Here is the first of an irregular series of comments for 1999 on pest management issues for the SW part of the state. This will be on a more or less weekly basis during the growing season. More if there are important current pest management topics (something has me exceptionally aggravated) and less if there is not much going on (or I wear out).
 
Corn Planting Depth

Searching for Nirvana:
As planting season approaches, producers should be making repairs and adjustments to their corn planters.
In order to save all of us some agony this spring, it is wise to do a good job at seed placement. Planting will pay big dividends if done correctly and cause a lot of heartache and unhappy bankers if done poorly. The benefits of early planting have been well discussed. Having to replant because of a poor stand or having to watch a non vigorous stand struggle through the season is to be avoided.

The most common mistake that I observe is poor seed placement with respect to depth. Some growers seem to be intent on planting at the magical 1 1/2-inch depth. Some are quite good at hitting it, at least temporarily. It is extremely difficult to plant a field uniformly at 1 1/2 inches. There are several reasons for this. Residue often bumps the depth of the unit(s) shallower. Loose seedbed conditions (But the field has never worked up this nice!) can easily settle 1/2 inch or more. One-pass spring tillage has created some extremely mellow soil conditions the past couple of springs. Seed depth that started out at 1 1/2 inches can easily be an inch or less after a good rain. The symptoms of this phenomenon are good corn in the wheel tracks and stunted corn in the rows off the tractor tires. If you try to plant at 1 1/2 inches you have a good chance (probably greater than 50 %) of ending up too shallow. High speed (> 5-6 mph) corn planting gets you done quicker but it also gets your corn planter working like a big surfboard.

So what's the big deal?
There are several problems with shallow planted corn. The potential for poor root development is increased. Soil near the surface is prone to dry out and is exposed to greater fluctuations in temperature than soil at a greater depth. This means that root development is at risk from dry, hot, or cold conditions. The seminal and nodal root (including brace root) systems can develop symptoms that look like herbicide or nematode injury (stunted, stubby and swollen) under these high stress conditions. Nodal roots develop closer to the surface when corn is planted shallow. Development can be aborted if hot dry conditions occur where roots are developing. These problems are not a big deal as long as the corn plant doesn't need water, nutrients or help standing up.

Herbicide injury can be increased in shallow planted corn. For example, injury from Prowl, 2- 4, D and dicamba can be increased from shallow planted corn. A combination of shallow planting and urea in contact with the seed can also provide some impressive symptoms.

One might think that shallow planting would speed emergence. It does for some seeds. Unfortunately, the environment is a lot more variable near the surface. Some seeds will be exposed to adverse conditions and probably be slower to germinate/emerge. They may sit there a long time if the spring turns dry. Corn planted at 2 inches will likely be more uniform in emergence than shallow planted corn. Late emerging corn functions like a weed since it is unable to compete well with the plants around it that emerged earlier. There is a pretty strong evidence poor stand and weeds reduce corn yields.

Some final thoughts
Planting is one of the most important steps in obtaining good corn yields. It takes time and seed costs were substantial the last time I checked. You only get one chance to do this right. Set your planter to plant at 1 3/4 to 2 inches and check your setting in each field. If you planter gets a bit shallower, you will be ok for depth. If you get a bit deeper you will still be ok. When checking planting depth in very mellow soil, you might want to step on top of the row to firm the topsoil before you measure depth. I have never calibrated this technique but it probably works well for people between 150 and 300 lbs. and size 9 to 14 shoes.

There seems to be an aversion to planting corn deep but no problem in planting soybeans to soil moisture, even when that first occurs at 4 inches.

Why would someone working in integrated pest management care about a fundamental agronomic issue like planting depth? Symptoms of shallow planting are usually blamed on herbicide injury and carryover, insect damage, disease problems and poor seed. Secondly, when a corn field is already afflicted with reduced yield potential due to shallow planting caused complications, managing pests becomes a lot less interesting and profitable.

Bruce Potter
IPM Specialist- SW Minnesota
University of Minnesota Extension Service
bpotter@tc.umn.edu

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