Soybean is the world’s most widely grown leguminous crop and is an important source of oil and protein for food and feed in addition to other industrial uses. However, herbicide-resistant and troublesome weed control challenges limit yield potential and threaten conservation tillage (CT) systems. Cover crops have been widely adopted as an integrated pest management component in CT systems to suppress weeds and maintain soybean yield potential. A 3-yr field experiment was conducted to estimate the influence of a cereal rye cover crop following CT on the critical period for weed control (CPWC) in soybean. The experiment was implemented in a split-plot design in which main plots as CT following cover crop (CT + CC), CT following winter fallow (CT + WF), and conventional tillage (CVT), and subplots were multiple durations of weed-free and weed interference. Results showed that the estimated CPWC of CT + CC and CT + WF treatments was 0 wk and >7 wk, respectively, in 2018. In 2019, the estimated CPWC was 0 wk, 5.0 wk, and 1.3 wk under CT + CC, CT + WF, and CVT treatments, respectively. In 2020, the estimated CPWC was 3.5 wk, >6.2 wk, and 0 wk under CT + CC, CT + WF, and CVT treatments, respectively. The presence of a cover crop delayed the CTWR and caused an early beginning of the CWFP compared with CT + WF treatment, and hence shortened the CPWC in 2018 and 2019. In conclusion, the CT + WF system did not reduce the weed competition and subsequent yield loss in soybean compared to the CT + CC system.

Limited information exists on the critical time of weed removal (CTWR) with the currently used soybean cultivars in Ontario. A study consisting of eight field experiments was conducted from 2017 to 2019 in Ontario, Canada, to determine the impact of delayed postemergence (POST) herbicide application on soybean yield based on average weed height at application, days after crop emergence (DAE), accumulated crop heat units (CHU) from the date of planting, and soybean growth stage. The regression model estimated the weed size at herbicide application that led to 1%, 2.5%, and 5% yield loss in soybean was 9, 14, and 20 cm under low weed density (averaging 73 to 134 plants m−2) and 3, 4, and 6 cm under high weed density (143 to 153 plants m−2) conditions, respectively. The estimated DAE at herbicide application time that led to 2.5%, 5%, 10%, and 25% yield loss in soybean was 24, 30, 37, and 53 DAE under low weed density and 8, 10, 14, and 23 DAE under high weed density, respectively. The predicted crop stage at herbicide application that resulted in 2.5%, 5%, 10%, and 25% yield loss in soybean was V4, V5, R2, and R5 under low weed density and VE, VC, V1, and V4 under high weed density, respectively. This study concludes that soybean yield loss is influenced by the weed density (low vs/high) and the time of the first POST herbicide application. When the first POST herbicide application was delayed until soybean was at the V2 stage the monetary loss was Can$20.46 and Can$221.20 ha−1 in low and high weed-density environments, respectively.

Nine field experiments were conducted from 2017 to 2019 in Ontario to determine the impact of early weed interference on corn yield based on corn growth stage, days after emergence (DAE), accumulated crop heat units (CHU), and weed size. The predicted weed size at herbicide application that resulted in a 1%, 2.5%, 5%, 10%, 25%, and 50% yield loss in corn was estimated to be 1, 4, 11, 53, non-estimable (N est.*), and N est.* cm under low weed density and 3, 5, 7, 11, 27, and N est.* cm under high weed density, respectively. The predicted DAE at herbicide application time that resulted in a 1%, 2.5%, 5%, 10%, 25%, and 50% yield loss in corn was predicted to be 14, 20, 27, 44, N est.*, and N est.* DAE under low weed density and 5, 7, 11, 17, 25, and 59 DAE under high weed density, respectively. The predicted CHU from planting at herbicide application time that led to a 1%, 2.5%, 5%, 10%, 25%, and 50% yield loss in corn was 468, 636, 821, 1,271, N est.*, and N est.* CHU from planting under low weed density and 207, 283, 385, 551, 972, and 1,748 CHU from planting under high weed density, respectively. The predicted crop stage at herbicide application that led to a 1%, 2.5%, 5%, 10%, 25%, and 50% yield loss in corn was V5, V6, V7, V11, N est.*, and N est.* under low weed density and V1, V2, V3, V4, V8, and V14 under high weed density, respectively. Results indicate that weeds must be controlled before they reach 7 cm in height, prior to 11 d after crop emergence, prior to 385 accumulated CHU from emergence, or prior to the V3 stage under high weed density to avoid greater than 5% yield loss.

Destructive growth analysis of field replacement series experiments with mayweed chamomile and dry field peas was used to determine the competitive relationship between the two species. Mayweed chamomile produced similar amounts of leaf area and dry weight per plant in a dry year and a wet year. On the other hand, dry field peas produced 20% more leaf area and 100% more dry weight per plant in the wet year compared to the dry year. Height, leaf area, and dry weight of peas reached maximum between bloom and pod-set, and then declined. Height and dry weight of mayweed chamomile increased steadily throughout the growing season. Mayweed chamomile leaf area reached a maximum at the beginning of flower stem elongation. Initially, the relative growth rate of mayweed chamomile was about three times greater than the rate for peas, but by 40 to 48 d after planting, rates were similar for both species. Relative yields and relative crowding coefficients for dry weight showed that peas were 3 to 20 times more aggressive than mayweed chamomile. Results of these experiments show that mayweed chamomile is a weak competitor against peas, but because it continues to grow after peas senesce, it could interfere with crop harvest.

In three replacement series experiments, winter wheat (Triticum aestivum L.), jointed goatgrass (Aegilops cylindrica Host. #3 AEGCY), and downy brome (Bromus tectorum L. # BROTE) were paired in all possible combinations to determine competitive relationships during vegetative growth. Under growth chamber conditions of ample fertility and soil moisture and day/night temperatures of 18/10 C, relative yield totals for the three species were similar, indicating that they compete for the same resources. Both winter wheat and jointed goatgrass had greater plant growth and higher relative crowding coefficients than downy brome, which indicated a hierarchy of relative competitiveness of winter wheat > jointed goatgrass >> downy brome. In other growth chamber studies, winter wheat was slightly more competitive than jointed goatgrass regardless of fertility levels. Winter wheat was the superior competitor at 18/10 C and −33 kPa (soil moisture), whereas jointed goatgrass was superior at 27/10 C and −300 kPa, conditions that are frequently encountered in the Pacific Northwest.

Replacement series experiments were conducted under greenhouse conditions to evaluate effects of population densities and proportions on the intraspecific and interspecific interference of either purple nutsedge or yellow nutsedge with tomato cv. Sunny, under nonlimiting conditions of water and nutrients. When grown with either nutsedge species for 40 d, tomato dry weight per plant increased and dry weight per plant of nutsedge decreased as their relative proportions decreased in mixture. Relative yield analysis indicated tomato is a stronger competitor than either nutsedge species. Both nutsedges appeared to be weak interspecific competitors but strong intraspecific competitors. Attenuated light on a purple nutsedge canopy showed that tomato additions can reduce light received by the weed compared to pure nutsedge stands.

A replacement series study was conducted in a greenhouse in 1998 and 1999 to evaluate the interference interactions among two rice cultivars and two red rice ecotypes. Plants were established in proportions of 3:0, 2:1, 1:2, and 0:3 (rice–red rice) plants/pot. Relative yield of Kaybonnet based on the shoot dry weight was lower than that of KatyRR or LA3, whereas PI 312777 was comparable to that of KatyRR and LA3. These results indicate that Kaybonnet was less competitive than PI 312777 when contrasted with KatyRR and LA3 red rice ecotypes. Kaybonnet (commercial rice cultivar) was dominated by both KatyRR (suspected rice × red rice cross) and LA3 (tall red rice ecotype) in tiller production, whereas PI 312777 (weed-suppressive cultivar) was comparable to either KatyRR or LA3. Both KatyRR and LA3 considerably reduced the leaf area of Kaybonnet. In contrast, PI 312777 reduced the growth of KatyRR, and its leaf area was comparable to that of LA3. The data suggest that high tillering capacity, as demonstrated by PI 312777, should be considered when breeding for rice cultivars that are competitive against weeds. This agronomic characteristic of rice may improve the success of reduced herbicide rate application programs.

Barnyardgrass (BYG) has been the most frequently reported troublesome weed in rice because it is an aggressive invader, is difficult to control, and reduces yields significantly. A replacement series study was conducted to determine how a naturally suppressive cultivar (T65∗2/TN 1; ‘PI 312777’), a nonsuppressive cultivar (‘Lemont’), and an F3 cross between the two (‘PI 312777 × Lemont’) would interfere with BYG in the southern United States. The rice cultivars did not differentially affect BYG height. The PI 312777 produced more tillers and greater shoot dry weight but was only moderately competitive (relative yield [RY]) or aggressive (relative crowding coefficient) against BYG. Competitiveness at 2:2 rice and BYG mixture proportion and replacement series illustrations on RYs for the number of tillers and shoot dry weights for the three rice cultivars indicated that PI 312777 suppressed BYG growth relatively better than the other two cultivars. Plant-for-plant, PI 312777 was more competitive than Lemont. PI 312777 × Lemont suppressed BYG relatively less than did PI 312777 and therefore would require significant genetic improvements before it is suitable for commercial use in a reduced herbicide production system.

Nearly all commercial sweet corn fields contain weeds that escaped management and, therefore, sweet corn often suffers yield losses due to weed competition. For this reason, field trials were conducted from 2009 to 2011 near Prosser, WA and Urbana, IL to evaluate the responses of weeds and four sweet corn hybrids to three levels of weed management; weed free, high intensity cultivation (HC), and low intensity cultivation (LC). Weed management level had the greatest impact on early season weed densities and HC reduced final weed biomass more than LC in 2 of 4 site-years. Two taller sweet corn hybrids with greater leaf area suppressed final weed biomass more than two shorter hybrids with less leaf area in 3 of 4 site-years. When grown with less intense weed management that resulted in more weeds, taller sweet corn hybrids with greater leaf area maintained yields better than shorter, less competitive sweet corn hybrids. Utilizing hybrids with greater tolerance to weeds and greater ability to suppress weeds could be a valuable component of an integrated weed management system.