(From grant submitted to Montana noxious weed trust fund, 1999)
Purpose: Wild oat infestations cause substantial yield reductions in small grains and other crops throughout the northern Great Plains. Montana researchers reported wild oat can reduce wheat yield up to 55% and barley yield up to 45%. To prevent and minimize the potential impact of wild oat on wheat and barley yield Montana farmers may spend as much as $20 per acre for herbicide to prevent yield loss due to wild oat competition. Nearly all of this herbicide was uniformly broadcast across entire fields.
Wild oat infestations are typically distributed in fields as many irregular patches rather than as one uniform coverage (see attached figures in the back of this proposal). This has motivated a few growers in Montana to consider reducing costs through use of variable rate techniques capable of applying herbicide to site-specific locations in fields. This technique relies upon global positioning system (GPS) and geographic information system (GIS) technologies to analyze crop yield variation and then apply treatments in accordance with productivity levels in specifically defined areas within a field.
Before growers can benefit from site-specific application they must develop detailed maps of their field’s weed distribution or rely on visual identification of the weeds at the time of management. Historically, mapping was performed by visually identifying perimeters of weed patches while crop scouting on foot or from a vehicle. Unfortunately, this conventional approach is not accurate enough for basing site-specific applications. However, in order to produce more accurate or detailed maps may require significant additional cost, offsetting any economic advantage with precision application of inputs. Ideally growers need an accurate, fast, and inexpensive method of determining weed infestation patterns at a resolution equal to, or greater than, the scale of herbicide application. Several alternative methods have been proposed that revolve around the use of GPS assisted surveying and aerial remote sensing.
Specially designed GPS equipment is now available to assist in crop scouting and weed mapping. By carrying a GPS receiver around areas containing wild oat, the perimeter of weed infestations can be accurately mapped and measured. GPS also can be used to geo-reference locations of mature wild oat from the combine during harvest. This is accomplished by tagging GPS location data as the combine header enters and exits patches of wild oat. In addition, color and color infrared films, and digital imagery are high resolution media that can provide synoptic and detailed views of entire fields. They have been successfully applied in mapping locations of wild oat infestations in wheat fields in North Dakota.
No one mapping method is both accurate and efficient. The first GPS method provides the most detailed information of patch boundaries, but is less efficient. In contrast, the combine method is more efficient, but provides less detail, and the information can only be used for weed control in the following year. Our experience in 1998 indicated that the aerial photograph method was not very useful for producing accurate wild oat maps, and so costly that it was not an economically efficient mapping method. We have not evaluated the aerial photography mapping approach from 1999 at this time.
The use of weed mapping for site-specific management will be fully implemented when the advantages of both GPS assisted mapping and aerial survey methods are linked. We propose to cooperate with crop consultants and farmers in (i) evaluating the accuracy and cost of conventional, GPS-assisted, and aerial mapping techniques, and (ii) quantify the economic return of site-specific management of wild oat. From the results, we hope to develop a site-specific management method that utilizes the weed map data
Benefits:
Wild oat control is essential for the successful production of dryland wheat in Montana with, typically, two applications of post-emergent herbicide being made to the entire crop. These weed control measures can range in cost from $10 to $30 per acre. However, recent MSU research has documented that in many fields dense infestations of wild oat tend to occupy a maximum of approximately 20% of the total area. Economic threshold models predict that herbicide costs could be reduced by at least 75% by only applying chemicals to those smaller areas within fields having weed levels that cause significant yield losses. Thus, if site-specific application reduces herbicide input by 75%, then a significant saving in input costs could be made while potentially reducing the environmental impact in terms of residues on crops and leached chemical into the groundwater.Specific Objectives and Methodology
Objectives:
Plan of Work:
This study will be organized into four phases: (i) initial weed mapping involving crop scouting and aerial photography (completed at 3 farms in 1998 and 4 farms in 1999), (ii) assessing mapping methods and deriving a final weed management map (tested in 5 fields in 1999) ; (iii) field experimentation involving economic comparison of conventional broadcast vs. site-specific herbicide application (tested in 5 fields in 1999), and (iv) dissemination of research results. Data are to be acquired in time for same season herbicide control, however, some data will be acquired in late summer for herbicide control in the following year. Experiments will be repeated in the same fields for one more years to quantify the response of wild oat patches to conventional broadcast versus site-specific management.
Crop scouting and aerial photography: Several 50 to 100 acre fields will be selected, one from each cooperating producer’s farm in Cascade, Chouteau, Liberty, and Gallatin Counties in the spring of 1998. Each field will be thoroughly surveyed for wild oat and this information used as a reference of comparison. After the reference data are collected, we will independently map wild oat patches by the following methods: conventional crop scouting, GPS-assisted scouting, interpretation of aerial imagery, and a combination of GPS-assisted scouting and aerial interpretation. Weed surveys will commence at the three leaf stage of the crop prior to application of post-emergence herbicide. Weed surveys will be conducted again just prior to harvest and sampled from the combine during harvest.
Accuracy and cost assessment: Accuracy of maps derived from GPS assisted survey and aerial imagery will be compared with maps derived from conventional field scouting. In addition, time and equipment required to conduct each method of mapping will be recorded so an economic analysis of each method can be completed. An accuracy assessment will be undertaken by measuring the correspondence between the reference map and the maps derived by each mapping method. Location and extent of weed areas should be
±15% of weed areas located on the reference map.Conventional broadcast versus site specific management: Each production field will be divided into a series of treatment application strips (see following figures for weed maps and accompanying as applied treatment maps). The first treatment will employ a variable-rate herbicide application program. The distribution of this plan will be based on spatial patterns in levels of wild oat infestations as derived from the most accurate mapping procedure. Herbicide recommendations will be based on published chemical guidelines. Higher rates will be applied in areas having heavy infestations of weeds whereas lower rates will be applied in areas having light infestations. The second treatment will employ a conventional broadcast application program. The herbicide rate will be equal to the mean rate used in the variable herbicide application program. A final treatment will be no herbicide and be contained within the broadcast treatments. Treatments will be applied to the same field area for the first and third year if the farm is in a summer fallow rotation, or every year if continuously cropped.
Herbicide treatments will be applied with a Spray Coupe equipped with a variable-rate controller and a GPS receiver. This GPS receiver will provide information for tracking and guiding the applicator along parallel lines. The strips will be harvested with farmer- owned combines that are equipped with an on-the-go grain flow sensor and GPS receiver. Measurement accuracy of a properly calibrated sensor is within
±1% and positional accuracy of a GPS receiver is within ±1 m. Yield and economic return will be contrasted between weed management programs. In addition, wild oat patches will be tracked over time to quantify their response to conventional broadcast treatment as opposed to site-specific applications.