Site-Specific Herbicide Applications

CROP PROTECTION
Site-Specific Herbicide Applications
by Heather Goudy, Francois Tardif, Ralph Brown and Ken Bennett, Department of Plant Agriculture and School of Engineering, University of Guelph

Precision agriculture is a popular conversation topic these days, with new technology such as yield monitors and satellite imagery. The advent of new tools such as Global Positioning Systems (GPS) and Geographic Information Systems (GIS) has opened the doors into a whole new era of crop management for Ontario growers.

One of the newest avenues of precision agriculture is site-specific herbicide application. Traditional broadcast herbicide applications are only economically effective when weeds are randomly or uniformly distributed in the field. However, in most cases weeds tend to be patchy or clustered. Site-specific herbicide applications only target herbicide applications for those areas of the field that are infested and therefore would represent an economic savings and environmental benefit to growers.

It has been suggested that site-specific herbicide applications could reduce herbicide inputs by as much as 40- to 60 per cent. Our University of Guelph team has decided to put some of these claims to the test, to address these fundamental questions: Will site-specific herbicide applications be as effective as broadcast applications in regards to weed control and yield? And, how stable are weed patches from year to year?

Historically, accurate site-specific herbicide applications were not possible due to the lack of technology. However, today the technology seems to be ahead of the science. Before this past summer, there was no field research being conducted testing the merits of site-specific herbicide applications. Two of us (Ralph Brown and Ken Bennett, of the School of Engineering) have developed a direct injection patch sprayer (DIS) that allows us to go to the field and test site-specific herbicide applications. The sprayer is very unique because of the immediate response time of the herbicide injection pump, as well as the sprayer’s ability to locate itself in the field. The sprayer has separate tanks for the carrier and the concentrated herbicide. The direct injection pump actually injects the concentrated herbicide slurry directly into the nozzle and is mixed only a fraction of a second before it is applied for those specific areas of the field that require treatment.

The process of patch spraying is quite involved and follows four main steps:

Detecting Weed Patches: The first step is to precisely locate where the weeds are in the field. Although aerial photography and remote sensing have been suggested methods to develop weed maps, the technology is not accurate enough to produce useful maps. We decided to use scouting on a small scale (6 x 6m grid) to get information that was precise enough for our research needs.
Formulating Weed Maps: Once the weeds have been counted, the weed data needs to be analyzed using spatial statistics and GIS software to create weed contour maps.
Prescription Maps: The weed contour maps are then divided up into manageable blocks and appropriate decisions about weed control are made for each block. The prescription map is essentially an on/off map, telling the sprayer when to spray and when not to. The block size is based on the accuracy of the weed map and the scale of the spray boom.
Patch Spraying: Finally, once the prescription map has been prepared, the operation is ready to move to the field. However, the sprayer must be able to locate itself in the field and be able to turn on and off accurately enough to match the management blocks you are targeting.

We conducted an experiment last summer on a four hectares (10 acre) no-till corn field. We scouted weeds when the corn was at the five-leaf stage of growth and then used the data to generate weed contour maps. Three weed species, perennial sowthistle, dandelion and field horsetail – were dominant in the field. The maps produced by the GIS program showed definite patches of the three weeds (figure 1). We devised different site-specific treatments that were compared to a traditional broadcast treatment. In a site-specific treatment, only those management units where weeds were present received herbicides (Figure 2). In a conventional treatment the whole area was sprayed, including bare soil. Our goal was to show weed control was effective when only patches are sprayed, compared to the whole field receiving herbicide.

Preliminary results indicate that our site-specific treatments were as effective at controlling weeds as the conventional treatment. This means our sprayer effectively targeted weedy areas and did not miss any patches. With this approach we used 26% less herbicide... and our corn yields were identical in all treatments.

Next year the study will be continued on several more fields, including conventional tilled fields with annual weeds. Patch stability will be followed for the remainder of the study to address issues about durability of weed maps in consecutive years. If site-specific herbicide applications continue to prove successful the implications for growers will be substantial. Variable rate applications, and the increased number of herbicides combinations used in one field will give the grower the ability to target each weed patch with the optimal herbicide at the optimal rate.