Using Knowledge of Weed Spatial Variability in Integrated Weed Management Systems

Abstract. Knowledge of the spatial variability of weeds and their propagules within a field has been touted as an important component of an integrated weed management system. To date, knowledge of weed spatial variability has been used almost entirely for the precision application of herbicides. Although precision application of herbicides is an important factor in controlling production costs and in environmental stewardship, weed scientists and others need to consider how else weed spatial variability information might be used in the management of weeds and crops.
Tillage practice. Knowing that certain large areas in a field are infested with weeds, for example, Canada thistle, field bindweed, jointed goatgrass, downy brome, that are particularly troublesome in no-till or conservation tillage systems would be important information to a grower considering adopting a particular tillage practice. Depending on the weed species present and their distribution, the grower may decide not to adopt no-till or conservation tillage at all, or he might decide not to adopt these tillage practices in only the portion of a large field infested with the troublesome weeds. Moreover, even if the entire field is- tilled, the depth of tillage can be adjusted easily while working the field. Deep tillage may be more appropriate for areas with Canada thistle, whereas shallow tillage may suffice for the remainder of the field.
Tillage implement. Growers could use a moldboard plow to deeply bury seeds of weeds such as wild oat, jointed goatgrass and downy brome in portions of a field, if they knew the spatial variability of weed seed in a field. On the other hand, growers may consider using shallow, tine-tillage in areas infested with deeply buried seeds of lambsquarters, pigweeds, nightshades and mustards. Seeds of these weeds are long-lived in the soil and deep plowing returns them to the surface where they are exposed to light and can germinate. Large-seeded broadleaf weeds, such as velvetleaf, suffer from poor establishment if not buried by about 1 to 2 inches of soil. No-tillage may be appropriate where patches of these types of weeds occur.
Crop rotation. From a weed management point-of-view, it may be okay to use a short interval between a given crop if there are no highly troublesome weed species present in the field or a portion of a large field. However, if particularly troublesome weeds are present, for example, perennial weeds, wild oat or jointed goatgrass, then long crop rotations that include spring and winter crops, warm-season and cool-season crops, and broadleaf and grass crops should be used. Dividing a field into differing crop-rotation management units according to the presence or absence of troublesome weeds represents a historical and common-sense application of knowledge of weed spatial variability.
Crop species and variety. Knowing the spatial variability of weeds in a field will provide growers with information to select the crop species and variety most competitive with a particularly troublesome weed or common group of weeds. For example, if nightshades are present, a large seeded legume would not be a good choice, or if wild oat is present spring barley rather than spring wheat should be grown, because barley is more competitive with wild oat. If downy brome is present, a tall winter wheat with rapid early growth would be preferred in those fields or portions of a field infested with this weed. If perennial weeds such as Canada thistle are present, a deep-rooted crop such as alfalfa would be more appropriate than a shallow-rooted crop such as field beans. If foxtails and barnyardgrass are present in large numbers, small-seeded vegetable crops such as carrots should be avoided until the weed seed populations can be reduced.
Seeding rate and row spacing. Generally speaking, planting crops at higher than normal seeding rates will enhance crop competition against weeds. Changing the seeding rate is relatively easy and could be used in portions of a field where weed seed populations are high. Narrow row spacings also enhance crop competitiveness, but would be difficult to change within a field. On the other hand, increasing row spacing for some crops to accommodate precision cultivation in certain areas could be accomplished by blocking every other drill opener. Some growers cross-seed areas with known high populations of weeds, thus increasing crop competitiveness.
Seedbed preparation date. Most growers need to prepare seedbeds and plant their crops as soon as possible to ensure not only high yields, but the timeliness of a myriad of other farming and family obligations. Nevertheless, purposefully delayed Seedbed preparation in weedy blocks of a field may have considerable beneficial effects on control. These effects are especially apparent in spring-sown crops for early-emerging broadleaf weeds such as ragweed and lambsquarters.
Timing of in-crop cultivation. Knowing the spatial variability of weeds within a field will help farm managers locate in-crop field cultivations for maximum weed control effectiveness. Because weed species appear at different times in a field, knowledge of weed periodicity would have to be known also to maximize the use of weed spatial variability for weed control with this strategy. The real estate adage “location, location, location” has to be appended for precision farming with “timing, timing, timing.”
Precision application of certain insecticides. Certain insects are associated with specific weeds. For example, stalk borer is strongly associated with giant ragweed infestations in corn fields, and cutworms that attack asparagus are strongly associated in early spring with patches of Canada thistle and field bindweed. Knowing the spatial variability of these weeds would allow growers to place insecticide more precisely, thus reducing production costs and insecticide use.
Determining the “true” weed pressure. Given equivalent total weed populations, crop yield losses are usually less in fields where weeds are aggregated rather than distributed uniformly or randomly. Consequently, knowing the spatial variability of weeds will allow a more accurate prediction of potential yield loss, and thus give farm managers the opportunity to change production and weed management practices to minimize these losses.
Plant residues. Knowing the location within a field of certain weeds will give farm managers the opportunity to spot-burn weed and crop residues to kill most of the weed seed on the soil surface. Also, this information will allow livestock grazing to be managed selectively so that especially noxious weeds will not be spread to the rest of the field, or so that plants poisonous to livestock can be avoided.
Postharvest quality management of crop products. If the spatial variability of weeds within a field is known, farm managers can segregate crop products to minimize the impact of weeds on crop quality and price received. Some applicable examples include: 1) not harvesting areas in mint fields infested with horseweed, a weed that produces a high quantity of oil that taints mint oil; 2) not harvesting areas in green pea fields infested with nightshade because there is a zero tolerance for nightshade berries in canned or frozen peas; 3) not harvesting areas in wheat field grown for seed that are infested with jointed goatgrass; and 4) avoiding areas in alfalfa fields infested with mustards that can impart off-favors in the milk from dairy animals fed weed-contaminated hay.
Increasing the awareness of specific weed problems. Knowledge of the spatial variability of weeds seems to be increasing the awareness among crop advisors and producers that the biology and ecology of weeds are important and that this information can be used for weed management and economic gain.
We hope these examples will stimulate more thought and research on how we can use weed spatial variability information to enhance weed management, reduce farmers dependence on herbicides, and improve farm profitability.