By Lyndon Kelley and Brenden Kelley
Applying water to cropland has significant economic and environmental benefits to weed management. Along with the benefits from irrigation, there are potential risks to groundwater if management is lacking or equipment is not properly monitored and maintained.
Soil-applied herbicides require water to incorporate into the weed seed germination area of the soil. Many soil-applied herbicides have warnings related to the ineffectiveness of the product if rainfall doesn’t occur within seven to 10 days of application.
An example of a herbicide that may be compromised without rainfall is Bicep II Magnum. The label states, “Dry weather following preemergence application of Bicep II Magnum or a tank mixture may reduce effectiveness. Cultivate if weeds develop in conventional tillage corn or sorghum. If available, sprinkler irrigate within 2 days after application. Apply 1/2-1 inch of water. Use lower water volume (1/2 inch) on coarse-textured soils and higher volume (1 inch) on fine-textured soils. If irrigation is not possible and rain does not occur within 2 days after planting and application, weed control may be decreased. Under these conditions, a uniform, shallow cultivation as soon as weeds emerge may improve control.”
If rainfall falls short of what is required on the label to incorporate herbicide, irrigation water applied in a timely manner can save the day and reduce the need for follow-up herbicide applications to control weed escapes. Postemergence herbicide applications that control weed escapes that are not controlled by the original preemergence herbicide program represents additional environmental risks and added costs. These additional costs range between $30 and $40 per acre plus an additional $10-$12 application fee. This means that a $3.50 average energy cost to pump an inch of irrigation water has about a tenfold economic return when used to incorporate herbicides.
Even post-emergence herbicide applications can benefit from timely irrigation. Drought-stressed weeds produce thicker cuticles that reduce herbicide absorption, ultimately reducing the effectiveness of postemergence herbicides. A half-inch application of irrigation water a couple of days before the postemergence herbicide application greatly improves uptake of the herbicide by weeds, increasing the herbicide effectiveness.
Even when the weather cooperates, irrigation has an economic benefit for herbicides and other crop inputs. A $50 per acre herbicide program resulting in 150-bushel per acre corn yield is $0.33 of herbicide investment per bushel. If the same field is irrigated, the $50 investment per acre divided by the irrigated yield of 250 bushel per acre results in a $0.20 per bushel herbicide investment. The increased and more stable yields accomplished by irrigation make most land-based crop inputs more cost-effective.
Irrigation may also pose potential herbicide risks to the environment. Herbicides vary greatly in their water-solubility and absorption to soil particles. Leachable herbicides are those that move easily with water and have greater environmental concern. One of our most common soil-applied herbicides with high water-solubility is atrazine. Due to its higher water-solubility, atrazine can leach into stream and drainage water if not applied correctly. To help manage these applications, atrazine is classified as a restricted use pesticide, which requires extra training for an applicator license, and the use of additional setbacks for products that contain atrazine. Irrigation, if not properly managed, has the potential to increase water in the soil profile. This increases the chance of moving the herbicide out of the upper layers of the soil where they are effective and pushing them towards groundwater where they could be a potential contaminant.
Correctly designed, well-managed and maintained irrigation has limited potential to increase herbicide movement out of the area where it is needed. The major factor in irrigation management for the prevention of herbicide movement is good irrigation scheduling. Irrigation scheduling is simply planning when the system is turned on and how much water is applied. Correctly done, irrigation applications should not overfill the water profile and leave enough water holding capacity within the rooting zone to accommodate a 1-inch rainfall. See the Michigan State University Extension resource, “Irrigation Scheduling Tools” for resources and details.
Two areas of irrigation design and maintenance that may result in overwatered areas and movement of herbicide are irrigation system uniformity and avoiding double irrigated areas. Double irrigated areas exist where two irrigation machine’s application areas overlap. The double irrigated areas have greater potential to oversaturate the soil root zone and move the herbicide out of the area where it is needed. Irrigation system uniformity is the concept that the machine should apply water within 10% of the desired application throughout the entire system. The calculated variable for uniformity is referred to as a Christiansen coefficient of uniformity of 90%. Simply put, if you set up the irrigator to apply 1-inch of water, it should provide a 1-inch depth of irrigation water to every square yard coverage area with less than a 10% deviation.
The challenge for both double irrigated areas and areas of over-application due to poor system uniformity is that these errors will occur in the same area every time the center pivot makes an application. Thus, the potential for herbicide leaching in those areas increases over the course of the growing season.
Timely application of irrigation water improves the incorporation of herbicides, activation of herbicides, systemic herbicide uptake by weeds and helps the crop out-compete weeds. But with the benefits of irrigation comes the risk of over or unevenly applying water, resulting in potential leaching pesticides out of the root zone, endangering water resources.
Source : msu.edu