Site-specific weed management Case R. Medlin March 2001

Site-specific weed management is currently the newest and least advanced of the precision farming technologies, but one day it could easily offer the greatest economical and environmental benefits. The slow development of site-specific herbicide application systems is not due to a lack of effort by university and industry researchers; rather it is due to the complexity of weed population dynamics over years. For example, soil fertility conditions remain relatively constant over a period of several years. So several years of site-specific fertilizer applications may be based on one soil nutrient map collected years earlier. However, the geography and composition of weed populations can change considerably within just one growing season and even more drastically across several growing seasons. Site-specific herbicide applications fall into two main categories (1) variable rate, soil-applied herbicide applications and (2) site-specific postemergence applications.

In general the goal of variable rate soil-applied herbicide applications is, “reduce the rate in some areas and increase the rate in other areas as needed,” however, the entire field still receives herbicide applications. Variable rate soil-applied applications can be implement by building herbicide maps from already existing soil maps. Labeled rates of most soil-applied herbicides vary by soil texture, soil organic matter, soil CEC, and/or soil pH. So varying the rate of soil-applied herbicides is a matter of building herbicide treatment maps from the existing soil information and loading them into a variable rate sprayer.

The goal of site-specific postemergence herbicide applications is to treat only those areas where weeds are present. With herbicides that are taken up through the foliage, any herbicide that reaches the soil is bound to the soil surface or degraded, rendering it useless for controlling the weed. Unfortunately, there are few automated systems for site-specifically applying postemergence herbicides. The first challenge is locating the weeds. Weed maps can be developed from knowledge of past years weed infestations, data collected at harvest with a yield monitor, data collected by in-season crop scouts, or by visual analysis of the field by the applicator. Since weeds tend to spread through seed and/or vegetatively, they are likely to infest the same areas of the field the following years, but could also spread to other areas. So the capability of returning to treat marked weed patches or visually identifying weeds on-the-go is critical for most successful site-specific weed management systems.

Potential benefits of site-specific herbicide management include (a) low herbicide input costs, (b) better herbicide placement resulting in higher net return on a herbicide application, (c) less crop injury from excessive herbicide applications in areas requiring lower rates, (d) better weed control from higher rates when needed, and (e) reduced applications to environmentally sensitive areas that may experience negative effects of herbicides.