Top Crop Manager

Features Agronomy Soybeans
Seed size a key factor in determining planting populations

Simple calculation can increase yield or prevent yield loss.

November 12, 2007  By Top Crop Manager

50aDetermining optimum soybean planting populations is an important step in helping
maximize yield and ensuring growers get the best return on seed investment.
But before you sit down to do population calculations this spring, first take
note of your seed size.

Although seed size varies between varieties, the most important thing to remember
is that seed size can vary within the same variety from seedlot to seedlot,
explains Monsanto Seed technology specialist Graham McGregor. "Any given
year can produce variations in seed size. That's dependent on the productivity
of the field, the population that the seed was grown in and any number of factors.
In a dry year, like 2003, you can have a much smaller seed, for example."

In 2004, the seed size situation was almost reversed, explains Ontario Ministry
of Agriculture and Food soybean specialist Horst Bohner. "This year, we're
in a different situation. We had moisture in July and August for the most part
and we had a beautiful September which helped finish off the beans nicely."
In general, Bohner feels soybean seed coming off fields last fall was 10 to
25 percent larger than the previous year, which means seed for 2005 planting
should be "closer to long-term averages."


When growers are sitting down to do rough seed calculations this winter, McGregor
advises them to check seed guides for seed-per-kilogram ratings. He cites the
First Line Seeds seed guide, for example, which provides a seed-per-kilogram
rating based on a two year average of production. In the 2005 First Line guide,
seed size varies from 5100 seeds per kilogram to 7800 seeds per kilogram for
smaller-seeded food varieties.

"Once you've done your rough calculation, you can ask your dealer how
seed size is shaping up. Sales reps are kept informed of how the seed looks
each year, whether it's larger than average or smaller than average. We won't
know for sure until it's in the bag or ready for bulk delivery. When the seed
is delivered, the actual seeds per kilogram will be printed on the bag tote
or bulk delivery receipt," says McGregor.

It is so important to keep an eye on seed size when determining populations
because both over-planting and under-planting can have implications for yield
and performance.

"If you're planting 75 pounds per acre and the seed is 10 percent smaller
than you expected, you have over-planted by 10 percent," says McGregor.
In this situation, depending on fertility and soil type, growers could see increased
lodging which can cause harvest difficulties. "Depending on growing conditions,
there could also be serious disease pressure which can lead to increased white
mould and that's going to cost you yield."

When incorrect seed calculation leads to 10 percent under-planting, growers
may lose one to two percent yield. "But if you under-plant by 20 percent,
you're simply losing end yield by not planting enough beans," explains
McGregor. "On one side of the equation you risk losing money to disease
and agronomics; on the other side you risk losing money by just not generating

When making seed calculations, Bohner encourages growers to think about growing
conditions and the number of plants they need to grow an efficient stand. It
is not uncommon to see 90 percent germination and then a 90 percent emergence
rate on that seed that did germinate. If conditions are tough, 30 to 40 percent
of seedlings may never make it to full grown plants. "In tillage trials
we conducted this year, we aimed for a population of 220,000 and we ended up
with 183,000 plants on average across 18 trials. It goes to show that you always
lose some."

Bohner suggests there are a number of scenarios where growers should increase
populations to generate the desired stand. "In a no-till situation I would
up the planting rate. I would do the same for early planting because it's a
more stressful environment, and for any kind of field that recently had sod
or grasses or has not had a good rotation. Fields with grasses and poor rotations
are more likely to have diseases or insects in there to consume the bean or
young seedling as it is trying to grow."

Growers also have to be mindful of soil type when making population decisions.
Both Bohner and McGregor agree that when it comes to heavy clays, growers need
to crank up the planting rate. "On clay soils you want to have sufficient
populations because you will lose more beans to diseases early on; you also
run the risk of crusting losses. With high populations you have more beans in
a foot of row and this is a benefit when it comes to emergence," says McGregor.
"Under clay loam and sandy loam conditions, you want to have your normal
populations and on your high fertility loams and muck soils you want to make
sure you don't over-plant."

Choosing a variety that is a good fit for your targetted population and row
width is also a key factor in a grower's population decision. "You really
want to plant the right variety for your specific field and planting conditions,"
explains Bohner. "In no-till, for instance, those beans will end up being
shorter because the conditions are more stressful. So you want to choose a no-till
variety which is a little taller and a little bushier. In a high fertility field
where there's a risk of white mould, you don't want to be going 7.5 inches with
a bushy variety – that's just asking for trouble."

Recently, Bohner says he has become more of a fan of 14 inch rows. "Overall,
I think you can reduce some of your seed costs and increase your overall stand.
That's because those beans will help push within the row and they work together
as a team almost to push that crust away. When you have seven-and-a-halfs, the
beans are more spread out and they sometimes have a harder time getting out.
When your conditions are perfect crusting is not an issue, but on a lot of the
heavier soils the ability to manage crusting can be the difference between a
good stand and an unacceptable stand," says Bohner.

When it comes to yield benefits, Bohner says the difference between 7.5 and
14 inch rows is pretty marginal. "All the research would say if there is
a bushel there, that's it. Once you move wider than that, to between 21 and
30 inches, the research is fairly consistent that you begin to lose yield potential." 


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