Weed Density and Soil Active Herbicides

Weed response to soil applied herbicides is dependent on weed density with herbicide efficacy decreasing with increasing weed density. While soil applied herbicides are applied to the soil and in some respects we are treating the soil, it is actually the weed that is being treated. The soil in this case is merely the medium supporting the weed. Soil texture, organic matter and pH influence the bioavailability of soil applied herbicides. Application rates of many soil applied herbicides varies with soil properties in order to 'provide' a constant quantity of biologically active herbicide to interact with the weed.

Even with herbicide rates adjusted to compensate for soil properties there is a weed density effect on herbicide activity. Since we are actually treating the weed rather than the soil the more weeds present the greater the amount of herbicide required for the same level of activity. This is due to competition between plants for the available herbicide even though this competition is harmful to the plant. With a constant amount of herbicide present as weed population increases the amount of herbicide available for each weed decreases resulting in decreased activity. Careful experiments provide insight into this relationship. As soybean planting density increased form 10 to 60 plants per square foot the effect of atrazine on soybean growth decreased (Table 1).

As soybean planting density increased from 2 to 10 plants per pot in a greenhouse experiment the uptake of radioactive labeled atrazine as indicated by disintegrations per minute (dpm)/plant decreased from 19,160 to 6,371 (Table 2).

Soybean plants at high planting density were exposed to less herbicide per plant and as a result were less affected. This plant density effect on herbicide activity demonstrated with atrazine, a root uptake herbicide, has also been demonstrated with alachlor, and EPTC shoot absorbed herbicides (Tables 3,4 and Figures 5,6).  

This effect of plant density on herbicide activity demonstrated in greenhouse experiments also is observed in the field. This response has important implications for weed management. Hartzler and Roth demonstrated that the herbicide Bicep (a combination of atrazine and metolachlor) provided a greater degree of giant foxtail control when the foxtail density was 13 plants per square foot than at 40 plants per square foot (Figure 7). Typically weed infestations are not uniform across agricultural fields. Rather some areas of fields have higher weed densities than other areas in the field. However weed management practices tend to be applied uniformly across fields. Rates of soil applied herbicides may be adjusted for soil properties to provide a constant amount of biologically available herbicide but are rarely adjusted based on expected weed density. As a result the herbicide has a differential effect on weed control across the field with efficacy greatest in areas of low weed density and lowest in areas of high weed density. This contributes to perpetuating the patchy distribution of weeds.