Duration Of Weed Interference Research Articles

Phosphorus influence on the critical period of weed control in sweet corn

Abstract Understanding the effect of phosphorus (P) fertilization on weed interference with sweet corn is important for deciding appropriate fertilization levels and weed control programs. Field experiments were conducted in 2020 and 2021 in Belle Glade, FL, to determine the influence of P fertilization levels (0 or residual P, 62.5, and 120 kg P2O5 ha−1) on the critical period of weed control (CPWC) in sweet corn on organic soils. Experimental plots were subjected to increased duration of weed interference and weed-free period treatments for each P fertilization level. The beginning and end of the CPWC based on 5% and 10% acceptable yield loss (AYL) levels were determined by fitting log-logistic and Gompertz models to represent the increasing duration of weed interference and duration of the weed-free period, respectively. The log-logistic curves did not estimate the beginning of the CPWC at 5% AYL for 0 and 125 kg P2O5 ha−1 because the estimated upper limits of the curves were lower than the 95% relative yield used for estimation of 5% AYL. Based on a 10% AYL level, the length of the CPWC in sweet corn under optimum P fertilization levels was estimated to be 27 d, from the 6- to 7-leaf stage until the silking stage of growth. Reducing P fertilization by 50% increased the CPWC to 36 d, from the 5-leaf stage until the silking to blister stage of growth. Lack of P fertilization increased the CPWC to 64 d, from sweet corn emergence until the blister to milk stage of growth. These results show that the beginning of the CPWC in sweet corn is delayed and the end is shortened as P fertilization level increases. Therefore reduction in P fertilization will require a more intensive weed management program for sweet corn because of the prolonged duration of the CPWC.

Cultivation system influenced the critical period for weed control in cotton field

BackgroundThe critical period of weed control (CPWC) refers to the period of time during the crop growth cycle when weeds must be controlled to prevent yield losses. Ultra-narrow row (UNR) is a method of planting of cotton in rows that are 25 cm or less apart. Amongst cultural techniques for weed control, the use of narrow row spacing is considered to be a most promising approach that can effectively suppress weed growth and provide greater yields in cotton. This cultivation system can shorten the length of the critical weed-crop interference duration and results in greater yield. The current research aimed to determination of critical time of weed control in cotton (Gossypium hirsutum L.) under conventional and ultra-narrow row spacing conditions. Field experiments were arranged as factorial experiment in a randomized complete block design with three replications. Factors were cultivation system (conventional (50 cm row spacing) and ultra narrow row (25 cm row spacing and weed treatment including 30, 45, 60, and 75 days weeding after emergence during the growing season (weed free), and 30, 45, 60, and 75 without weeding (weed infested) in the growing season along with weedy and weed-free from sowing to harvesting. A four-parameter log-logistic model was fit to the two sets of relating relative crop yield to data obtained from increasing durations of weed interference and lengths of weed-free period.ResultsIn both years and cultivation systems, the relative yield of cotton decreased with the increasing duration of weed-interference but increased with the increasing duration of weed-free period. Ultra-narrow row cultivation delayed the beginning of the CPWC in cotton. Under ultra-narrow row condition, the CPWC ranged from 21 to 99 days after germination in 2021 and 23 to 91 days in 2022 based on the 5% acceptable yield loss. Under conventional cultivation CPWC ranged from 17 to 102 days after emergence in 2021 and 18 to 95 days after emergence in 2022.ConclusionsUnder both conventional and Ultra-narrow row conditions, weed interference reduces seed yield. Under ultra-narrow row condition, weed interference until 21.1–23.5 days after cotton emergence and under conventional condition, weed interference until 16.9–18.5 days after cotton emergence had not significant reduction on cotton yield.

The Critical Period of Weed Control Influences Sunflower (Helianthus annuus L.) Yield, Yield Components but Not Oil Content

Field studies were conducted in the northeastern part of the Republic of Croatia to determine the influence of the critical period of weed interference on sunflower (Helianthus annuus) yield, yield components, and oil content. For this purpose, different durations of competition were established, allowing weeds to infest the crop for increasing periods of time after planting or maintaining plots weed-free for increasing periods of time after planting. The beginning and the end of the critical period of weed control (CPWC), based on a 5% and 10% loss of sunflower yield, were determined by fitting the four-parameter log-logistic equations to the relative seed yield. The total weed biomass increased progressively in relation to the increase in the competition. The beginning of the CPWC period, based on a 5% acceptable yield loss, ranged from 141 to 234 growing degree days (GDD), which corresponded to the two-to-four true leaf development stage (the V2–V4 growth stages) across both sites and years. The crop had to be kept weed-free until a period when sunflower inflorescence began to open and flower (the R4–R5 growth stage) or from 1365 to 1932 GDD. The sunflower yield and yield components varied between the years and among locations. An increasing duration of weed interference negatively affected crop height, head diameter, and 1000-kernel weight, but not seed oil content.

Critical Period of Weed Control in Cotton (Gossypium hirsutum L.) at Three Nitrogen Levels*

The critical period for weed control is that part of the cropgrowth cycle during which the crop must be kept weed-free to preventunacceptable yield losses. Field studies were conducted for twoconsecutive years, 2004 and 2005, in the Thessaly plain in central Greeceto determine the critical period for weed control in cotton at three nitrogenlevels (50,100, and 150 kg N ha-1 ).Treatments of increasing duration ofweed interference and weed-free period were imposed at biweeklyintervals from 0 to 10 weeks after crop emergence (WAE). Based on anarbitrarily acceptable yield loss level of 5% and 10%, the beginning andend of the critical period were determined by fitting the polynomialregression to relative yield data representing increasing duration of weedinterference and weed-free period. Plant height, shoot and root dryweights and yield of cotton were reduced by prolonged delays in weedremoval at all nitrogen levels in 2004 and 2005. The results showed thatweeds reduced cotton shoot and root dry weights, respectively, by 55%and 38 % at 50 kg N ha-1 , 50% and 42 % at 100 kg N ha-1 and 40% and 46% at 150 kg N ha -1 , when allowed to compete for two weeks. At 10%acceptable yield loss level, the critical period was 8.2 weeks, starting 0.4WAE and ending 8.6 WAE at 50 kg N ha -1 , and was decreased to 7weeks, starting 1 WAE and ending 8 WAE at 100 kg N ha-1 . The criticalperiod decreased to 6.1 weeks, at 150 kg N ha -1 , starting 1.3 WAE andending 7.4 WAE. These findings showed that the supply of nitrogen to thecrop and weeds significantly influenced the crop-weed interferenceduration. This information will be useful for providing weed controlrecommendation to cotton producers.

Weed Community in Soybean Responses to Agricultural Management Systems

Weed infestation is a major cause of the poor yield of soybeans (Glycine max (L.) Merr.); therefore, proper weed management represents one of the most important and expensive steps in soybean production. Field experiments were established in northeastern parts of Croatia, in the Vukovar-Syrmia county from 2014 to 2016, arranged in a split-plot design with four replications. Two different studies were conducted: the first study was to determine the weed interference, weed biomass accumulation, yield, and yield components of soybeans growing in three different rows spacing (25, 50, and 70 cm), and the second study aimed to simulate a risk analysis by building models of probabilities for generating profit as a result of weed control. The weed community in soybean during the study period comprised 34 dicot and grass species of a varied perennation. Compositional differences in the weed community tended to be affected most by the year (humid–arid environment), followed by row spacing. There were no differences in the weed biomass accumulation with a reduction in row spacing from 70 to 50 and 25 cm. The dominant weed species Amaranthus retroflexus, Ambrosia artemisiifolia, Chenopodium album, Datura stramonium, Setaria viridis, and Sorghum halepense formed the main biomass and were spread over all row spacings. There was a significant influence of row spacing, the duration of weed interference, and year on soybean yield and yield components. Weed infestation until the second trifoliate (V2) stage had no detrimental effect on soybean yield, regardless of the row spacing. The number of pods per plant significantly decreased at the same V2 stage in 25 and 50-cm rows, but in 70-cm soybean rows, this process started later, at four unfolded trifoliate leaves (V4 stage). A 1000 kernel weight was less sensitive to weed infestation and was significantly decreased at full flowering (R2 stage) in 25 and 70 cm rows, while it already decreased at the V4 stage in 50 cm rows. The probability distribution of achieving a profit showed the best results for soybeans growing in 70 cm rows, with preemergence herbicide application and two inter-row cultivation.

Effects of Irrigation Regimes and Duration of Weed Interference on Grain Yield of Wheat (Triticum aestivum L.) in Middle Egypt

TWO EXPERIMENTS were performed at Farm of Faculty of Agriculture, El-Minia University Egypt, during two consecutive winter seasons 2017-2018 and 2018-2019 to evaluate the impact of weed removal period and different irrigation regimes on weeds, critical period of weed competition and yield and its components on wheat crop. A strip-plot design with three replications was used. Four irrigation regimes were in the horizontal plots and eight treatments of weed competition (4 weed-free periods and 4 weed competition periods) were in vertical plots. Results showed that omitting two irrigation (IR4) significantly decreased total density and dry weight of weeds, wheat traits, i.e. plant height, spike length, number of grains spike-1, 1000-grain weight, number of spikes m-2, grain yield and grain ability. Weed infestation for whole season significantly decreased all studied traits compared with weed removal even once after wheat sowing (DAS). Maximum yield losses of wheat due to weed infestation in whole season were 28.52 and 28.17% compared with weed-free treatment in the 1st and 2nd seasons, respectively. Cubic model was the best model for weed-free and weed competition over all treatments of the two seasons. The critical periods for weed control were 28 to 52 and 28 to 67 DAS in the two seasons, respectively.

Integrating emergence and phenology models to determine windows of action for weed control: A case study using Senna obtusifolia

• Weed phenology can be modeled as a function of seedling emergence timing. • Phenology models can be used to determine windows of action for weed control. • Windows of action can complement the critical period of weed control concept. The success of integrated weed management strategies is contingent on the accuracy of control actions in both time and space. While emphasis has been given to spatial accuracy, timing accuracy has been largely neglected. Weed control timing must consider not only the total duration of weed interference with the crop, as done by the traditional critical period of weed control (CPWC) based on yield protection only, but also weed growth, size, and susceptible phenological stages. In this study, we expand upon the idea of using weed emergence models for timing weed control by integrating them with phenology probability models for key weed growth stages to optimize timing of control actions, here referred as Critical Control Windows (CCW). Combining the CCW with thresholds for yield loss due to weed interference and thresholds for weed survival risk makes it possible determining the frequency with which control actions should be implemented to maintain crop yield and weed populations at desired levels. Using Senna obtusifolia as a study case, vegetative and reproductive phenological stages were modeled as a function of seedling emergence for different cohorts. Chronological and thermal-time models provided robust predictions of S. obtusifolia phenology. CCW did not always coincided with CPWC for several crops when considering 10-cm tall plants as the threshold for control. In general, for summer row crops, CCW required 2 postemergence control actions and sometimes 1 action outside the CPWC. The results of the present research illustrate how predictive models can be used to develop CCW that will complement the traditional CPWC. These two concepts when used complementary can increase not only timing accuracy, but also efficiency of weed control.

The impact of hemp (Cannabis sativa L.) plant density on critical period of weed control.

The impact of hemp (Cannabis sativa L.) plant density on critical period of weed control.

Critical Period of Weed Control in Snap Bean on Organic Soils in South Florida

Field studies were conducted in 2011 and 2012 in Belle Glade, FL, to evaluate the critical period of weed control (CPWC) in snap bean grown on organic soils in the Everglades Agricultural Area (EAA) of South Florida. Treatments consisting of increasing duration of weed interference and weed-free period were imposed at weekly intervals from 0 to 7 weeks after emergence (WAE) of snap bean. The beginning and end of the CPWC based on 2.5%, 5%, and 10% snap bean acceptable yield loss (AYL) levels were determined by fitting log-logistic and Gompertz models to represent increasing duration of weed interference and weed-free period, respectively. Based on 2.5% yield loss, the CPWC was 7.2 weeks long, beginning 1.2 (cotyledon and unifoliate leaf) and ending 8.4 WAE (mid-pod set, 50% of pods reached maximum length). At 5% yield loss, the CPWC was 5.0 weeks, beginning 1.7 (first to second trifoliate leaf) and ending 6.7 WAE (mid-flower to early pod set, 50% of flowers open and one pod reached maximum length). At 10% yield loss, the CPWC was 3.0 weeks, beginning 2.2 (second trifoliate leaf) and ending 5.2 WAE (early flowering, one open flower). Based on these results, the beginning of CPWC was hastened, whereas the end was delayed at different yield loss levels showing that acceptable weed control in snap bean on organic soils in the EAA is required throughout much of the growing season to minimize yield loss.

Determination of Critical Period of Weed-Crop Competition in Rice ( Oryza sativa L.) in Bench Maji and Kaffa Zone, South Western Ethiopia

Accurate assessment of crop weed interference period is an essential part for planning an effective weed management for cropping systems and, hence, can be considered the first step to design weed control strategy. The experiment was conducted at Guraferda and Gojeb, Southern Ethiopia during the 2016 main cropping season to assess the critical period of weed-crop competition and yield loss in rice. There were 14 treatments comprising: two series i.e. early (weedy up to 10, 20, 30, 40, 50, 60 and 70 days after crop emergence) and late (weed free up to 10, 20, 30, 40, 50, 60 and 70 days after crop emergence) competition periods were compared with two checks namely complete weed free and weedy check. The treatments were arranged in randomized complete block design with three replications for each set. The major weed species competing vigorously with rice were Cyperus assimilis, Setaria pumila, Phalaris paradoxa and Xanthium spinosum. With increasing duration of weed interference, weed dry weight, and the number of days of rice plant required to reach physiological maturity were increased whereas number of tillers per plant, panicle length, thousand seed weight, grain yield, aboveground biomass, and harvest index of the rice crop were reduced. Uncontrolled weed growth significantly reduced rice grain yield by 68% compared to the grain yield obtained from the weed-free check plots. The beginning and the end of critical period of weed crop competition were based on 5 and 10% acceptable yield loss levels, which were determined by fitting logistic and Gompertz equations to relative yield data, representing increasing duration of weed-interference and weed-free periods. In conclusion, the results of the study revealed that, to reduce the loss in the grain yield of rice by more than 10% and higher economic return, it is important to keep the crop weed-free between 30 to 70 days after crop emergence at Guraferda, in Bench Maji Zone and 28 to 47 days after crop emergence at Gojeb in Keffa Zone.

Critical period of weed control in radish (<i>Raphinus sativus</i> L.)

A field study was carried out at Crop Farm of the Eastern University of Sri Lanka to determine the critical period for weed control and the effects of weed interference in radish in the regosols of the Batticaloa District. An increasing series of the duration of weed interference and length of the weed-free periods were imposed weekly from 0 to 40 days after emergence (DAE). The beginning and the end of CPWC were based on 5, 10 and 20% acceptable yield loss levels (AYL), which were determined by fitting logistic and Gompertz equations to relative yield data, representing increasing duration of weed-interference and weed-free periods. The critical period for weed control increased to 31 days, starting at 7 days and ending at 38 days, at 5% yield loss level. At 10% and 20% yield loss levels, the CPCW decreased to 12 and 8 days respectively, starting at 18 DAE and ending at 30 DAE, and beginning at 13 DAE and ending at 21 DAE, respectively.

The critical period for weed control in three corn (Zea mays L.) types

Knowledge of the crop-weed competition period is vital for designing effective weed management strategies in crop production systems. Field studies were conducted at the Agricultural Research Institute, Kahramanmaras, Turkey in 2013 and 2014 to determine the critical period for weed control (CPWC) in three corn (Zea mays L.) types (field corn, popcorn, and sweet corn). A four parameter log-logistic model described the relationship between relative crop yield to both increasing duration of weed interference and length of weed-free periods. The relative yield of corn was influenced by duration of weed-infested or weed-free period, regardless of corn types. Increasing periods of weed interference significantly reduced corn yields in both years. In field corn, the CPWC ranged from 175 to 788 growing degree days (GDD) in 2013 which corresponded to V2–V12 growth stages, and 165–655 GDD (V1–V10 growth stages) in 2014 based on the 5% acceptable yield loss (AYL) level. In popcorn, the CPWC ranged from 92 to 615 GDD (VE–V10 growth stages) in 2013 and 110–678 GDD (V1–V10 growth stages) in 2014. In sweet corn, the CPWC ranged from 203 to 611 GDD in 2013 (V2–V10 growth stages) and 182–632 GDD (V2–V10 growth stages) in 2014. The practical implication of this study is that weed management should be initiated around V1 stage and maintained weed-free up to V12 stage in all corn types to prevent yield losses greater than 5%. These findings could help corn producers improve the cost effectiveness and efficacy of their weed management programs.

Row spacing impacts the critical period for weed control in cotton (Gossypium hirsutum)

The knowledge on the critical crop-weed competition period is important for designing an efficient weed management program. Field studies were conducted in 2012 and 2013 at the Agricultural Research Institute, Kahramanmaras, Turkey to determine the effects of three row spacing (50, 70 and 90 cm) on the critical period for weed control (CPWC) in cotton. A four parameter logistic equation was fit to data relating relative crop yield to both increasing duration of weed interference and length of weed-free period. The relative yield of cotton was influenced by the duration of weed-infested or weed-free period, regardless of row spacing. In cotton grown at 50 cm row spacing, the CPWC ranged from 117–526 growing degree days (GDD) (V2–V11 growth stages) in 2012 and 124–508 GDD (V2–V10) in 2013 based on the 5% acceptable yield loss level. At 70 cm row spacing, the CPWC ranged from 98–661 GDD in 2012 (V2–V13) and 144–616 GDD (V2–V12) in 2013. At 90 cm row spacing, the CPWC ranged from 80–771 GDD in 2012 (V1–V14) and 83–755 GDD (V1–V14) in 2013. In order to obtain a 95% weed-free yield, the weed management should start at 16 days after crop emergence (DAE) and continued until 52 DAE (V2–V11) for crops grown in 50 cm row spacing, 15 and 60 DAE (V2–V13) for 70 cm row spacing and 11 and 67 DAE (V1–V14) for crops grown in 90 cm row spacing. This suggests that cotton grown in narrow row spacing (50 cm) had greater competiveness against weeds compared with wider row spacing (70 and 90 cm). Cotton growers can benefit from these results by improving cost of weed control through better timing of weed management.

Nitrogen application influenced the critical period for weed control in cotton

The critical knowledge of crop-weed competition period is important for designing weed management strategies in cropping systems. Field experiments were conducted in 2012 and 2013 growing seasons at the Agricultural Research Institute, Kahramanmaras, Turkey to determine the effect of three nitrogen (N) rates (0, 150 and 300 kg N ha−1) on the critical period for weed control (CPWC) in cotton. A four parameter log-logistic model was fit to the two sets of relating relative crop yield to data obtained from increasing durations of weed interference and lengths of weed-free period. In both the years and regardless of the N application rate, the relative yield of cotton decreased with increasing duration of weed-interference and increased with increasing duration of weed-free period. The application of N fertilizer delayed the beginning of the CPWC in cotton. At the 0 kg N ha−1, the CPWC ranged from 44 to 537 GDD (growing degree days) (7–51 days after crop emergence [DAE]) in 2012 and 87–605 GDD (14–57 DAE) in 2013 based on the 5% acceptable yield loss. At the 150 kg N ha−1, the CPWC ranged from 124 to 597 GDD (18–56 DAE) in 2012 and 183–551 GDD (22–49 DAE) in 2013. Depending on the N application rate, weed-free conditions need to be established as early as one week after crop emergence and maintained as late as eight weeks after crop emergence to avoid more than 5% loss in cotton yield. These findings could help cotton producers improve the cost effectiveness and efficacy of their weed management programme under different N application rates.

Determination of critical period for weed control in intensive and non-intensive sugarcane (<i>Saccharum officinarum</i> L., Poaceae) production systems in center Côte d’Ivoire

Field experiments were conducted in Zuenoula and Yamoussoukro for determining Critical Period for Weed Control (CPWC) in sugarcane. The treatments consisted in two sets of weed interference. In the first set, the crop was kept weed-free until 31, 61, 92, 123 days after planting (DAP) in Zuenoula and until 32, 69, 98 and 162 DAP in Yamoussoukro. In the second set, weeds were permitted to grow within the crop until the above-mentioned DAP. The CPWC was determined for 5, 10, 15 and 20% acceptable yield loss levels by fitting Logistic and Gompertz nonlinear equations to relative yield data. In both locations, increasing the duration of weed interference decreased sugarcane yield significantly. In Zuenoula, the CPWC was from 28 to 117, 30 to 93, 32 to 75 and 34 to 59 DAP to prevent yield losses of 5, 10, 15 and 20%, respectively. In Yamoussoukro, the CPWC ranged from 38 to 163, 39 to 112, 40 to 99 and from 40 to 91 DAP to prevent yield losses of 5, 10, 15 and 20%, respectively. Results suggest weed control between 28 and 117 DAP in Zuenoula and between 38 and 163 DAP in Yamoussoukro to provide maximum yield.Keywords: Weed interference, Gompertz, Logistic, weed control, yield.

Weed Interference Effects on Leaves, Internode and Harvest <span style="font-size: 12.0pt; font-family: 'Times New Roman',serif"> Index of Dry Bean (<i>Phaseolus vulgaris</i> L.)

The development of appropriate weed management strategies and efficient use of herbicides relies upon understanding weed-crop interactions. A field study was carried out to assess the effect of weed interference on leaves, internode and harvest index of dry bean (Phaseolus vulgaris L.). The experiment was established under a randomized complete block design with two types of weed interference treatments: plots with weeds and plots without weeds at different time intervals (0, 10, 20, 30, 40 and 50 days after crop emergence). The sigmoid Boltzmann model was used to quantify the crop traits as influenced by weed interference. Prolonged delays in weed removal reduced gradually the number of leaves of the crop. Weed interference decreased dry weight of leaves as well, so that the lowest value of it (33.49 g plant-1) was observed in full season during weed-infested treatment. Infestation of weeds affected the length of the crop internodes. While the weed interference duration increased, the length of the internodes decreased. Harvest index was also sensitive to weed competition. As the crop was kept weed-infested from the emergence for increasing periods of time, harvest index decreased to a value of 28.01%. A significant negative correlation between total biomass of weeds and dry bean traits (number of leaves, leaves dry weight, internode length and harvest index) was observed. Therefore, weeds are able to adversely affect dry bean growth through constraining environmental resources and impairing leaves as the photosynthetic areas.

Weed Control in Soybean as Influenced by Residual Herbicide Use and Glyphosate-Application Timing Following Different Planting Dates

Soybean planting has occurred earlier in the Midwestern United States in recent years; however, earlier planting subjects the crop to longer durations of weed interference. This may change the optimum timing of POST glyphosate applications, or increase the need for residual herbicides applied PRE to optimize yield. A field study was conducted in 2012 and 2013 near Arlington, WI to determine the effect of planting date, residual herbicide use, and POST glyphosate timing on weed control and soybean yield. Planting dates were late April, mid-May, and early June. A PRE application of sulfentrazone plus cloransulam was applied to half the plots following each planting date. Glyphosate was applied POST to all plots at the V1, V2, V4, or R1 soybean growth stage. Planting date and glyphosate timing did not affect soybean yield in this study. However, averaged across years, planting dates, and POST glyphosate timings, yield increased from 3,280 to 3,500 kg ha−1when a PRE herbicide with residual soil activity was used. In POST-only treatments, delaying the planting date to June decreased weed density at POST application timing from 127 to 5 plants m−2(96%) and from 205 to 42 plants m−2(80%) in 2012 and 2013, respectively. Where a PRE was used, total weed density at POST application timing was always less within planting date, and also declined from early to late planting date 26 to 3 plants m−2(89%) and 23 to 6 plants m−2(74%) in 2012 and 2013, respectively. In conclusion, both PRE herbicide use and delayed soybean planting were effective strategies to reduce the number of in-crop weeds exposed to POST glyphosate and should be considered as strategies to reduce the number of weeds exposed to POST herbicides for resistance management.

The Effect of Weed Competition on Germination Indices and Seed Vigor of Chickpea

ABSTRACT- This experiment was carried out during the growing seasons of 2010 and 2011 at the Agricultural Research Farm of Bu-Ali Sina University, to study the effect of weed competition on subsequent germination and seed vigour of chickpea. 12 weed interference durations were evaluated in a randomized complete block design with three replications. Experiments consisted of two sets of treatments. In the first set, the crop was kept weed-free until 0, 14, 28, 42, 56 and 70 days after sowing and in the second set, weeds were permitted to grow within the crop until 0, 14, 28, 42, 56 and 70 days after sowing. Results of the two year experiments showed that the traits germination index, germination rate, mean germination time, percentage of germination, abnormal seedling percentage and seedling dry weight were affected by different duration of weed interference. Thereafter, the maximum value of germination rate and the minimum value of mean germination time belonged to the full weed-free condition. The increasing length of weed competition decreased seedling dry weight and increased abnormal seedling percentage. This finding is supported by a reduction in the seed weight. In other words, a decrease in seed storage contents resulted in a decrease in seedling growth and an increase in the percentage of abnormal seedlings.

Effect of weed interference on Zea mays: Growth analysis

In order to investigate the effect of weed competition on corn, growth trials were conducted in Shushtar, Iran, using a comparative growth analysis. In this study, two sets of treatments were imposed, based on the phenological stages of corn development, using a randomized complete block design with three replicates. The results showed that barnyardgrass and redroot pigweed were the most dominant weeds in these trials. Increasing the duration of weed interference reduced the corn leaf area index gradually. The reduction in the corn leaf area index led to a decline in the crop growth rate as a result of a reduction in the ability of corn to capture light and its photosynthetic ability. The total dry matter and relative growth rate of corn were decreased when the weeds were removed later as a result of a reduction in the leaf area index. According to these results, the leaf area index has a critical role in weed–crop competition.

Phosphorus Application Influences the Critical Period of Weed Control in Lettuce

Field studies were conducted in 2010 and 2011 at Belle Glade, FL, to evaluate the influence of phosphorus (P) applications (98, 196, and 293 kg P ha−1) on the critical period of weed control (CPWC) in lettuce. Natural populations of mixed weed species were allowed to interfere with lettuce in a series of treatments of both increasing duration of weed interference and the duration of weed-free period imposed within 98, 196, and 293 kg P ha−1levels added to the soil. The beginning and end of the CPWC for each P fertilization level based on a 5% acceptable marketable fresh lettuce yield loss level was determined by fitting log-logistic and Gompertz models to represent the increasing duration of weed interference and the duration of weed-free period, respectively. The CPWC in lettuce was estimated to be 4.6, 3.4, and 2.3 wk at 98, 196, and 293 kg P ha−1, respectively. The beginning of the CPWC was delayed at the highest P fertilization level (293 kg P ha−1), whereas the end of the CPWC was hastened at the same P fertilization level. Our study shows that inadequate levels of P fertilization in lettuce result in the need for more-intensive weed management practices to attain acceptable yields.