Farm Progress

Corn Source: Shallow and small root systems are a key reason why yield varies so much this fall.

September 7, 2017

4 Min Read
BIG DIFFERENCES: Widely varying corn yields will be evident in much of Iowa this fall. The yield range won’t only vary from field to field, but also within fields.

By Joel DeJong

The 2017 crop season has again been an interesting one. Here in northwest Iowa, rainfall during the growing season was spotty, and it’s been a year when the “sins” of planting have haunted fields all season. I’m amazed at the differences in yield potential this year. Some fields look quite good, while others will be collecting crop insurance checks. There will likely be as wide of a yield range in northwest Iowa this year as we’ve seen in several years. The yield range will not only vary from field to field but within fields as well.

As an ISU Extension field agronomist, I get to spend most of my summer looking at fields with problems: not the good fields. In this column I’ll review some of the corn problems observed that added greatly to variable yield expectations in and between fields.

The easiest spots to pick out are related to low rainfall amounts and soil types. As we know, sandy soils hold less moisture than a silt loam soil, and when rainfall is sparse, those spots show up first. In my neighborhood, daily leaf rolling of some corn plants started before the end of June.

Like most years recently, the 2017 planting season was a collection of starts and stops. It was hard to find a window when soil conditions were fit, and the weather forecast did not include significant rainfall and cold temperatures. Because of this, we had some fields that were not planted in ideal conditions, particularly as we got near mid-May. In fact, even in late July we could see the seed furrow open where the planters ran.

Rooting depth makes a difference
In much of Iowa, we often assume the rooting depth of corn is about 5 feet. I’ve conducted meetings where we looked at soil pits in deep loess soils and measured roots as deep as 9 feet, although not every year. A foot of good Iowa soil can hold around 2 inches of moisture for the crop at field capacity. So the deeper the root system, the less we have to rely on timely rainfall.

However, many of the problems I’ve observed between and within fields are related to how well you established that root system. If you have small root systems but timely rain, you are OK. However, small root systems combined with limited rainfall causes stress on the corn plants.

Remember, the first root a corn plant develops comes from the seed. The root system maximizes its size when the corn plant is very young. To grow a good crop we need to get the secondary, or nodal, root system established. The first set of nodal roots doesn’t grow from the seed; it originates from that first “shoot” about 1 inch below where the shoot first sees sunlight.

Timing of rainfall impacts root development
We are often concerned about good seed-to-soil contact, and in recent years we’ve become more concerned about sidewall compaction: the area where these nodal roots establish. This year I observed problems with that zone of nodal root establishment in many fields. Since a number of fields in this part of Iowa were planted too wet, under the dry conditions we experienced here after planting, we dried out the area where these roots would normally grow into the soil and get established. The soil dried, got hard, and restricted root growth. A good soaking rain would have lubricated the soil and allowed growth, but we did not get that rainfall this year.

Another situation I observed was where a corn-on-corn field was tilled several times to “make a good seedbed.” When I visited that field in July, the soil was still so soft that I sunk into the ground with every step. The seed depth measured about 1.75 inches, which should have been fine. However, the first nodal roots weren’t emerging any deeper than a quarter of an inch below the soil surface.

Little rain fell between planting and emergence in this field, and then a fast inch fell. I think this rainfall packed some of that looser soil after the depth of nodal roots had been established (emergence), therefore shallowing up these roots. Surface soil dries out faster than deeper soil, which impacted the root development in this field.

Do you know where your roots are?
This year, I also observed the normal things that limit corn root growth; such as soil compaction that physically restricts root growth, shallow planting which pushes nodal root growth toward the surface and corn rootworm feeding that destroyed much of some root systems.

These are not all of the reasons why some fields, or parts of fields, performed the way they did in 2017. Timely and adequate rainfall during the growing season reduces the need for a well-established root system. In many parts of Iowa this year we needed good roots. Did your cornfields have them? Have you dug and looked?

DeJong is an ISU Extension field agronomist covering northwest Iowa. Contact him at [email protected].


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