For many disease and insect pests there are well-established thresholds to help determine the most critical management times to oid yield loss. These thresholds are the backbone of an Integrated Pest Management program, helping growers reduce pesticide applications or other pest management practices by only managing pests when economically necessary.
Fewer threshold models he been developed to help make decisions on when to take weed control actions, with even more limited adoption1. This is due to several factors:
An entire weed community, composed of many species, is competing directly with the crop for the same resources (light, nutrients, water) throughout the growing season.Weed species compete with each other for the same resources that they are competing with the crop for.Crop-weed competition is highly variable, depending on both the crop and the weed species.Soil seedbank dynamics, which are a legacy of previous weed control efficacy, complicate threshold development. This becomes even more complicated since seed from different weed species survive and germinate in different environmental conditions.There is a non-linear relationship between weed density and yield loss.Perhaps the most widely accepted weed-control threshold model is the Critical Period of Weed Control (CPWC)2. The CPWC is defined as the timeframe within a crop growth cycle, during which weeds need to be controlled in order to maintain crop yields3.
How the CPWC is Determined
The CPWC is determined from two separate, but related, timeframes. Somewhat confusingly, they all he similar names, but I will try to help you keep them organized in your mind.
The first concept is called the Critical Timing of Weed Removal (CTWR) (Fig. 1). It is the maximum amount of time after planting your crop that weed competition can be tolerated until the crop begins to experience unacceptable levels of yield loss from weed competition. It determines the beginning of the CPWC. I like to think of it as an answer to the question: How long after planting can I wait before I need to start weeding?
Figure 1. Yield relative to the amount of time after planting your crop that weed competition is permitted. The longer weeds are permitted to compete with the crop, the lower yield will be. Point A is the Critical Timing of Weed Removal based on 5% yield loss.The second concept is called the Critical Weed-Free Period (CWFP) (Fig. 2). It is the minimum amount of time after planting that the crop must be maintained weed-free before the crop will no longer experience unacceptable levels of yield loss from weed competition. It helps determine the end of the CPWC and answers the question: When can I stop weeding after planting?
Figure 2. Yield relative to the amount of time after planting your crop that weed competition is prevented. The longer weeds are managed after crop planting, the higher yield will be. Point B is the Critical Weed-Free Period based on 5% yield loss.Combining these two timeframes gives you the shortest period of time that is most critical for preventing competition from weeds (Fig. 3).
Figure 3. Combining the Critical Timing of Weed Removal with the Critical Weed-Free Period gives you the Critical Period of Weed Control. The Critical Period of Weed Control (point A to point B) is the timeframe within a crop growth cycle, during which weeds need to be controlled in order to maintain crop yields.Constraints
The factors that make developing thresholds for weed management complicated are all still at play in the development of the CPWC, so it is important to emphasize that this period is only suitable as a guideline for when to prioritize weed management efforts. Just weeding during the CPWC will be more or less effective at maintaining crop yield on your farm depending on many factors, the most influential being: your weed species composition, weed density, environmental conditions, past management, and soil fertility. It is also less useful for managing perennial weeds, which he large underground storage structures, so thinking about perennial weed management is necessarily longer-term4. With that ceat in mind, below is a list of critical time thresholds for several different crops. Sometimes the authors calculated the CTWR or the CWFP instead of the CPWC. Some authors also explored factors that may be especially influential for the critical time threshold for that crop. For example, carrots he a much longer CWFP when planted early in the season than when they are planted late in the season. You may also notice that for annual crops, the CPWC is calculated based on growing degree days or time since being planted, but for perennials, it is calculated based on the time of year. These critical timeframes can also be calculated based on crop growth stage.
Table 1. List of critical timeframes for given crops.
Crop
Time
Type of Period
Impacting variables
Apples5, 6
May – July
Just before bloom – variable end date
CPWC
Planting date
Broccoli7
15 DATa (only one weeding event performed during the trial)
CTWR
Carrots8
Sown in April: 930 GDDb base 5°C (12-leaf stage)
Sown in mid- to late-May: 414 – 444 GDD base 5°C (4-leaf stage)
CWFP
Later planting reduced the CWFP
Dry beans9
3 – 5 or 6 WAPc
CPWC
Lettuce10
fertilized at 87 lbs P acre-1: 4.6 WAP
fertilized at 175 lbs P acre-1: 3.4 WAP
fertilized at 261 lbs P acre-1: 2.3 WAP
CPWC
Phosphorous fertility
Sweet potatoes11
2 – 6 WATd
CPWC
Later planting led to higher yields and may reduce the CPWC
Sweet corn12
Planted early: 160 – 320 GDD base 10°C (18 DAEe – V8f growth stage)
Planted late: 662 – 134 GDD base 10°C (53 DAE – V3g growth stage)
CPWC
Later planting reduced the CPWC
Tomatoes13
Grafted: 2.2 – 4.5 WAT
Non-grafted: 3.3 – 5.8 WAT
CPWC
Grafting shifted the CPWC 1 week earlier, but did not affect the length of the CPWC
aDAT: Days After Transplanting
bGDD: Growing Degree Days
cWAP: Weeks After Planting (direct seeded)
dWAT: Weeks After Transplanting
eDAE: Days After Emergence
fV8: Corn with eight visible collars
gV3: Corn with three visible collars
Works Cited
Wilkerson, G. G., Wiles, L. J. & Bennett, A. C. Weed Management Decision Models: Pitfalls, Perceptions, and Possibilities of the Economic Threshold Approach. Weed Sci. 50, 411–424 (2002).Nieto, H. J., Brondo, M. A. & Gonzalez, J. T. Critical Periods of the Crop Growth Cycle for Competition from Weeds. Pest Artic. News Summ. Sect. C Weed Control 14, 159–166 (1968).Knezevic, S. Z., Evans, S. P., Blankenship, E. E., Acker, R. C. V. & Lindquist, J. L. Critical period for weed control: the concept and data analysis. Weed Sci.50, 773–786 (2002).Ramesh, K., Kumar, S. V., Upadhyay, P. K. & Chauhan, B. S. Revisiting the concept of the critical period of weed control. J. Agric. Sci. 159, 636–642 (2021).Breth, D. & Tee, E. Critical weed control requirements in young high-density apple orchards. NY Fruit Q. 25, 3–9 (2017).Gut, D., Barben, E. & Riesen, W. Critical period for weed competition in apple orchards: Preliminary results. ISHS Acta Hortic. 422 273–278 (1996) doi:https://doi.org/10.17660/ActaHortic.1996.422.49.Latif, A. et al. Evaluation of critical period for weed crop competition in growing broccoli crop. Sci. Hortic. 287, 110270 (2021).Swanton, C. J., O’Sullivan, J. & Robinson, D. E. The Critical Weed-Free Period in Carrot. Weed Sci. 58, 229–233 (2010).Burnside, O. C. et al. Critical Periods for Weed Control in Dry Beans (Phaseolus vulgaris). Weed Sci. 46, 301–306 (1998).Odero, D. C. & Wright, A. L. Phosphorus Application Influences the Critical Period of Weed Control in Lettuce. Weed Sci. 61, 410–414 (2013).Seem, J. E., Creamer, N. G. & Monks, D. W. Critical Weed-Free Period for ‘Beauregard’ Sweetpotato (Ipomoea batatas). Weed Technol. 17, 686–695 (2003).Williams, M. M. Planting date influences critical period of weed control in sweet corn. Weed Sci. 54, 928–933 (2006).Chaudhari, S. et al. Critical Period for Weed Control in Grafted and Nongrafted Fresh Market Tomato. Weed Sci. 64, 523–530 (2016).