Remember
last year? 22% of Minnesota’s corn acres were planted in mid April. The
seedbed was ideal and the soil temperature was above
normal for mid April. But, it turned cold and stayed that way
for 30 days. As
a result seed laid in the ground for 30 to 40 days. The
end result was uneven emerging plants and stands that
were substantially lower than the desired plant populations. In
addition, stands were not uniform in plant spacing. Yet – we
had a record state average corn yield in Minnesota of
156 bushels per acre!
So,
when will you plant corn this year? With
the poor start to the season last year several were asking
if we are planting corn too early in Minnesota. My answer is clearly no! The air temperatures and subsequent soil
temperatures last year were not normal. Figure
1 shows the long-term soil temperature at the two-inch
depth and the soil temperatures for last year. For
the historic average, the soil temperature at the two-inch
depth shows an increase in temperature during the month
of April and early May. The
temperature reaches 50 degrees about April 20, which is
the temperature the seed needs to reach to support and
promote germination and seedling growth. The
soil temperature fluctuates around this average temperature
with part of the day when the temperature is above average
and part of the day when the temperature is below average. As the warming trend continues, there
is a greater portion of the day when the seed zone soil
temperature is above 50 degrees. That’s why early-planted corn requires more time to emerge
than does later-planted corn.
The soil
temperature was above the long-term average last year in
mid April and then fell below the normal warming trend until
late May. There
were warming and cooling periods for the next 30 days. And
even though stands were lower than expected, one can be surprised
that such a high percentage of the seed did continue to survive,
germinate, and emerge. With a normal spring, we shouldn’t
have the long, extended time for emergence that we had last
year.
And we can’t
forget the importance of early planting for maximum yield. I
believe that early planting was a major reason for the high
yields last year. Of course the rainfall and timing of
the rains were important, but early planting sets the stage
for high yields and minimizes the first yield-limiting barrier. Production
costs are fixed and independent of when corn is planted;
planting date is a no cost production practice. So there is greater profitability with early planting and
higher yields. The
yield relationship with planting date is given in Table 1.
Are
uniform stands important and do late emerging plants contribute
to yield? Growers worried about the poor stands
and uneven emergence last year and rightly so because cornfields
are not as pretty when plants are different heights with
gaps between plants and not growing uniformly. I would also prefer to see uniformly
emerged plants is every field because each plant has an
equal chance at competition for water, nutrients, and light. So how important is this?
Results from
a study to delay emergence of various portions of the plants
are given in Table 2. When 25% of the stand (full stand was
30000 plants per acre) was missing, the yield was 90% of
potential or 10% of the yield was lost. When the stand was full (30000ppa) but
25% emerged 10 days later than the rest of the stand, yield
was reduced only 6%, so the late emerging plants contributed
4% to yield. The yield contribution was not equal to the other plants (25%
of the plants only produced 4% of the yield), but they did
contribute as long as the delay in emergence was not too
long. When emergence was delayed 20 days for
25% of the stand, there was no contribution to yield (yield
was 90% of potential which was the same yield obtained with
25% of the stand missing).
When
stands are extremely non-uniform, late emerging plants contribute
more to yield. For example, yield is only 70% of potential
with a 50% stand (30% loss). If
those 50% of the plants emerge 10 days later the yield loss
is only 8% and only 20% if they emerge 20 days later, so
the later emerging plants contribute to yield.
And,
an important point to remember is that the yield potential
of an early planted field with a poor stand is usually
better than the yield potential of a later planted field
with a full stand of uniformly spaced and similar height
plants. That’s
not what we want to look at in June and July, but has a
better profitability potential. And I haven’t mentioned that there
would be replant costs and later maturity with higher drying
costs in the fall!
And
then there’s the question of planting depth with early
corn planting. Certainly the seed zone will be warmer
the closer the seeds are to the soil surface because the
soil warms from the top down. So one might want to plant shallow with
early planting to promote more rapid germination and emergence. I
recommend planting corn 2-inches deep regardless of planting
date. I think there will be fewer potential
problems later with a 2-in depth. With
normal weather, the soil is on a warming trend and the
extra few days to emerge for corn planted 2-in deep versus
1 ½ - in deep is not worth the risk of poor root development
that can occur with shallow planting. Sometimes
the seedbed settles, particularly with rain. When
this occurs, the seed doesn’t move much from where
it was placed in the soil, but the planting depth is now
shallower because the surface settled. If the surface settles enough, then the
crown of the plant is very close to the surface and may
result in reduced and poor secondary root development. And
these roots are the permanent roots of the corn plant. So, plant corn 2-in deep.
Table
1. Effect of Planting Date on Corn Yield.
Planting Date Potential Yield (%)
April 25 100
April 30 99 May 5 97 May 10 94 May 15 91
May 25 88 May 30 83
Table 2. Effect of Uneven
Emergence and Missing Plants on Corn Yield.
Stand Condition Yield
(%) Stand Loss 25% Missing 90 Stand Loss 50% Missing 70 Stand Loss 75% Missing 49
Late Emerging 10 Days Later 20
Days Later
25% Late 94 90 50% Late 92 80 75% Late 93 78
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