This week I’m focusing on how pigweed control can vary according to population density, how the control can vary with soil moisture and how soil moisture and population density can be related.

These are tough lessons we learned in the days before Roundup Ready, and a lot of growers are learning the lesson for the first time or they are having to re-learn it.

A scattered pigweed infestation can be relatively easy to control if you do things just sort of right. In a scattered infestation, the pigweeds are growing with some spacing, so spray coverage is easier. And you can get more active herbicide on a weed with some spacing than you can where the density is higher.

Scattered infestations do not deplete soil moisture nearly as quickly as more dense infestations.

In a lot of areas now, the pigweed population levels are getting back to where they were before Roundup Ready and this can make herbicides and weed control programs behave differently.

First, in a dense infestation, 70 percent control from a soil-applied herbicide goes from looking pretty good in the scattered infestation to looking like a failure. In the dense infestation, spray coverage becomes much more of an issue. All of a sudden you have pigweeds under the soybean canopy and pigweeds stacked in layers at different growth stages. We saw fields this year with 6-inch to 12-inch pigweeds and a solid carpet of emerging pigweeds underneath. In these situations it is very difficult to get enough spray on all of them.

The higher the population density, the less herbicide you can get on any individual plant. When Dick Oliver first hit me with the “active site” concept years ago, I was very skeptical and thought it was just a “college professor thing.” It did not take long for me to figure out he was exactly right.

When you combine the coverage problems with his active site concept, it is easy to understand why high pigweed populations simply can overpower herbicide treatments.

The other part of the population density equation is soil moisture. Apart from population density, moisture can be your best friend or worst enemy. Soil residual herbicides often work well with excellent moisture. We went through a period this year when residual herbicides worked very well.

At present, we are in an extended dry period with no rain in the current forecast. Under these conditions, they are not going to work.

Soil moisture has a tremendous impact on postemergence weed control. The higher the soil moisture is, the better any postemergence herbicide will work. The relationship between plant population density and soil moisture is high populations deplete soil moisture much more quickly.

When it is longer between rains, a solid infestation of pigweeds can deplete soil moisture in a matter of a week or less after emergence. Then as time goes along, you get bigger, drought-stressed weeds and have to deal with the population density issues mentioned above. That is a triple whammy.

I have brought up some of these issues because they can explain why a pigweed program that can look good in some situations can fail in others. It is because of the range of situations that are out there that I believe a LibertyLink program gives you the best opportunity for control — especially in the tougher situations.

Do not forget, however, that the same factors mentioned above affect that program as well. If the pre-emergence herbicide does not get activated, the Ignite timing needs to be earlier. In higher populations or as soil moisture is depleted, the timing needs to be earlier. In lighter infestations with better soil moisture, you may be killing 4-inch pigweeds nicely. As the situation gets tougher, you may need to be spraying 1-inch to 2-inch pigweeds.

Spray coverage gets much more important as the population increases.

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