Introduction
Drought severity and duration has been increasing and is expected to continue with ongoing climate change (Backlund et al. Reference Backlund, Janetos and Schimel2008; Dai Reference Dai2013). These impacts extend to other abiotic factors such as intense storms, highly variable rainfall patterns and resulting soil moisture, and rates of evaporation, among other factors (Skendžić et al. Reference Skendžić, Zovko, Živković, Lešić and Lemić2021). While these changes may affect plant growth, it is not clear how they impact the competitive interactions between crops and weeds. Studies have shown that weeds are directly affected by climate change, as they may expand their geographic range and change their biology and ecology (Malarkodi et al. Reference Malarkodi, Manikandan and Ramaraj2017). In part, some weeds have a wide range of developmental and physiological mechanisms that enable them to survive under adverse environmental conditions (Sun et al. Reference Sun, Khattak, Abbas, Nawaz, Hameed, Javed and Du2023). Therefore, it is predicted that they will be more competitive and more adaptive than most crops in elevated temperatures caused by climate change (Ramesh et al. Reference Ramesh, Matloob, Aslam, Florentine and Chauhan2017). This poses a major challenge to existing weed management strategies, as they may not be effective in the future. Understanding how weeds interact with crops is important in developing cost-effective and sustainable pest management practices in water-stressed environments (Manuchehri et al. Reference Manuchehri, Fuerst, Guy, Shafii, Pittmann and Burke2020; Swanton et al. Reference Swanton, Nkoa and Blackshaw2015).
Like other crop production systems, corn (Zea mays L.) production in the Intermountain West region of the United States faces biotic and abiotic challenges, with drought being one of the most severe abiotic issues (Sammons et al. Reference Sammons, Whitsel, Stork, Reeves and Horak2014). Drought has been projected to lead to economic losses both within the United States and globally, because corn is highly sensitive to water stress, especially 2 wk pre- and post-silking (Tollenaar and Lee Reference Tollenaar and Lee2011). Drought is also associated with severe infestations of pests (e.g., weeds and arthropods) that thrive in these conditions, which further contributes to increased economic losses (Skendžić et al. Reference Skendžić, Zovko, Živković, Lešić and Lemić2021). For instance, redroot pigweed (Amaranthus retroflexus L.), velvetleaf (Abutilon theophrasti Medik.), spiny pigweed (Amaranthus spinosus L.), spider mites (Tetranychus urticae), and sap feeders increase in prevalence within crop fields as drought conditions increase (Chauhan and Abugho Reference Chauhan and Abugho2013; Costea et al. Reference Costea, Tardif and Brenner2003; Gill et al. Reference Gill, Bui, Clark and Ramirez2020; Huberty and Denno Reference Huberty and Denno2004; Oerke Reference Oerke2006; Patterson and Highsmith Reference Patterson and Highsmith1989). Therefore, adoption of strategies for conserving moisture is critical for maintaining high yields in this region (Fuentes et al. Reference Fuentes, Flury, Huggins and Bezdicek2003).
The use of new technologies, including efficient irrigation systems like mobile drip irrigation systems and the use of drought-tolerant crops, has helped in conserving moisture (Blum Reference Blum2011; Kisekka et al. Reference Kisekka, Oker, Nguyen, Aguilar and Rogers2017). However, there is a lack of understanding about whether the use of these new technologies alters the critical period of weed control (CPWC) in corn production systems. The CPWC is the period in which crops must be kept weed-free to support growth and prevent yield loss due to weed interference (Van Acker et al. Reference Van Acker, Swanton and Weise1993; Zimdahl Reference Zimdahl1988). The CPWC is estimated from two factors: the critical weed-free period (CWFP) and the critical time of weed removal (CTWR), which show the beginning and end of the CPWC, respectively. The CWFP is the minimum duration of a weed-free period required following planting to prevent yield losses greater than 5%. The CTWR is the maximum period a crop can be exposed to early-season weed competition before it reaches a yield loss threshold (Hall et al. Reference Hall, Swanton and Anderson1992). This study hypothesizes that drought-tolerant (DT) corn hybrids have a shorter CPWC compared with drought-susceptible (DS) corn hybrids when exposed to water-stress conditions. Knowledge of the CPWC in crops is essential to developing successful weed management systems, as it allows weed control measures to be utilized effectively (Swanton and Wise Reference Swanton and Wise1991). For example, it provides guidelines for the use and timing of preemergence and postemergence herbicide applications (Knezevic et al. Reference Knezevic, Evans and Mainz2003). Therefore, CPWC provides generalized guidelines for the timing of weed control practices based on the observed mean yield loss (Knezevic et al. Reference Knezevic, Evans, Blankenship, Van Acker and Lindquist2002).
The CPWC is highly variable, as it is influenced by various factors, including crop characteristics (like drought-tolerance traits), weed characteristics, environmental variables (Tursun et al. Reference Tursun, Datta, Sakinmaz, Kantarci, Knezevic and Chauhan2016), cropping practices (e.g., crop planting density and row spacing) (Osipitan et al. Reference Osipitan, Adigun and Kolawole2016), soil nutrients (Odero and Wright Reference Odero and Wright2013), and the preemergence weed control program (Knezevic et al. Reference Knezevic, Elezovic, Datta, Vrbnicanin, Glamoclija, Simic and Malidza2013; Knezevic and Datta Reference Knezevic and Datta2015). For example, a weed-free period of 50 d from planting to prevent corn yield losses was recorded in Mexico, compared with a 21- to 28-d weed-free period recorded in the United States (Knake and Slife Reference Knake and Slife1969; Nieto et al. Reference Nieto, Brondo and Gonzalez1968). Moreover, other studies have also reported various CPWC values in corn. For instance, Gantoli et al. (Reference Gantoli, Ayala and Gerhards2013) reported a CPWC starting at the 4- to 6-leaf stage and continuing until the 10-leaf stage or flowering of corn. A CPWC of 1- to 5-leaf stage was reported by Isik et al. (Reference Isik, Mennan, Bukun, Oz and Ngouajio2006). On the other hand, Hall et al. (Reference Hall, Swanton and Anderson1992) reported that the beginning of the critical period ranges from 3- to 14-leaf stages of corn development, while the end of the critical period occurs on average at the 14-leaf stage.
Knowing the CPWC is an important tool in integrated weed management, as it is useful in reducing the need for and utilization of chemicals as well as their associated impacts on the environment (Harker et al. Reference Harker, O’Donovan, Turkington, Blackshaw, Lupwayi, Smith and Willenborg2016). There is a need to understand the CPWC for DT corn hybrids especially under water-stress environments. Our objective was to evaluate the CPWC of DT and DS corn at different irrigation levels.
Materials and Methods
Experimental Design and Procedure
A two-season field experiment was established in a split-split plot design using two glyphosate-resistant corn hybrids (drought-tolerant DKC 47-27 DroughtGard Double Pro® and drought-susceptible DKC 46-36; Bayer Crop Science, Whippany, NJ, USA), two irrigation levels (optimal and reduced irrigation level), and six weed removal timings. The experiments were conducted in 2021 and 2022 at Greenville West Research Farm at Utah State University, Logan, Cache County, UT, USA (41.76918°N, 111.82170°W). Season 1 (2021) and Season 2 (2022) experiments were conducted in the same field under tillage. Before this experiment, the field was under alfalfa (Medicago sativa L.), and no herbicides had been applied. The soil type in this field is Millville silt loam (coarse-silty, carbonatic, mesic Typic Haploxerolls), with a slope of 0% to 2%, organic matter of 0.77%, and pH of 8.1. The mean air temperatures and rainfall experienced during the growing seasons were 32 C and 345.44 mm, respectively, in 2021, and 29 C and 466.85 mm, respectively, in 2022.
Corn was planted with a four-row planter (7300 4-row planter, John Deere, Montgomery City, MO, USA) on May 13, 2021, and May 17, 2022, in plots measuring 6.1 by 3.05 m at a density of 66,172 seeds ha−1 in both seasons. Each plot had 4 rows of corn spaced 0.77 m apart. The plots were arranged in a randomized complete block design (RCBD) within the varied irrigation levels. Each plot received a single treatment of irrigation level, corn hybrid, and timing of weed removal. There were a total of 48 plots per replicate, representing each treatment combination, with each treatment replicated four times. The irrigation level was considered the whole-plot factor, corn hybrid was the split-plot factor, and time of weed removal was the split-split-plot factor. Timing of weed removal was based on corn development stages, which were determined by observing the number of leaf collars in the weed-free control plots (Ritchie et al. Reference Ritchie, Hanway and Benson1997). Weed removal treatments consisted of increasing duration of weed interference and increasing duration of weed control or increasing length of weed-free period. In treatments of increasing duration of weed interference, weeds were in competition from their time of emergence until corn reached the following growth stages: V3, V6, V9, V12, and VT (V representing vegetative). After this time, weeds were removed, and plots were kept weed-free for the remainder of the season. Treatments of increasing duration of weed control were kept weed-free until corn reached the following growth stages: V3, V6, V9, V12, and VT. After this time, weeds were left to grow for the remainder of the season. Each set of treatments had weed-free and weedy controls.
The naturally occurring weed population was used, and weeding was accomplished using glyphosate (PowerMax®, 540 g ae L−1, Bayer Crop Science, St Louis, MO, USA) at the rate of 2.34 L ha−1 followed by hand weeding for all weed removal treatments. Herbicide application was achieved using a CO2-pressurized backpack sprayer calibrated to deliver 187 L ha−1 at 276 kPa with four AIXR 1103 nozzles (TeeJet® Technologies). Each treatment received one application of glyphosate and subsequent hand weeding. Irrigation was achieved through use of overhead sprinklers (Teejet® Technologies), with fully irrigated plots receiving water once a week and reduced-irrigation plots receiving water biweekly for 5 h at each irrigation time. Blocks receiving different irrigation levels were placed 13 m apart to ensure that the two distinct irrigation levels were achieved. Moisture was monitored using Acclima soil sensors (TDR-310H, Acclima, Meridian, ID, USA) installed 30 cm deep. The sensors recorded data after every 6 h daily throughout the growing season. Volumetric water content was monitored weekly, and plots were watered accordingly to achieve desired moisture levels.
Weed infestations were evaluated at the completion of each treatment (considering time as a factor in the weedy treatments), the weeds were classified to species and the number of each species within a 0.25-m2 quadrat was recorded. The quadrat was placed at three different locations within each plot between the two middle corn rows. To determine the aboveground dry weight, weeds within the quadrats were clipped at the soil surface and dried at 70 C for 3 d to maintain a constant moisture content. The heights of two random plants of each weed species within the quadrat were measured and averaged. Additionally, corn heights from three randomly selected plants per plot were recorded by measuring corn in the middle row from the soil surface to the tip of the plant. Stem diameter was also measured at 10 cm from the soil surface (considered minimum diameter) and at maximum corn diameter. Crops were harvested from the two center rows measuring 6.1 m on October 1, 2021, and September 29, 2022, using a GEHL 865 pull-type forage harvester, CB865 (Gehl Company). The wet biomass was measured immediately, and a subsample of the plant material was collected, dried at 70 C, and weighed to determine the biomass.
Statistical Analyses
To determine the CPWC, yield data from the individual plots were calculated as the percentage of the corresponding weed-free plot yields. The PROC MIXED of SAS v. 9.4 (SAS Institute, Cary, NC, USA) was used to analyze the relative yield data by assessing the effect of the length of the weed-free period and increasing duration of weed interference on relative corn yield. The 5% level of probability was used. When two-way interaction effects were significant, the SLICE procedure of SAS was used to separate significant effects. When no significant interactions were observed, differences within significant main effects were determined using Tukey’s honest significant difference post hoc test. Normality tests were also completed through assessment of normal probability plots of the histograms of the residuals.
A three-parameter exponential decay equation was used to describe the effect of increasing the duration of weed interference on yield, which determines the beginning of the CPWC (Ratkowsky Reference Ratkowsky1990):
$$Y = c + \left( {d - c} \right){\rm{exp}}\left( { - X/e} \right)$$
where Y is the relative yield (percent of season-long weed-free yield), X is the duration of weed interference measured as the number of days after planting, d is yield with no weed interference, c is minimum yield possible due to weed competition, and e affects the rate of yield loss as weed interference duration increases.
The following asymptotic regression equation was used to describe the increasing duration of weed control on corn yield (Cousens Reference Cousens1988):
$$Y = a - \left( {a - b} \right){\rm{exp}}\left( { - cX} \right)$$
where Y is the relative yield (percent of season-long weed-free yield), X is the duration of weed interference measured as the number of days after planting, b is Y at X = 0, a is maximum yield when weeds are controlled all season, and c is a rate constant that controls how quickly yield improves with increasing weed-free period.
Acceptable yield loss levels of 5% were used to determine the CPWC. Season was considered a random effect, and because significant (P ≤ 0.05) effects due to season were observed, the two seasons were analyzed separately. Parameter estimates were also obtained for each hybrid and each irrigation level separately (Table 1).
Table 1. Parameter estimates for the exponential decay and asymptotic regression equations for drought-tolerant (DT) and drought-susceptible (DS) corn in Logan, Utah in 2021 and 2022.a
a Asterisks (*) denote significant difference at: ***0.001; **0.01; and *0.05.
Results and Discussion
Effect of Irrigation Level, Hybrid, and Timing of Weed Removal on Corn Yield, Height, and Stem Diameter
In Season 1 (2021), the DT corn hybrid had a higher mean yield with optimal irrigation (52 Mg kg ha−1) compared with the DS corn hybrid (46.3 Mg kg ha−1). However, reducing the irrigation level significantly reduced the corn yield of DT corn, with DS corn hybrids having similar yield under both optimal and reduced irrigation (Table 2; Figure 1A). Contrary to our findings, Kamara et al. (Reference Kamara, Menkir, Badu-Apraku and Ibikunle2003) observed that under water stress, DT corn genotypes showed a higher corn yield than DS corn genotypes. Both corn hybrids displayed higher yields under optimal irrigation compared with reduced irrigation. Similar results were observed by Kebede et al. (Reference Kebede, Sui, Fisher, Reddy, Bellaloui and Molin2014), who also observed a significant corn yield reduction when water supply was reduced by 50%. In addition, Aguilar et al. (Reference Aguilar, Borjas and Espinosa2007) observed a reduction in corn yield of up to 17% due to limited irrigation. This could be attributed to the fact that drought was introduced when corn was still at the seedling stage, which could have led to damage of the structure of the photosynthetic membrane, resulting in lower chlorophyll content than in those plants that did not experience water stress at the seedling stage (Song et al. Reference Song, Jin and He2019). Furthermore, under reduced irrigation, nutrient availability, uptake, and transport is reduced and competition from weeds for limited resources is high, therefore contributing to low corn yields (Djaman et al. Reference Djaman, Irmak, Martin, Ferguson and Bernards2013). Similar to Season 1 observations, both corn hybrids showed higher yields under optimal irrigation compared with reduced irrigation (Table 3; Figure 1B) in Season 2. Unlike Season 1, in Season 2, both corn hybrids had higher yields under optimal irrigation compared with reduced irrigation (Figure 1C and 1D). This could be attributed to the fact that higher rainfall was experienced in Season 2 compared with Season 1.
Table 2. ANOVA of the effect of the two corn hybrids (drought tolerant and drought susceptible), irrigation level (optimal and reduced irrigation), and time of weed removal on corn yield, stem diameter, corn height, conductance, and water potential in Utah in 2021.
a Num df, numerator degrees of freedom; Den df, denominator degrees of freedom.
Figure 1. (A) Mean yields (±SE) of drought-tolerant (DT) and drought-susceptible (DS) corn hybrids at optimal and reduced irrigation levels in 2021 in Logan, Utah, (B) mean yield (±SE) response of both hybrid types to different irrigation levels in 2022 in Logan, Utah, (C) mean yield (±SE) response of DT hybrid to different irrigation levels (optimal and reduced) in 2022 in Logan Utah, (D) mean yield (±SE) response of DS hybrid to different irrigation levels in 2022 in Logan, Utah. Bars labeled with the same letter are not significantly different (P ≥ 0.05).
Table 3. ANOVA of the effect of the two corn hybrids (drought tolerant and drought susceptible), irrigation level (optimal and reduced), and time of weed removal on corn yield, stem diameter, corn height, conductance, and water potential in Utah in 2022.
Plots that were left weedy throughout the growing season had lower corn yields compared with those that were kept weed-free throughout the season. Up to 5% and 42% yield differences were observed between weed-free plots and plots that remained weedy throughout the field season in 2021 and 2022, respectively (Figure 2A and 2B). This suggests that weeds were competing with crops for a limited resource (water) leading to the lower corn yields. These results are similar to those of Evans et al. (Reference Evans, Knezevic, Lindquist, Shapiro and Blankenship2003), who also observed that weedy plots had lower yields compared with weed-free plots. Tursun et al. (Reference Tursun, Datta, Sakinmaz, Kantarci, Knezevic and Chauhan2016) also observed similar results in their study evaluating the CPWC in the three corn hybrids. In addition, Cerrudo et al. (Reference Cerrudo, Page, Tollenaar, Stewart and Swanton2012) and Cox et al. (Reference Cox, Hahn and Stachowski2006) observed that when weed removal was delayed, dry matter accumulation per unit area from emergence to maturity was reduced. The difference in corn yield response to weeds between 2021 and 2022 might be due to higher weed density in 2022 than in 2021, subsequently producing lower corn yields.
Figure 2. (A) Response of drought-tolerant (DT) and drought-susceptible (DS) corn hybrid mean yields (±SE) to different time of weed removal in 2021 in Logan, Utah. (B) Response of corn hybrid mean yield (±SE) to different time of weed removal in 2022 in Logan, Utah. Black bars represent corn yield at critical time of weed removal (CTWR); gray bars represent corn yields at critical weed-free period (CWFP). V represents vegetative stage; H represents time of harvesting.
In Season 1, both corn hybrids were taller when exposed to optimal irrigation compared with reduced irrigation across all timing of weed removal, suggesting that reducing irrigation level led to a reduction in corn height (Figure 3A). This is because under reduced irrigation, cell enlargement is affected by water deficit and plant growth and height is inhibited due to reduction in cell wall extensibility (Seleiman et al. Reference Seleiman, Al-Suhaibani, Ali, Akmal, Alotaibi, Refay and Battaglia2021). We observed similar results in our parallel study examining the competitive ability of A. retroflexus with the DT corn hybrid in a rainout shelter (Odemba et al. Reference Odemba, Creech, Ransom, Yost and Ramirez2025). In addition, Gopalakrishna et al. (Reference Gopalakrishna, Hugar, Rajashekar, Jayant, Talekar and Virupaxi2023) observed that inbred corn was shorter under water-stress conditions compared with well-watered conditions. Both hybrids responded uniformly at each weed removal stage with no significant difference observed in corn heights between the two hybrids at each stage. However, corn height progressed with time, displaying taller plants with later weed removal stages (Figure 3B). In Season 2, the two corn hybrids responded differently to timing of weed removal, with DS corn being taller than DT corn across all timings (Figure 3C). This could be attributed to the fact that DT corn has been bred to be short to help in reducing water consumption, especially under water-stress conditions (Bapela et al. Reference Bapela, Shimelis, Tsilo and Mathew2022). Similar trends were observed for corn height under different irrigation levels, with optimal irrigation plots having taller corn than corn growing under reduced irrigation (Figure 3D).
Figure 3. (A) Mean corn height (±SE) response of drought-tolerant (DT) and drought-susceptible (DS) corn hybrids under different timing of weed removal and different irrigation levels (optimal and reduced) in 2021 in Logan, Utah, (B) mean corn height (±SE) response of DT and DS corn hybrids under different timing of weed removal in 2021 in Logan, Utah, (C) mean corn height (±SE) response of DT and DS corn hybrids under different timing of weed removal in 2022 in Logan, Utah, (D) mean corn height (±SE) response of the DT and DS corn hybrids under different timing of weed removal and different irrigation levels (optimal and reduce) in 2022 in Logan, Utah. Bars labeled with the same letter are not significantly different (P ≥ 0.05). V represents vegetative stage; H represents time of harvesting.
In Season 1, DT corn hybrid had wider stems compared with DS corn at the minimum stem diameter (Figure 4A). This suggests that DT corn was not affected with water stress as much as DS corn. Under reduced irrigation, DT corn had wider stems compared with DS corn hybrid. Both hybrids had wider stems under optimal irrigation compared with reduced irrigation (Figure 4B). Previous studies have also reported effects of water stress on morphological characteristics. A study by Specht et al. (Reference Specht, Chase, Macrander, Graef, Chung, Markwell and Lark2001) reported a reduction in soybean [Glycine max (L.) Merr.] stem under water-stress conditions.
Figure 4. (A) Mean stem diameters (±SE) of drought-tolerant (DT) and drought-susceptible (DS) corn hybrids in 2021 in Logan, Utah. (B) Mean stem diameter (±SE) of DT and DS corn hybrids under different irrigation levels (optimal and reduced) in 2021 in Logan, Utah. Bars labeled with the same letter are not significantly different (P ≥ 0.05).
Weed Composition and the CPWC
Overall, we observed a total of 13 different weed species in both growing seasons, with most of the weeds being broadleaf plants (69%) and the remainder grasses (31%) (Table 4). In all instances, the CWFP and CTWR curves intersected at a point where CTWR preceded the end of the CWFP (Figures 5 and 6).
Table 4. List of weed species observed during the two seasons in Logan, Utah.
Figure 5. Critical periods of weed control (CPWC) (vertical lines) in 2021 for (A) drought-tolerant (DT) corn under optimal irrigation, (B) drought-susceptible (DS) corn under optimal irrigation, (C) DT corn under reduced irrigation, and (D) DS corn under reduced irrigation, in Logan, Utah. 5% RYL, 5% acceptable relative yield loss; CTWR, critical time of weed removal; CWFP, critical weed-free period.
Figure 6. Critical periods of weed control (CPWC) (vertical lines) in 2022 for (A) drought-tolerant (DT) corn under optimal irrigation, (B) drought-susceptible (DS) corn under optimal irrigation, (C) DT corn under reduced irrigation, and (D) DS corn under reduced irrigation, in Logan Utah. 5% RYL, 5% acceptable relative yield loss; CTWR, critical time of weed removal; CWFP, critical weed-free period.
CTWR
In 2021, the beginning of CPWC was delayed for DT corn compared with DS corn under both optimal and reduced irrigation (Figure 5A and 5C). This could be attributed to the fact that DT corn had an early-season rapid growth, allowing it to suppress growth of weeds, thus leading to delays in weed pressure. CPWC began at 39 d after planting (DAP) for DT corn growing under optimal irrigation and at 35 DAP for DT corn growing under reduced irrigation, suggesting that optimal irrigation delayed the beginning of CPWC. Unlike DT corn, under optimal irrigation, the CPWC began earlier for DS corn (18 DAP) and late for DS corn growing under reduced irrigation (26 DAP) (Figure 5B and 5D). These results suggest that optimal irrigation for DT corn gave it a competitive advantage over weeds delaying CPWC. On the other hand, optimal irrigation boosted more weed growth compared with DS corn, leading to a need for early weed removal.
In 2022, contrasting results were observed for the beginning of CPWC for DT corn. It began earlier for DT corn growing under both optimal and reduced irrigation (12 DAP and 2.5 DAP, respectively) than for DS corn under both irrigation levels (Figure 6A and 6C). For DS corn growing under optimal irrigation, CPWC began at 25 DAP compared with 10 DAP when growing under reduced irrigation, suggesting that optimal irrigation delayed the beginning of CPWC (Figure 6B and 6D). In this season, the beginning of CTWR began early because of high soil moisture levels at the beginning of the season due to spring rainfall, which facilitated more rapid establishment of high weed density than in 2021 (31% and 26% volumetric water content in 2022 and 2021, respectively) (Figure 7). This led to an early need for weed control to minimize yield loss. The beginning of CPWC for DT corn under optimal irrigation in 2022 was similar to that observed by Tursun et al. (Reference Tursun, Datta, Sakinmaz, Kantarci, Knezevic and Chauhan2016) for sweet corn.
Figure 7. Seasonal (from June 3, 2022, to September 2, 2022, and from June 3, 2021, to August 11, 2021) volumetric soil water content at 30-cm soil depth for irrigation treatments in the field study in Logan, Utah in 2021 (A) and 2022 (B).
CWFP
The end of CPWC varied across the hybrids and different irrigation levels. In the 2021 season, the end of CPWC was shortened by growing DT corn compared with DS corn under both optimal and reduced irrigation (58.5 DAP and 40 DAP, respectively) (Table 5A and B). This could be attributed to DT corn being more competitive than weeds and suppressing their growth, hence the shorter period for weed control.
Table 5. Critical period of weed control (CPWC) for drought-tolerant (DT) and drought-susceptible (DS) corn in Logan, Utah in 2021 and 2022.
Asterisks (*) denote significant difference at: ***0.001; **0.01; and *0.5.
Results in 2022 were similar to those in 2021, with the DT corn having an earlier end of CPWC compared with DS corn under both reduced and optimal irrigation (40 DAP and 51 DAP, respectively). DS corn showed a similar end of CPWC (60 DAP) under both reduced and optimal irrigation (Table 5A and B). The differences in beginning of CPWC between the two seasons could have been due to the time of weed emergence, with weeds emerging earlier in 2022 than 2021. Also, there was more frequent rainfall in 2022 than in 2021, leading to higher moisture content than in 2021, hence a high weed count early in the season leading to a need for early weeding. The CPWC was short under reduced irrigation because corn was able to outcompete weeds for the limited resources and formed a canopy earlier, thus better suppressing weed growth. For DS corn, end of CWFP was delayed, likely because the weeds were more competitive than DS corn, thus delaying corn canopy formation.
The results from this study show that water management practices and corn hybrids influence how corn interacts with weeds. The beginning and end of CPWC differed between the two corn hybrids as well as between the two irrigation levels in both seasons. CPWC for DT corn under both optimal and reduced irrigation levels was shorter than that for DS under the same conditions in both seasons. In addition, increasing duration of weeds in the field significantly reduced corn yields. This knowledge is important for effective weed management, especially in water-stressed environments, as it shows that DT corn hybrids reduce the period for weed control and provide flexibility in the timing for effective weed control under water stress. However, because CPWC is site specific and highly influenced by the hybrids grown, continuing research is needed for recommendations for different regions and different hybrids.
Acknowledgment
We would like to thank Eric Galloway for assisting with land preparation and harvesting, Cody Beckley for assisting with herbicide applications, our lab technicians for assisting with setting up the experiment and data collection, Xin Dai for assisting with data analysis, and Fabiane Mundim for reviewing the paper.
Funding statement
This study was funded by USDA-NIFA Agriculture and Food Research Initiative competitive grant no. 2019-67014-29369.
Competing interests
The authors declare no conflicts of interest.
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