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Plant populations in soybeans: how low is too low?

Studies show savings in seed and similar yields.

November 14, 2007
By Top Crop Manager


24aAs commodity prices continue to struggle, growers are becoming more likely
to question certain practices and standards, whether it is to increase revenues
or decrease costs. Planting populations in soybeans is one of those issues:
How low can they go?

At the 2006 Southwest Agricultural Conference, Horst Bohner, soybean specialist
with the Ontario Ministry of Agriculture, Food and Rural Affairs, fielded a
question from Eric Kaiser, a no-till grower from Napanee, Ontario. Kaiser asked
about minimum planting populations in soybeans, noting he had read an article
that called for the equivalent of one soybean plant for every eight to 8-1/2
inches, or about 64,000 plants per acre, to achieve maximum yield. That differs
greatly from provincial recommendations which call for a minimum of 150,000
plants per acre.

According to Bohner, there are a lot of parameters: soil type, use of a planter
versus a drill, planting date, row widths, weed pressure and the season's weather,
among others. While it may be true that lower seeding rates work if emergence
is excellent, the problem is that often, only 50 to 70 percent of soybean seeds
emerge. Very low seeding rates are risky unless everything goes perfectly.


Whether the producer is into IP, conventional or Roundup Ready systems is another
key issue in the debate. "It gets interesting when we're talking about
Roundup Ready seed, or some specialty certified IP soybean," explains Bohner,
adding that the issue for bin-run is almost irrelevant. "With cheap seed,
just plant extra for insurance but with Roundup Ready seed, you're in the range
of $6.00 for 25,000 seeds, so you're talking about a bushel in today's economics
when you raise or lower the population by 25,000. If we could get away with
175,000 as opposed to 225,000, which is our recommendation now in 7-1/2 inch
rows, that would be about $12 depending on seed size. With the cost of Roundup
Ready seed, we can't afford to plant extra."

Putting numbers to the test
In field studies across the province, Bohner has been testing different populations
at 7-1/2 inch and 15 inch row spacings. Twelve trials were conducted in 2005
and 20 different locations were being tested in 2006, with most in Middlesex,
Huron, Perth, Wellington and Niagara region. "What we're testing at all
of the sites is 175,000, 200,000 and 225,000 at both widths," he says.
At a significant number of those sites, he also is testing 150,000 and 250,000
seeds per acre. Most are done using a seed drill but they are comparing drills
to planters, as well. "Chances are we're not going to go below 150,000
seeding rate, and the research we have is pretty conclusive that once you've
dropped below that, it's just too much. Above 250,000, I can't see that we're
going to do that, although we might on some clays."

The principle favours something in the middle, between 150,000 and 250,000
seeds per acre depending on soil type, planting date and equipment used. Any
lower and Bohner cites problems in emergence and subsequent pod set per acre.
Any higher and growers open themselves to challenges from lodging and white
mould. "From 2005's numbers, at 150,000 seeding rate, we got clobbered
in 7-1/2 inch rows. On 15 inch rows, it wasn't so bad," says Bohner. "But
until we find out different, my research so far shows 200,000 seeds per acre
provides maximum yield in both 7-1/2 inch and 15 inch rows. Yes, there's some
contradictory research, but it can be often be explained by geography, weather,
planting equipment and other factors."

A successful contradiction
Where Eric Kaiser is concerned however, there is considerable contradiction,
and with all the right results. Kaiser and his son Max, farm near Napanee, Ontario,
on what are often cited as some of heaviest clays in the province. Yet test
plots on his 900 acre no-till farming operation have shown he can get comparable
yields from populations well below the 150,000 minimum suggested by Bohner.

The secret to success for Kaiser is understanding what works best for his farm.
Despite the heavy Napanee clay, Kaiser has been able to beat the odds, and grow
no-till beans at lower populations. He credits much of his success to something
he read about spacing, leaving eight to 8-1/2 inches between plants in any direction.
"The difficulty is achieving the uniformity required, and at some point
you can get down to 90,000 or 100,000 and still achieve enough uniformity that
you won't compromise much below that 100 percent yield goal," says Kaiser.
Like Bohner, he notes that with conventional or bin-run, the economics become
irrelevant. "With Roundup Ready, it gets relevant pretty quick. When I
did the arithmetic five years ago, the difference between 200,000 and 150,000
was about $12 an acre."

At that rate, Kaiser reasoned he would be able to pay for a new No-Till planter
in about 20 years, just on the savings in soybean seed alone. In fact, he and
his son Max have found it outperforms a seed drill in attaining even emergence.
"It's very difficult to get uniformity with the seed distribution system
in a seed drill, or with a planting depth uniformity that a seed drill will
provide, particularly in heavy clay soils, which we have," explains Kaiser,
who rejected a no-till drill and purchased a new planter in 2000. With their
own modifications, he and Max now use it to plant six 30 inch rows of corn and
17 nine inch rows of soybeans and wheat. "Right away we realized it provided
us with uniform spacing and a uniform planting depth, which will maximize the
germination potential, and if you can get better seed placement, better germination
and uniform spacing, you can move towards lower populations."

Plant or drill for the best?
With the planter design that drops the seeds individually instead of bunching
them, Kaiser saw an opportunity to reduce planting populations. The first year
of his trials, he planted 170,000 on the assumption it would work, and it did.
"In 2006, we have approximately a third of our acreage at 130,000, and
while we've been planting 150,000 in between, we've never seen a negative yield
response with populations as low as 110,000."

In 2005, Eric and Max planted some plots for Pioneer with populations as high
as 250,000. But in a dry year, the lowest population plot had emergence problems
in one area, compromising that result. Other plots validated the concept. In
2006, the Kaisers put in six plots at Bohner's request, with 110,000, 130,000,
150,000, 170,000, 200,000 and 225,000. Emergence rates of 68 to 82 percent are
in the normal range giving emerged stands of 92,000 to 170,000, depending on
planting rates. By September 24, 2006, one of two farm plots had been harvested
with results consistent with expectations.

Kaiser's belief in lowering planting populations is unwavering. And he advocates
growers in other regions to try it for themselves. He may be an exception to
the rule, but given his soil type, location and his commitment to no-till, he
sees no reason for not trying. "If it was going to have difficulty anywhere,
it would have difficulty where we are because most of our land is very heavy
clay," says Kaiser. "You can achieve a lot of the things you require,
so far as planting conditions are concerned in Middlesex loam, that you can
never get in Napanee clay so if we can make it work in heavy clay soils, anybody
can make it work." -30-