Variable-density planting can be a cost-effective tool for weed control

High-density crop planting is a proven approach for suppressing patches of weeds that escape other controls. Unfortunately, though, the cost of seed keeps many growers from considering this dense planting strategy to address weed control.
Researchers writing in the journal Weed Science describe a bioeconomic model growers can use to overcome the cost barrier. The model is based on a two-year study conducted in maize, cotton and soybean crops. Researchers explored whether the cost of higher-density plantings in areas with weed escapes could be balanced by lower-density plantings elsewhere in the field.
Completed between 2019 and 2020 summer seasons, the planting occurred at the Upper Coastal Plain Research Station in Rocky Mountain, NC. Each crop had six planting arrangements. Local conditions of eastern North Carolina and the settings of the tractor-mounted planter used.

1. Normal density in all rows as the control
2. 75 percent of normal density planting in all rows
3. 50 percent of normal density planting in all rows
4. 25 percent of normal density planting in all rows
5. Sequential arrangement of alternating 25 percent and 75 percent planting densities (hereafter 75-25-75-25),
6. A 25-75-75-25 pattern of planting densities (hereafter 75-25-25-75).

The study demonstrated distinct physiological differences in each crop that impact the balance needed to achieve a cost-neutral result. For example, maize grown at a 75 percent planting density produced a 229 percent increase in yield, a 43 percent increase in return and a 79 percent increase in profit as compared to areas planted at a 2X density. Cotton planted at a 25 percent density produced a 1,099 percent increase in yield, a 46 percent increase in return and a 62 percent increase in profit as compared to two times the planting density.
These results decrease the low-density area needed to compensate for higher-density plantings for weed suppression. By contrast, soybean crops exhibited a one-to-one ratio for the same measures, regardless of planting density.
“By using our optimization model, growers can now adopt variable planting strategies at a large scale to increase weed suppression while maintaining or even reducing their costs,” says Sandra Ethridge of North Carolina State University. “They can balance denser plantings in areas where weeds have been identified with lower-density plantings in areas known to be weed-free or at lower risk.”