Integrated Weed Management In Corn, Soybean, And White Beans
Integrated Weed Management (IWM) uses all available weed control options in the best possible way to manage weed populations. Such options include crop rotation, cover crops, intercropping, manipulation of nitrogen fertility, planting pattern, tillage systems, critical period of weed control, alternative weed management strategies in conservation tillage systems and economic thresholds. All these practices are components of an IWM system and none of these control measures on their own can be expected to provide acceptable levels of weed control. Therefore, instead of banking on a particular method of weed control, an IWM system uses a mixture of methods of weed control; for example, reduced rates of herbicides can be combined with mechanical tillage for improved weed control. In general, IWM systems exert various selection pressures on the weeds so that the economic impact of the weeds can be minimized. By following the principles of an IWM system we can reduce the use of herbicides applied into the environment and at the same time provide optimum economic returns to the grower.Crop rotation
Crop rotation involves alternating different crops in a systematic and recurring sequence on the same land. It is an important strategy for developing a sound long term weed control program. Weeds tend to thrive with crops of similar growth requirements as their own and cultural practices benefitting crop plants may also benefit the growth and development of certain weeds. Hence, monoculture results in weed flora dominated by one or several weed species that are adapted to the growing conditions continuously maintained for the crop. When diverse crops are used in a rotation, weed suppression may be obtained by various factors. For example, a densely planted, fast-growing crop suppresses weeds by competing for resources. Variation in planting date of crops from one growing season to another disrupts weed life cycles through tillage or cultivation or early crop emergence.
Cover crops
Cover crops are an important component of IWM systems. The inclusion of plowdown crops like rye, red clover, buckwheat and oilseed radish or overwintering crops like winter wheat and forages in the cropping system can suppress weed growth. Fast growing crops or crops exhibiting allelopathic properties suppress weeds through crop interference with weeds. Highly competitive crops may be grown as short duration "smother" crops within the rotation. Additionally, cover crop residues left on the soil surface can suppress weeds by shading and cooling the soil. When choosing a cover crop, consideration should always be given to how the cover crop will affect the following crop.
Intercropping
Intercropping involves spatial diversification of cropping systems, e.g., growing a smother crop between rows of the main crop. Intercrops are able to suppress weeds however, use of intercropping as a strategy for weed control should be approached carefully. The intercrops can greatly reduce the yields of the main crop if competition for water or nutrients is strong.
Manipulation of nitrogen fertility
Nitrogen fertilizer applied within crop rotation sequences can affect crop-weed competition in subsequent crops. For example, nitrate is known to promote seed germination and seed production in some weed species. Nitrogen fertilization under weed-infested conditions may result in increased weed growth instead of increased crop yield. Supplementing nitrogen needs of the crop by using legume residues as opposed to chemical nitrogen fertilizer may result in weed suppression.
Planting patterns can supplement weed management
Altering planting patterns are a part of an IWM strategy. Crop density, diversity, spatial arrangement and choice of crop species and cultivars can affect weed growth. For example, studies have shown that narrow row widths and a higher seeding density will reduce the biomass of later-emerging weeds by reducing the amount of light available for weeds located below the crop canopy. Similarly, fast growing cultivars can have a competitive edge over the weeds.
Conservation tillage is a viable alternative
Concerns about weed management problems in conservation tillage generally arise as it is perceived that tillage systems can alter the weed species composition and populations. Although a number of predictions have been made regarding changes in weed species following the adoption of conservation tillage, it is difficult to generalize as these variations can be affected by the management practices adopted and local conditions.
Tillage systems can alter the soil seedbank dynamics and depth of burial of weed seeds. Such alterations can be beneficial from a weed management standpoint. For example, it is more beneficial to have weed seeds distributed closer to the soil. Keeping weeds closer to the soil surface improves weed control because weeds germinate and emerge more uniformly, the dormancy of the weed seeds are reduced, number of years of survival of the weed is reduced and better control can be obtained by a burndown or postemergence. Similarly, weed seeds closer to the soil surface are more likely to be predated and damaged by insects, animals, other predators and disease causing organisms. Studies have found that almost 75% of the weed seedbank was concentrated in the upper 5 cm of soil in no-till fields (Figure 1). In the moldboard plow system however, the seedbank was more uniformly distributed over depth and was larger than the other systems.
Season-long weed control is not necessary
The critical period of weed control is an important concept in IWM systems. This period has been defined as an interval in the life cycle of the crop when it must be kept weed-free to prevent yield loss. The critical period of weed control consists of two sub-periods; (1) the critical weed-free period: which is the maximum length of time weeds emerging with a crop can be allowed to grow until they begin to cause unacceptable yield losses, and (2) the critical weed-removal period: which is the minimum length of time weeds that emerge after a crop must be removed so that unacceptable yield losses do not occur. This concept helps in determining the most effective time for non-residual postemergence herbicides application, reduces the practice of season-long residual herbicides, and eliminates unnecessary late application of herbicides. The critical periods are defined relative to a crop growth stage to account for environmental variation. The critical period is based on a yield loss due to weed interference of no more than 5%. In other words, the crop has to be weed-free during these stages to prevent a yield loss of more than 5%.
Corn: The critical period extends from 3 to 10 leaf stage. Corn kept weed-free for approximately 34 DAP or until 8-10 leaf stage had a yield loss of 5% or less. Excellent weed control must be achieved from the 4th leaf to the 10th leaf stage (tip of the 10th leaf emerging above the whorl). See Figure 2.
Soybean: The critical period extends from the 1st to the 2nd trifoliate stage of soybean growth (V2 to V3). Soybean kept weed-free for approximately 15 DAE had a yield loss of 5% or less. See Figure 3.
Whitebean: The critical period extends from the 2nd trifoliate to the 1st flower (V3 to R1) stage of whitebean.
The critical weed-free period and the critical time of weed removal are shown in Table 3. If weeds have been controlled throughout the critical period, the weeds which emerge later will not affect yield and can be controlled prior to harvest with a harvest aid to burn down the weeds and desiccate the crop.
Table 1: Critical weed-free period and critical time of weed removal in corn, soybean and white bean.| ------------ Yield loss ---------- | |||||
| 5.0% | 5.0% | 10.0% | |||
| CORN | Critical weed-free period | DAP
Growth stage |
42
10-14 leaf |
34
8-10 leaf |
26
5-8 leaf |
| Critical time of weed removal | DAP
Growth stage |
36
3-10 leaf |
42
10-14 leaf |
48
14 leaf-tassel |
|
| SOYBEAN | Critical weed-free period | DAE
Growth Stage |
22-30
V3-R1 |
11-19
V2-V3 |
0-15
Planting-V2 |
| Critical time of weed removal | DAE
Growth Stage |
9-38
V2-R3 |
16-50
V3-R5 |
22-74
V4-harvest |
|
| WHITEBEAN | Critical weed-free period | DAP
Growth Stage |
64
R3-R7 |
45
R1 |
31
V3 |
| Critical time of weed removal | DAP
Growth Stage |
27 | 32 | 37 | |
| V3 ------------------------------ R1 | |||||
How to interpret Table 1: The critical weed-free period and the critical time of weed removal can be approximated from the above table. For example, the critical weed-free period for white bean is approximately 45 DAP at a yield loss level of 5% or less. In other words, white bean kept weed-free for approximately 45 DAP had a yield loss of 5% or less. Similarly, the critical time of weed-removal in white bean is approximately 32 DAP at a yield loss level of 2.5%. In other words, weeds emerging with the white bean crop required control up to 32 DAP when the crop was at the 2nd trifoliate to early flowering, to prevent a 2.5% yield loss.
Alternative weed management strategies can be implemented
Alternative methods of weed control are available for all tillage systems. In general, studies have shown that banding herbicides will increase profitability. For example, selective application of herbicides in 30-cm bands on 76-cm wide corn rows represented an approximate 60% reduction in total herbicide applied into the environment.
The need for weed control should be based on economic thresholds
Weed economic thresholds help in determining if weed density and interference is sufficient to justify control measures, i.e., if the yield loss avoided is greater than the cost of application. Redroot pigweed is a major problem weed in corn and soybeans, and ragweed is a common problem in white beans in Ontario. Economic threshold studies conducted with redroot pigweed and ragweed have found that the time of weed emergence in relation to the crop stage is more important than weed density when evaluating weed control options. Crop yield loss depended on the relative time of weed seedling emergence.
Corn: Corn could tolerate a weed density of 4 plants/m of row when redroot pigweed emerged at the 6th leaf stage of corn, but only 0.5 plants/m when they emerged at the 4th leaf stage of corn (Table 2).
Soybean: Soybean could tolerate a weed density of 4-8 plants/m when redroot pigweed emerged at the unifoliate stage of the crop but only 0.5 plants/m of row when they emerged with the crop (Table 2).
White bean: White bean crop could tolerate a ragweed density of only 0.5 plants/m of row when the weed emerged with the crop, but when the ragweed emerged at the 2nd trifoliate stage of the crop the white beans could tolerate 4 plants/m of row before it started experiencing economic yield loss (Table 2).
How to interpret Table 2: The weed densities are based on a threshold of 5%. For example, a grower scouts his/her white bean field and finds ragweed plants emerging at an average density of 4 plants/m of row, at the 2nd trifoliate stage of crop growth. Expected yield losses would be approximately 5%. Then based on his expected yield estimate, the yield loss in dollars can be estimated based on current market value. If the yield loss is $20/acre and the herbicide application will cost $25/acre, then do not spray.
Table 2: Economic threshold levels of redroot pigweed in corn and soybean, and ragweed in white bean.| Estimated densities of redroot pigweed required Crop to exceed a 5% yield loss threshold | ||
| CORN | Crop Stage | Weed density (plants/m of row) |
| 3 leaf stage | 0.5 | |
| 6 leaf stage | 4 | |
| 9 leaf stage | -- | |
| SOYBEAN | Emergence | 0.5 |
| Unifoliate | 4 to 8 | |
| 2nd trifoliate | -- | |
| Estimated densities of ragweed required to exceed a 5% yield loss threshold> | ||
| WHITE BEAN | Emergence | 0.5 |
| Unifoliate | 1-4 | |