,
avoid some heat and moisture stress and fill for a longer period of time. My
personal opinion is that fear of frost damage has caused growers to delay planting
of soybeans and the frequency of frost damage over the years has not been great
enough to warrant late May planting.

Yield loss due to weeds – Delaying
weed control in soybeans past the V2 stage (fully developed leaves at both the
unifoliate and first trifoliate nodes) usually causes yield reductions of five
to 10 percent, if there is significant weed pressure. Growers of Roundup Ready
soybeans should not allow weed populations to progress beyond this stage without
control being applied. We learned that lesson the hard way this year. We had
two trials with the same treatments except that one had essentially no weed
pressure while the other was dirty and was sprayed with Roundup after the weeds
had been allowed to develop until the V3 stage in the soybeans. The yield difference
was over 10 bushels per acre. Next year we'll be smarter! -30-

*Dave Hume is a Senior Agri-Coach, Agri-Trend Agrology



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