African mustard (Brassica tournefortii Gouan), turnipweed [Rapistrum rugosum (L.) All.], and African turnipweed (Sisymbrium thellungii O.E. Schulz) are common broadleaf weeds in chickpea (Cicer arietinum L.) crops, particularly under dryland region conditions in eastern Australia. Information on crop yield losses and the seed production potential for these weeds in chickpea are limited. Field studies were conducted in the winter seasons of 2020 and 2021 in eastern Australia with different densities of the three weeds (B. tournefortii, R. rugosum, and S. thellungii) in chickpea. Based on the sigmoidal model, chickpea yield was reduced by 50% at 11 plants m−2 of B. tournefortii. Based on hyperbolic models, a 50% yield reduction of chickpea occurred at 5 and 25 plants m−2 of R. rugosum and S. thellungii, respectively. Based on the linear model, B. tournefortii, R. rugosum, and S. thellungii produced a maximum of 448,000, 206,700, and 869,400, seeds m−2, respectively. At chickpea harvest, the low seed retention (<55%) of B. tournefortii and S. thellungii suggests limited opportunities for harvest weed seed control, and the seed rain of these weeds may enrich the weed seedbank in the soil. At crop harvest, the seed retention of R. rugosum was found to be greater than 90%, suggesting that it is a suitable candidate for harvest weed seed control. This study demonstrated that R. rugosum could cause a greater reduction in chickpea yield compared with B. tournefortii and S. thellungii. Furthermore, restricting seed rain of B. tournefortii and S. thellungii by not allowing the plants to produce seeds is recommended to reduce their weed seedbanks in the soil. The information generated from this study could aid in strengthening integrated weed management in chickpea.

Small-seeded vegetable crop production is challenged by poor emergence, stand establishment and canopy development, as well as a lack of effective weed control options. The potential use of plant growth regulators such as gibberellic acid (GA) could enhance early emergence and growth rates while potentially synchronizing weed germination with control tactics. In response, a controlled environment study was conducted to investigate the effects of GA on garden beet, cabbage, carrot, and onion. At 7 d after seeding (DAS) carrot emergence was greater when carrot seeds were treated with 2, 4, 8, 16, or 64 ppm GA compared with nontreated seeds. Total carrot emergence over the study period was 14% greater when seeds were treated with 4 ppm GA compared with nontreated seeds. Treatment of cabbage with as low as 2 to 4 ppm GA increased cabbage emergence rate and total plant emergence over the study period relative to nontreated seeds. Onion response to GA treatment was variable and unremarkable and was hypothesized to be influenced by seed dormancy because emergence was also low with the nontreated seeds. The GA rates that stimulated vegetable crop seed germination and emergence were then explored with three common weed species to determine whether a similar response was observed. If so, GA could be used to stimulate weed emergence in synchrony with management tactics. Palmer amaranth emergence was strongly affected by GA treatment, whereby the total number of emerged plants was 48% greater when 4 ppm GA was applied than in the nontreated check. Velvetleaf emergence at 3 DAS with the 4 and 8 ppm GA was 2.9 and 3.0 plants pot−1, respectively, compared to no emergence in the nontreated pots. Redroot pigweed emergence was not affected by GA treatment at any rate.

Junglerice [Echinochloa colona (L.) Link] is increasing its prevalence in eastern Australia by adapting to Australia’s changing climatic conditions and conservation agricultural systems and by evolving resistance to glyphosate. Information is limited on the growth and seed production dynamics of E. colona when it interferes with mung bean [Vigna radiata (L). R. Wilczek], a major potential export crop for eastern Australia. This field study examined the interference of E. colona in mung bean for two summer seasons (2020 and 2021) at Gatton, QLD. Different infestation levels (0, 2, 4, 8, 16, and 32 plants m−2) of E. colona were assessed for their potential to cause yield reductions in mung bean. Seed yield of mung bean was highest in the weed-free plots (2,767 kg ha−1) and declined by 20%, 27%, 34%, and 43% at weed infestation levels of 4, 8, 16, and 32 plants m−2, respectively. Echinochloa colona biomass in mung bean varied from 11 to 137 g m−2 as weed density increased from 2 to 32 plants m−2. Based on a three-parameter hyperbolic rectangular decay model, crop yield loss was 52% and 57%, respectively, when weed density and weed biomass approached maximum. Echinochloa colona at the highest density (32 plants m−2) produced a maximum of 15,140 seeds m−2, and this seed production was reduced by 50% at a weed density of 10 plants m−2. Echinochloa colona plants retained 63% to 68% seeds at mung bean maturity, indicating a great opportunity for harvest weed seed control. This study suggests that a high infestation of E. colona in mung bean fields could cause a substantial yield loss and increase the weed seedbank.

The influence of weeds on cranberry yield and quality is not well known and cannot be extrapolated from other cropping systems given the unique nature of both cranberry production and the weed species spectrum. The work presented here addresses this need with four common weed species across multiple production seasons and systems in Wisconsin, Massachusetts, and New Jersey: Carolina redroot, earth loosestrife, bristly dewberry, and polytrichum moss. The objectives were to use these representative species to quantify the impact of weed density, groundcover, and biomass on several cranberry yield components and related interactions with other cranberry pests, and to determine whether these relationships were consistent enough across seasons to be reliably used in weed management decision-making. The relationships between Carolina redroot and bristly dewberry growth measures and marketable cranberry yield were highly significant (P ≤ 0.001 in 12 of 13 regressions) and consistent across growing seasons, but not significant for similar regressions with earth loosestrife. In particular, the strong relationship between Carolina redroot and bristly dewberry visual groundcover observations and cranberry yield suggests a simple way for growers and crop scouts to reliably estimate yield loss. The relationship between polytrichum moss biomass and cranberry yield was also significant in both years, but not consistent between years. Weed competition also affected cranberry quality, in that Carolina redroot density was strongly related to the percentage of insect-damaged fruit and bristly dewberry growth reduced cranberry color development. On a practical level, this information can be used to educate growers, consultants, agrichemical registrants, and regulators about the impacts of weeds on cranberry yield and quality, and to economically prioritize management efforts based on the weed species and extent of infestation.

The utilization of remote sensing in agriculture has great potential to change the methods of field scouting for weeds. Previous remote sensing research has been focused on the ability to detect and differentiate between species. However, these studies have not addressed weed density variability throughout a field. Furthermore, the impact of changing phenology of crops and weeds within and between growing seasons has not been investigated. To address these research gaps, field studies were conducted in 2016 and 2017 at the Horticultural Crops Research Station near Clinton, NC. Two problematic weed species, Palmer amaranth (Amaranthus palmeri S. Watson) and large crabgrass [Digitaria sanguinalis (L.) Scop.], were planted at four densities in soybean [Glycine max (L.) Merr.]. Additionally, these weed densities were grown in the presence and absence of the crop to determine the influence of crop presence on the detection and discrimination of weed species and density. Hyperspectral data were collected over various phenological time points in each year. Differentiation between plant species and weed density was not consistent across cropping systems, phenology, or season. Weed species were distinguishable across more spectra when no soybean was present. In 2016, weed species were not distinguishable, while in 2017, differentiation occurred at 4 wk after planting (WAP) and 15 WAP when weeds were present with soybean. When soybean was not present, differentiation occurred only at 5 WAP in 2016 and at 3 WAP through 15 WAP in 2017. Differentiation between weed densities did occur in both years with and without soybean present, but weed density could be differentiated across more spectra when soybean was not present. This study demonstrates that weed and crop reflectance is dynamic throughout the season and that spectral reflectance can be affected by weed species and density.

Prevalence of wild oat (Avena fatua L.) and sterile oat [Avena sterilis ssp. ludoviciana (Durieu) Gillet & Magne; referred to as A. sterilis hereafter], winter-season weeds, is increasing in the eastern grain region of Australia. Biological attributes of these weeds enable them to survive in a wide range of environments and under different weed infestation levels. The interference of A. fatua and A. sterilis in a wheat (Triticum aestivum L.) crop was examined in southeast Queensland, Australia, through field studies in 2019 and 2020. Different infestation levels (0, 3, 6, 12, 24, and 48 plants m−2) of A. fatua and A. sterilis were evaluated for their potential to cause yield losses in wheat. Based on a three-parameter logarithmic model, the A. fatua and A. sterilis infestation levels corresponding to 50% wheat yield loss were 15 and 16 plants m−2, respectively. The yield reduction was due to a reduced spike number per unit area because of an increased weed infestation level. At the highest weed infestation level (48 plants m−2), A. fatua and A. sterilis produced a maximum of 4,800 and 3,970 seeds m−2, respectively. Avena fatua exhibited lower seed retention (17% to 39%) than A. sterilis (64% to 80%) at wheat harvest, as most of the seeds of A. fatua had shattered at crop maturity. Our results implied that there is a good opportunity for harvest weed seed control if the paddock is infested with A. sterilis. This study suggests that in the absence of an integrated weed management strategy (using both chemical and nonchemical options), a high infestation of these weeds could cause a severe crop yield loss, increase weed seed production, and replenish the weed seedbank in the soil.

Field studies were conducted from 2005 to 2009 in Idaho and Oregon to 1) evaluate the competitive effect of volunteer potato on sugar beet yield (volunteer potato competition experiment), and 2) determine the optimum timing of volunteer potato removal from glyphosate-tolerant sugar beet fields using glyphosate (volunteer potato removal timing experiment). The volunteer potato competition experiment consisted of eight potato densities, including the untreated check: 0, 6,741, 10,092, 13,455, 16,818, 20,184, 26,910, and 40,365 tubers ha−1. The volunteer potato removal experiment consisted of 10 removal timings (including the untreated check) ranging from the 10-cm rosette stage to mid-tuber bulking. There was a linear decrease in sugar beet root and sucrose yield as volunteer potato density increased (P < 0.001) such that with every volunteer potato tuber per square meter, sugar beet root yield decreased by 15% and sucrose yield decreased by 14%. At the highest volunteer potato density (40,365 tubers ha−1), sugar beet root yield was 29,600 kg ha−1 (compared to 73,600 kg ha−1 for the untreated), representing a 60% reduction in sugar beet root yield. In the removal timing study, a one-time application of glyphosate at the 10-cm rosette, hooking, and tuber initiation stages provided 74% to 98% reduction in volunteer potato tuber biomass. Delaying volunteer potato removal beyond the tuber initiation stage reduced sugar beet root and sucrose yield (12% to 20%), resulting in an economic loss of $104 to $161 per hectare. The best potato removal timing that optimizes the trade-off between improved control and potential for sugar beet yield reductions is before or at the tuber initiation stage.

Some well-managed organic soils are known to have higher crop yield potential than conventionally managed soils due to the greater soil quality and the ability to tolerate weed competition. However, low available soil mineral N and P in some organic systems may mask such soil quality–related benefits. We hypothesize that when plant-available N and P are not limiting, tillage-based highly diverse organic crop rotations have less yield loss (better crop tolerance) due to weed competition and higher crop yields than no-till conventional systems with low-diversity rotations. A greenhouse study was carried out in Saskatoon, Canada, using long-term (18-yr) organically managed soils (ORG) and no-till conventional soils (CONV) with three crop rotation diversities (LOW, MEDIUM, and HIGH) to compare the crop tolerance to weed competition under standard soil nutrient management conditions and under excess supply of mineral N and P. Under fertilized conditions, crop biomass increased by 50% and 69% in ORG and CONV systems, respectively. Weed biomass was similar between ORG and CONV systems under nonfertilized conditions but was 14% greater in CONV when excessive N and P were supplied. Crop biomass loss (crop tolerance) was not different among cropping systems under excess fertilizer or under standard fertilizer levels. Even with greater weed biomass under fertilized conditions, the CONV system showed crop tolerance similar to that of the ORG system. Under nonfertilized conditions, the crop biomass yield was 43% lower in ORG compared with CONV, and even after mineral N and P were applied, ORG systems showed less (17%) crop biomass than CONV. Further, differences in crop tolerance were not identified among crop rotations under both fertilizer levels. Overall, this study revealed that there were no yield benefits or better crop tolerance to weed competition in organically managed soils compared with no-till conventional soils, even under nonlimiting soil macronutrient conditions.

Knowledge of crop–weed interference effects on weed biology along with yield penalties can be used for the development of integrated weed management (IWM) tactics. Nevertheless, little is known about the beneficial effects of soybean [Glycine max (L.) Merr.] density, an important aspect of IWM, on late Palmer amaranth (Amaranthus palmeri S. Watson) establishment time. Two field experiments were conducted in 2014 and 2015 to investigate how various soybean densities and A. palmeri establishment timings in weeks after crop emergence (WAE) affect height, biomass, and seed production of the weed but also crop yield in drill-seeded soybean. Soybean density had a significant impact on dry weight and seed production of A. palmeri that established within the first 2 wk of crop emergence, but not for establishment timings of the weed 4 wk and later in relation to crop emergence. Differential performance of A. palmeri gender was observed, regarding greater biomass production of female than male plants under crop presence, and merits further investigation. Grain yield reductions were recorded at earlier A. palmeri establishment timings (i.e., 0 and 1 WAE) compared with 8 WAE establishment timing in 2014 and 2015. High soybean densities resulted in greater soybean yields compared with low soybean density, but no grain yield benefits were observed between medium and high soybean densities. Crop budget analysis revealed the benefits of moderate seeding rate (i.e., 250, 000 seeds ha−1) increases in comparison to lower (i.e., 125,000 seeds ha−1) or high (i.e., 400,000 seeds ha−1) on crop revenue, net income returns, and breakeven price. Earlier A. palmeri establishment timings (i.e., 0, 1, and 2 WAE) resulted in lower crop revenue and net income returns compared with later establishment timings of the weed.

Potential yield losses in grain sorghum due to weed interference based on quantitative data from the major grain sorghum-growing areas of the United States are reported by the WSSA Weed Loss Committee. Weed scientists and extension specialists who researched weed control in grain sorghum provided data on grain sorghum yield loss due to weed interference in their region. Data were requested from up to 10 individual experiments per calendar year over 10 yr between 2007 and 2016. Based on the summarized information, farmers in Arkansas, Kansas, Missouri, Nebraska, South Dakota, and Texas would potentially lose an average of 37%, 38%, 30%, 56%, 61%, and 60% of their grain sorghum yield with no weed control, and have a corresponding annual monetary loss of US $19 million, 302 million, 7 million, 32 million, 25 million, and 314 million, respectively. The overall average yield loss due to weed interference was estimated to be 47% for this grain sorghum-growing region. Thus, US farmers would lose approximately 5,700 million kg of grain sorghum valued at approximately US $953 million annually if weeds are not controlled. With each dollar invested in weed management (based on estimated weed control cost of US $100 ha−1), there would be a return of US $3.80, highlighting the return on investment in weed management and the importance of continued weed science research for sustaining high grain sorghum yield and profitability in the United States.

Annual sowthistle (Sonchus oleraceus L.) is a broadleaf weed that is increasing in prevalence in the northern cropping regions of Australia. Being a member of Asteraceae family, this weed possesses many biological attributes needed to thrive in varying environments and under differing weed management pressures. Interference of this weed in a wheat (Triticum aestivum L.) crop was examined through field studies in 2016 and 2017. Different densities of S. oleraceus were evaluated for their potential to cause yield loss in wheat: 0.0 (weed-free), low (9 to 15 plants m−2), medium (29 to 38 plants m−2), and high (62 to 63 plants m−2). Based on the exponential decay model, 43 and 52 plants m−2 caused a yield reduction of 50% in 2016 and 2017, respectively. Yield components such as panicles per square meter and grains per panicle were affected by weed density. At the high weed infestation level, S. oleraceus produced a maximum of 182,940 and 192,657 seeds m−2 in 2016 and 2017, respectively. Sonchus oleraceus exhibited poor seed retention at harvest, as more than 95% of seeds were blown away by wind. Adverse effects on crop, high seed production, and wind-blown dispersal may lead to an increased prevalence of this weed in the absence of an integrated weed management strategy utilizing both herbicides and nonchemical options.

Turnipweed [Rapistrum rugosum (L.) All.] and Mexican pricklepoppy (Argemone mexicana L.) are increasingly prevalent in the northern cropping regions of Australia. The effect of different densities of these two weeds was examined for their potential to cause yield loss in wheat (Triticum aestivum L.) through field studies in 2016 and 2017. There was 72% to 78% yield reduction in wheat due to competition from R. rugosum. Based on the exponential decay model, 18.2 and 24.3 plants m−2 caused a yield reduction of 50% in 2016 and 2017, respectively. Rapistrum rugosum produced a maximum of 32,042 and 29,761 seeds m−2 in 2016 and 2017, respectively. There was 100% weed seed retention at crop harvest. Competition from A. mexicana resulted in a yield loss of 17% and 22% in 2016 and 2017, respectively; however, plants failed to set seeds due to intense competition from wheat. Among the yield components, panicles per square meter and grains per panicle were affected by weed competition. The studies indicate a superior competitiveness of R. rugosum in wheat and a suppressive effect of wheat on A. mexicana. The results indicate that a wheat crop can be included in crop rotation programs where crop fields are infested with A. mexicana. High seed retention in R. rugosum indicates the possibility to manage this weed through seed catching and harvest weed seed destruction.

Parthenium hysterophorus is a prolific invasive weed species, which infests many crops in over 40 countries around the world. A 2-year field study was carried out to quantify the potential impacts of this weed on direct-seeded rice. Parthenium weed was allowed to compete for 2, 4, 6 or 8 weeks after crop emergence, while full season weedy and weed-free plots were maintained as controls. Parthenium weed plants grew taller and attained more biomass as the competition duration prolonged. The yield and yield-related attributes of rice were negatively affected with increasing competition duration. The season-long competition caused the highest reductions in panicle number (28–34%), panicle length (26–27%), grains per panicle (22–23%) and grain yield (33 and 38%) of rice in both years. Weed competition for 2–8 weeks caused 5–34% and 6–33% losses in rice grain yield during both years, respectively. Importantly, Parthenium weed control after 8 weeks of competition did not improve rice yield significantly. The results suggested that Parthenium weed should be controlled in rice fields between 4 and 8 weeks after crop emergence under direct-seeded conditions to avoid over 10% yield losses.

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is one of the most troublesome and competitive weed species in rice (Oryza sativa L.) grown under conventional flood (FLD) irrigation and is expected to be similarly damaging under alternate-wetting-and-drying (AWD) irrigation. Several indica rice genotypes have been shown to suppress E. crus-galli under FLD management, but very little is known about the nature and potential of weed suppression using AWD irrigation. In this 3-yr field study, we evaluated seven diverse rice genotypes for their weed suppression and grain yield potential under FLD and AWD irrigation. The E. crus-galli dry biomass at midseason was 11% less under AWD compared with FLD. Overall, plots of an indica variety and a hybrid resulted in less E. crus-galli dry biomass under AWD compared with FLD. Grain yield in weed-free AWD plots averaged 12% less than in weed-free FLD plots. Grain yield of the tropical japonica (TRJ) type, ‘Bengal’, was 32% lower under AWD than FLD, whereas grain yields of the two indica genotypes, PI 312777 and ‘Rondo’, and a hybrid were similar in both irrigation systems. Grain yield reduction in E. crus-galli–infested AWD plots averaged greater than 90%. Thus, E. crus-galli greatly reduced the grain yield of all rice genotypes tested in both FLD and AWD systems in this study. Grain yield of the indica and hybrid genotypes was relatively less affected by the AWD treatments compared with the TRJ genotypes, particularly with Bengal, suggesting that the weed-suppressive genotypes, PI 312777 and Rondo, would be better suited to AWD irrigation systems and for inclusion in weed-suppression rice-breeding programs in the southern United States.

Information about weed biology and weed population dynamics is critical for the development of efficient weed management programs. A field experiment was conducted in Fayetteville, AR, during 2014 and 2015 to examine the effects of Palmer amaranth (Amaranthus palmeri S. Watson) establishment time in relation to soybean [Glycine max (L.) Merr.] emergence and the effects of A. palmeri distance from the soybean row on the weed’s height, biomass, seed production, and flowering time and on soybean yield. The establishment time factor, in weeks after crop emergence (WAE), was composed of six treatment levels (0, 1, 2, 4, 6, and 8 WAE), whereas the distance from the crop consisted of three treatment levels (0, 24, and 48 cm). Differences in A. palmeri biomass and seed production averaged across distance from the crop were found at 0 and 1 WAE in both years. Establishment time had a significant effect on A. palmeri seed production through greater biomass production and height increases at earlier dates. Amaranthus palmeri that was established with the crop (0 WAE) overtopped soybean at about 7 and 10 WAE in 2014 and 2015, respectively. Distance from the crop affected A. palmeri height, biomass, and seed production. The greater the distance from the crop, the higher A. palmeri height, biomass, and seed production at 0 and 1 WAE compared with other dates (i.e., 2, 4, 6, and 8 WAE). Amaranthus palmeri establishment time had a significant impact on soybean yield, but distance from the crop did not. The earlier A. palmeri interfered with soybean (0 and 1 WAE), the greater the crop yield reduction; after that period no significant yield reductions were recorded compared with the rest of the weed establishment times. Knowledge of A. palmeri response, especially at early stages of its life cycle, is important for designing efficient weed management strategies and cropping systems that can enhance crop competitiveness. Control of A. palmeri within the first week after crop emergence or reduced distance between crop and weed are important factors for an effective implementation of weed management measures against A. palmeri and reduced soybean yield losses due to weed interference.

Strip-intercropping of functionally diverse cover crop mixtures including cereal rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth) is one mechanism by which nitrogen (N) banding can be applied to an organic, strip-tilled system to increase crop competitiveness over weeds. We hypothesized that by targeting hairy vetch, a low C:N legume, to the tilled strip directly in row with future crop establishment, and cereal rye, a high C:N grass, to the untilled strip directly between future crop rows, that N would be preferentially available to the crop. We conducted a field study between 2011 to 2013 in southwest Michigan to examine the effects of rye–vetch mixture spatial arrangement (strip intercropping vs. full-width mixture) on (1) soil inorganic N; (2) weed biomass; and (3) sweet corn (Zea mays L.) biomass, yield, and competitiveness against weeds. We found that as the proportion of vetch biomass in the crop row (in-row, IR) increased, we also saw increasing levels of IR soil inorganic N and greater early sweet corn N uptake and growth relative to weeds. However, sweet corn yield and final biomass were more responsive to vetch biomass across the whole plot (WP) and did not respond to rye and vetch segregation into strips. Increasing vetch WP biomass increased sweet corn final biomass across both years, but only increased corn competitiveness against weeds in 1 out of 2 years and decreased sweet corn competitiveness in the other year. Strip-intercropping of cereal rye and hairy vetch has potential to increase soil N availability to the crop, thereby increasing early crop competitiveness, which may lower weed management costs.

Blessed milkthistle is considered to be a noxious weed in irrigated and rainfed areas of Pakistan due to its strong allelopathic effects on food crops. For sustainable wheat production, it is necessary to know the critical time for weed removal (CTWR) for blessed milkthistle to allow wheat growers to get maximum benefit from control of this weed. A field study was conducted in 2014 and 2015 at the College of Agriculture, University of Sargodha, Pakistan, to investigate the CTWR of blessed milkthistle in wheat. The field experiments were designed with seven treatments; weed free (control); 2, 3, 4, 5, and 6 wk after emergence (WAE); and weedy check. At 6 WAE, a significant reduction was noted in plant height (8% and 17%), number of productive tillers per square meter (16% and 16%), spike length (23% and 54%), grains per spike (13% and 34%), 1,000-grain weight (14% and 37%), grain yield (20% and 21%), and biological yield (24% and 50%) compared with control (weed-free plots) during 2014 and 2015, respectively. The logistic model supports the field study results and suggests that blessed milkthistle’s CTWR for wheat is 1 to 5 WAE based on acceptable yield losses of 5% to 15% during both years. The experimental results and logistic model indicate that blessed milkthistle should be controlled within 1 to 5 WAE to get better wheat crop harvests without compromising farmers’ profits. To our knowledge, this is the first study ever in Pakistan regarding the CTWR in terms of WAE of blessed milkthistle and could help other scientists create weed control strategies for other areas of the country.

Field studies were conducted in North Carolina to determine the critical period for Palmer amaranth control (CPPAC) in pickling cucumber. In removal treatments (REM), emerged Palmer amaranth were allowed to compete with cucumber for 14, 21, 28, or 35 d after sowing (DAS) in 2014 and 14, 21, 35, or 42 DAS in 2015, and cucumber was kept weed-free for the remainder of the season. In the establishment treatments (EST), cucumber was maintained free of Palmer amaranth by hand removal until 14, 21, 28, or 35 DAS in 2014 and until 14, 21, 35, or 42 DAS in 2015; after this, Palmer amaranth was allowed to establish and compete with the cucumber for the remainder of the season. The beginning and end of the CPPAC, based on 5% loss of marketable yield, was determined by fitting log-logistic and Gompertz equations to the relative yield data representing REM and EST, respectively. Season-long competition by Palmer amaranth reduced pickling cucumber yield by 45% to 98% and 88% to 98% during 2014 and 2015, respectively. When cucumber was planted on April 25, 2015, the CPPAC ranged from 570 to 1,002 heat units (HU), which corresponded to 32 to 49 DAS. However, when cucumber planting was delayed 2 to 4 wk (May 7 and May 21, 2014 and May 4, 2015), the CPPAC lasted from 100 to 918 HU (7 to 44 DAS). This research suggested that planting pickling cucumber as early as possible during the season may help to reduce competition by Palmer amaranth and delay the beginning of the CPPAC.

Two field studies were conducted in Louisiana to determine the impact of Nealley’s sprangletop on rough rice yield under multiple environments in 2014, 2015, and 2016. The first study evaluated optimal timings of Nealley’s sprangletop removal for optimizing rough rice yields. The second study evaluated the impact of Nealley’s sprangletop densities on rough rice yield. Nealley’s sprangletop was removed with applications of fenoxaprop at 122 g ai ha–1 at 7, 14, 21, 28, 35, and 42 d after emergence (DAE). Nealley’s sprangletop removal at 7 and 14 DAE resulted in higher rough rice yields of 7,880 and 6,960 kg ha–1, respectively, when compared with the rice from the season-long Nealley’s sprangletop competition with a 6,040 kg ha-1 yield. Delaying herbicide application from 7 DAE to 42 DAE resulted in a yield loss of 1,740 kg ha–1. Over the 35-d delay in application, rough rice yield loss from Nealley’s sprangletop interference was equivalent to 50 kg ha–1 d–1. Nealley’s sprangletop densities were established at 1, 3, 7, 13, and 26 plants m–2 by transplanting Nealley’s sprangletop when rice reached the one- to two-leaf stage. At Nealley’s sprangletop densities of 1 to 26 plants m–2, rough rice yields were reduced 10 to 270 kg ha–1, compared with the rice from weed-free plots. Based on regression analysis, Nealley’s sprangletop densities of 1, 35, 70, and 450 plants m–2 reduced rough rice yield 0.14%, 5%, 10%, and 50%, respectively.

Slow carrot emergence and canopy development render the crop a poor competitor with weeds. In this study, the ability to suppress weeds and maintain yield in the presence of weeds was compared among nine carrot varieties that included those selected by plant breeders for rapid vegetative canopy development compared to traditional varieties. Two weed management treatments were compared: handweeding for 21 d after carrot seeding versus handweeding for the entire carrot season. In years and locations with low to moderate weed pressure, such as in the 2014 study, differences among carrot varieties in weed competitiveness or tolerance were less apparent and therefore less relevant. Maximum carrot yield loss to weed competition among varieties was 28% in 2014. Yield loss in the presence of weeds was 15% or less with six of the nine carrot varieties. However, when weed pressure was intense in the 2015 study, both carrot plant density and carrot canopy development were inversely related to weed biomass. Carrot yield loss in the presence of weeds ranged from 38 to 87%. Despite correcting seeding populations for differences in germination among carrot varieties, carrot stand establishment varied greatly and would likely affect subsequent weed control measures such as timely cultivation or herbicide application. Future research efforts are warranted that consider carrot stand establishment factors and their relationship with integrated weed management programs.