Weed Management Programs Research Articles

Multiple herbicide resistance in barnyardgrass (Echinochloa crus-galli) in direct-seeded rice in the Philippines

Barnyardgrass (Echinochloa crus-galli [L.] Beauv.), a troublesome weed in rice production, has been confirmed for the first time to be resistant to both chloroacetamide (butachlor)- and acetanilide (propanil)-group herbicides commonly used in direct-seeded rice in the Philippines. Resistance screening showed that 17 of 18 sampled populations (94%) were resistant to butachlor + propanil. Dose–response assays using butachlor + propanil on the populations resulted in all but one not being controlled at twice the recommended rate for field application (1.4 kg ai or 2 l/ha) and seven populations not being controlled by 4 l/ha (2.8 kg ai). Dose–response assays using butachlor and propanil separately indicated that the latter populations were resistant to both of the herbicides. LD50 values for butachlor + propanil populations ranged from 0.6 l/ha (0.42 kg ai) to 2.9 l/ha (2.03 kg ai) indicating 1.9 to 9.1 times less sensitivity than a known susceptible population. Thus, what was perceived by farmers to be reduced herbicide efficacy is actually herbicide resistance. Hence, this new development in major herbicides has become a threat in rice production and will alter weed management in direct-seeded rice. Concerted efforts in raising awareness on herbicide resistance and implementing an effective integrated weed management program are needed.

Managing invasive weeds under climate change: considering the current and potential future distribution of Buddleja davidii

Kriticos DJ,Watt MS, Potter KJB, Manning LK,Alexander NS &Tallent‐Halsell N (2011). Managing invasive weeds under climate change: considering the current and potential future distribution of Buddleja davidii. Weed Research 51, 85–96.Summary Buddleja davidii is both a prized garden ornamental and an invasive shrub that rapidly colonises disturbed ground. Originally from China, B. davidii has been widely distributed by horticulturalists and has subsequently invaded much of Europe and New Zealand, and to a lesser degree the Americas and Australia. The present and future climate suitability for B. davidii was assessed using a process‐oriented climate suitability model. There appears to be a considerable scope for further invasion, with the most suitable areas occurring adjacent to existing naturalised populations in the north‐eastern United States, north‐eastern Europe, south‐eastern Australia and south‐eastern New Zealand. Under future climates, the potential distribution and climate suitability for B. davidii increases most noticeably in the northern United States and southern Canada, northern and eastern Europe, and to a lesser extent in the south‐western part of the South Island of New Zealand. Elsewhere, there are projected poleward range shifts (South America) or range contractions out of subtropical areas (Africa and Australia). Climate‐based potential distribution models can help adapt weed management programmes to expected climate changes by: (i) classifying areas for the different types of weed management, (ii) supporting strategic control initiatives to prevent the spread of a weed, (iii) informing the reallocation of resources away from controlling a weed where climate suitability is expected to diminish in the future and (iv) identifying opportunities for relatively inexpensive preventative management to be applied to minimise future weed impacts.

Popcorn ( Zea mays L. var. everta) yield and yield components as influenced by the timing of broadcast flaming

Farmers are interested in producing popcorn under organic production systems and propane flaming could be a significant component of an integrated weed management program. The objective of this study was to collect baseline information on popcorn tolerance to broadcast flaming as influenced by propane dose and crop growth stage at the time of flaming. Field experiments were conducted at the Haskell Agricultural Laboratory of the University of Nebraska, Concord, NE in 2008 and 2009 using five propane doses (0, 13, 24, 44 and 85 kg ha −1) applied at the 2-leaf, 5-leaf and 7-leaf growth stages. Propane was applied using a custom-built research flamer driven at a constant speed of 6.4 km h −1. Crop response to propane dose was described by log-logistic models on the basis of visual estimates of crop injury, yield components (plants m −2, ears plant −1, kernels cob −1 and 100-kernel weight) and grain yield. Popcorn response to flaming was influenced by the crop growth stage and propane dose. Based on various parameters evaluated, popcorn flamed at the 5-leaf showed the highest tolerance while the 2-leaf was the most susceptible stage. The maximum yield reductions were 45%, 9% and 16% for the 2-leaf, 5-leaf and 7-leaf stages, respectively. In addition, propane doses that resulted in a 5% yield loss were 23 kg ha −1 for the 2-leaf and 7-leaf and 30 kg ha −1 for the 5-leaf stage. Flaming has a potential to be used effectively in organic popcorn production if properly used.

Competition of sorghum cultivars and densities with Japanese millet (Echinochloa esculenta)

Field studies were conducted at two locations in southern Queensland, Australia during the 2003–2004 and 2004–2005 growing seasons to determine the differential competitiveness of sorghum (Sorghum bicolor L. Moench) cultivars and crop densities against weeds and the sorghum yield loss due to weeds. Weed competition was investigated by growing sorghum in the presence or absence of a model grass weed, Japanese millet (Echinochloa esculenta). The correlation analyses showed that the early growth traits (height, shoot biomass, and daily growth rate of the shoot biomass) of sorghum adversely affected the height, biomass, and seed production of millet, as measured at maturity. “MR Goldrush” and “Bonus MR” were the most competitive cultivars, resulting in reduced weed biomass, weed density, and weed seed production. The density of sorghum also had a significant effect on the crop's ability to compete with millet. When compared to the density of 4.5 plants per m2, sorghum that was planted at 7.5 plants per m2 suppressed the density, biomass, and seed production of millet by 22%, 27% and 38%, respectively. Millet caused a significant yield loss in comparison with the weed‐free plots. The combined weed‐suppressive effects of the competitive cultivars, such as MR Goldrush, and high crop densities minimized the yield losses from the weeds. These results indicate that sorghum competition against grass weeds can be improved by choosing competitive cultivars and by using a high crop density of >7.5 plants per m2. These non‐chemical options should be included in an integrated weed management program for better weed management, particularly where the control options are limited by the evolution of herbicide resistance.

Evaluation of Legume Cover Crops and Weed Control Programs in Conservation-Tillage, Enhanced Glyphosate-Resistant Cotton

Research was conducted at Marianna, AR, for 2 yr to determine whether hairy vetch and Austrian winter pea cover crops would aid weed management programs in conservation-tilled, enhanced glyphosate-resistant cotton. Both cover crops were easily established and produced rapid growth in early spring, with biomass production of 435 to 491 g m−2by Austrian winter pea and 415 to 438 g m−2by hairy vetch. The effect of cover crops on weed control was short-lived in both years, with herbicide programs being the major determinant of weed control and seed-cotton yield. Averaged over cover crops, seed-cotton yields when the initial in-crop glyphosate application was delayed to the four-node cotton stage were up to 710 kg ha−1less than in a PRE herbicide program. In 1 of 2 yr, seed-cotton yields were greater in PRE-treated plots compared with a program where initial weed management was delayed to the one-leaf stage of cotton. As a result of rapid decay of hairy vetch and Austrian winter pea biomass following cotton planting and the lack of adequate Palmer amaranth, pitted morningglory, and goosegrass control in the absence of herbicides, it appears there may be minimal weed management benefits from the use of hairy vetch and Austrian winter pea in Midsouth cotton production.

An assessment of the accuracy and consistency of human perception of weed cover

Andújar D, Ribeiro A, Carmona R, Fernández-Quintanilla C & Dorado J (2010). An assessment of the accuracy and consistency of human perception of weed cover. Weed Research 50, 638–647. Summary When integrated weed management approaches are used, visual estimations of weed cover are commonly employed to quantify weed infestation levels in crops. This study assessed the reliability of visual estimation of weed cover, considering various factors which may influence the accuracy and consistency of human perception. Measurements were based on visual estimates of digital images acquired in maize fields. A total of 750 images were assessed independently by four experienced observers using three cover-abundance scales, varying the assessment date or the order in which the images were assessed. A good relationship was generally found between the visual estimates of weed cover and some objective parameters (actual weed cover, weed biomass). The analyses of reliability and repeatability showed no significant differences between the estimations performed by four different observers or by the same observer at different times, independently of the scale used. Nevertheless, the comparison between visual estimations performed by the observers and actual weed cover showed that observers tended to overestimate weed cover at low weed densities and underestimate it when densities were high. Although visual assessments of weed cover were relatively accurate to quantify weed infestation level, our results showed that human perception was not reliable enough to determine actual weed cover close to the class boundaries, both in the lower range (e.g. 5%, 10%) and in the middle range (e.g. 50%) of the scale. This may be a serious limitation for the use of visual methods in threshold-based weed management programmes.

Strategy to Manage Herbicide Resistant Eleusine indica in an Oil Palm Nursery in Jerantut Area

The present studies were conducted in an oil palm nursery in the Jerantut area where the main weed Eleusine indica was suspected to be resistant to glufosinate-ammonium. In Experiment 1, sodium chlorate -7.5 kg per hectare and glufosinate-ammonium -3.3 L per hectare gave moderate and poor control (60% and 40%) at 28 days after spraying respectively. Glyphosate at normal rate recommended for soft grass weed control ( lx) was ineffective. In Experiment 2, glyphosate at highest rate (x8) gave good weed control (79% scorched at 28 days after spraying). Fluazifop-butyl was ineffective at the 3 rates tested. Glufosinate-ammonium required higher rates (x4 and x8) to achieve good control at 14 days after spraying. Some of these herbicide mixtures gave high percentage weed scorching but the costs can be higher. Pre-emergent herbicide (ametryn, atrazine) in mixture with fluazifop-butyl and sodium chlorate were promising and can be included to manage resistant weed populations by reducing its seeds bank. Other than that, planters should practice integrated weed management programme including cultural practices such as grass cutting/mowing and rotation of crops or nursery site. Keywords: Eleusine indica, herbicides, biotype, resistance, integrated weed management.

Homeowner Considerations Prior to Selecting a Weed Control Product

HS1177, a 3-page fact sheet by Andrew W. MacRae and Marina D’Abreau, coaches homeowners to consider weed type and location in the landscape in selecting a herbicide as part of an effective weed management program. Published by the UF Department of Horticultural Sciences, May 2010.

Growth stage impacts tolerance of winter wheat ( Triticum aestivum L.) to broadcast flaming

Organic wheat producers are interested in testing propane flaming as part of an integrated weed management program for organic wheat production. Therefore, the objective of this study was to collect baseline information on winter wheat tolerance to broadcast flaming as influenced by its growth stage at the time of flaming and dose of propane. Field experiments were conducted at the Haskell Agricultural Laboratory of the University of Nebraska, Concord, Nebraska in 2007–2008 and 2008–2009 utilizing six doses of propane applied at four growth stages including: four leaves-4L, three tillers-3 T, shoot elongation-SE and boot stage-BS. The propane doses were 0, 12, 31, 50, 68 and 87 kg ha −1 and were applied using a custom built flamer driven at a constant speed of around 6 km h −1. Crop response to propane doses was described by log-logistic models based on visual estimates of crop injury, various yield components (spikes m −2, kernels spike −1 and 1000-kernel weight) and grain yield. Overall response to flaming was influenced by the growth stage of wheat and propane dose. In general, wheat at 3 T was the most tolerant and at BS was the most susceptible stage to broadcast flaming. Flaming negatively affected all yield components of wheat. Reduction of grain yield increased with increase in propane dose at each growth stage. The maximum yield losses of about 21%, 32%, 63% and 74% were obtained with the highest propane dose of 87 kg ha −1 applied at 3 T, SE, 4L and BS growth stages, respectively. Due to unacceptable yield loss, the use of broadcast flaming in winter wheat at the tested growth stages is not recommended.

Integrating Herbicide Use and Perennial Grass Revegetation to Suppress Weeds in Noncrop Areas

AbstractNoncropland such as levees, roadsides, field borders, fencerows, and wildlife areas are vulnerable to weed invasion. Many sites have undergone frequent human disturbance, such as manipulation from surrounding land uses, and lack competitive, desirable vegetation. This study addressed the importance of revegetation in an integrated weed management program including revegetation for noncrop areas. The study evaluated 14 cool-season perennial grasses (seven native species and eight introduced species) for their establishment, vigor, and ability to suppress weeds. It also evaluated the impact of herbicides on weed control and grass establishment. Treatments were applied at three noncrop sites in Northeast California that were heavily infested with weeds. Chemical weed control during the year of seeding and the following year was critical for perennial grass establishment. Weed cover was greater than 50% whereas average seeded grass cover was less than 6% in untreated plots at all sites 2 yr after seeding. In contrast, average seeded grass cover at all sites was 22 to 31% 2 yr after seeding for treatments where herbicide use resulted in wide-spectrum weed control and grass safety. Increasing perennial grass cover decreased total weed cover across perennial grass species 1and 2 yr after seeding. Individual grass species' cover differed among sites. Two introduced grasses (tall wheatgrass and crested wheatgrass) and three native grasses (western wheatgrass, bluebunch wheatgrass, and thickspike wheatgrass) showed broad adaptation and had > 20% cover at all sites 2 yr after seeding. In herbicide-treated plots, these grasses reduced total weed cover by 43 to 98% compared to unseeded plots 2 yr after seeding.

Evaluation and Economics of a Machine-Vision Guided Cultivation Program in Broccoli and Lettuce

Machine-vision cultivator guidance systems are commercially available to growers, but little work has been done to determine if these guidance systems can improve integrated weed management systems in vegetable crops. Studies were conducted in 2005 and 2006 in broccoli and lettuce to evaluate band-applied DCPA or pronamide, respectively, and four noncultivated bands ranging from 5.1 to 12.7 cm. DCPA or pronamide were applied in bands centered on the seed line at 0, 7.6 or 12.7 cm wide. A commercial machine-vision system was used to guide a commercial cultivator. Generally, weed densities and hand-weeding times were less where the DCPA band in broccoli or the pronamide band in lettuce were 7.6 or 12.7 cm wide compared to no herbicide. Weed densities were lowest in both crops where the noncultivated band width was 5.1 cm compared to 12.7-cm noncultivated bands. For broccoli in both 2005 and 2006, net returns above production costs were generally higher in the 7.6- and 12.7-cm-wide DCPA bands compared with the no-herbicide band. In lettuce in both years, the no-pronamide treatment had higher net returns, when compared with the 7.6- and 12.7-cm pronamide bands. Lettuce yields and higher net returns in the no-pronamide treatment compared to the 7.6- and 12.7-cm pronamide bands may be due to slight yield reduction from pronamide. Results suggest that pronamide was not needed during the dry months of the year when weed management tools such as hand-weeding and cultivation work very well. However, in periods of rainy weather when cultivation and hand-weeding are not possible, then pronamide would likely provide the greatest economic benefit. Given the large impact of cultivation on vegetable weed management programs, the greatest potential benefit of machine-vision guided cultivators is if they facilitate more timely and effective cultivation.

Rangeland Invasive Plant Management

lands provide a valuable resource for wildlife and recreational activities, as well as livestock grazing. The establishment and spread of invasive plants on rangeland are often symptomatic of long-term cultural practices.3 Invasion of many rangeland sites by non-native plants has been facilitated by multiple factors including overgrazing by domestic livestock, purposeful introduction of non-native plants for agricultural purposes, unintentional introduction of non-native plant seeds as a contaminate in crop seed, changes in fire frequency and intensity, and climatic changes that converge to cause shifts in plant community composition. There are over 300 rangeland weeds in the United States, but perhaps 30 have significant negative impact in the western United States.2 Of these, some of the most problematic include downy brome or cheatgrass {Bromus tectorum), medusahead {Taeniatherum caput-medusae), leafy spurge {Euphorbia esula)> perennial pepperweed {Lepidium latifolium), Dalmatian toadflax {Linaria dalmatica), and several members of the Asteraceae, including yellow starthistle {Centaurea solstitialis), diffuse knapweed (C. diffusa), spotted knapweed {C. stoebe subsp. micranthos), musk thistle {Carduus nutans), Canada thistle {Cirsium arvense), hawkweeds {Hieracium spp.), and Russian knapweed {Acroptilon repens). These and other invasive plants can have adverse effects on rangeland and pastures by disrupting vegetation dynamics and reducing ecosystem capacity to recover after disturbance.3 The financial impacts of invasive plants in rangelands are substantial and have been estimated to cost ranchers US$5 billion annually just for control.4 Other impacts to the livestock industry not included in the estimate are the negative effects of invasive plants on yield and quality of forage, livestock poisoning, interference with grazing, supplemental costs associated with managing and producing livestock, and land values. In addition to the livestock industry, invasive rangeland species can decrease wildlife habitat and forage, deplete soil and water resources, and reduce plant and animal diversity. Mechanical, cultural, biological, and chemical methods have been developed to manage rangeland invasive plants. These methods include mowing, prescribed burning, appropriate grazing management, herbicides, biocontrol agents, and revegetation with desirable plants.1 The merits of each control measure and potential for complementary or synergistic interactions when applied in appropriate sequences and combinations should be considered when developing integrated weed management programs. For example, summer burning resulted in a dramatic increase in yellow starthistle germination the following fall and winter.5 Thus, using an herbicide treatment in the year following a prescribed burn gave far better control of yellow starthistle compared with an herbicide treatment followed by prescribed burning. Successful integrated strategies require careful planning that incorporates a long-term approach consisting of prevention programs, education activities, and management approaches. These are designed to diminish negative impacts of invasive plants by improving the invaded plant community to increase its ecosystem function, niche occupation, and competitiveness. These qualities can increase the communities' resistance to reinvasion by other invasive plants.3

Inter‐population variation in seed longevity for two invasive weeds: Chrysanthemoides monilifera ssp. monilifera (boneseed) and ssp. rotundata (bitou bush)

Schoeman J, Buckley YM, Cherry H, Long RL & Steadman KJ (2010). Inter‐population variation in seed longevity for two invasive weeds: Chrysanthemoides monilifera ssp. monilifera (boneseed) and ssp. rotundata (bitou bush). Weed Research50, 67–75.SummarySeed longevity has a major influence on the success of weed management and eradication programmes. A correlation between responses to a controlled aging test performed at 45°C, 60% relative humidity (RH) and seed persistence in the field has recently been suggested. Here we investigated whether collections of differing quality of two closely related invasive weeds, Chrysanthemoides monilifera ssp. monilifera and ssp. rotundata (boneseed and bitou bush, respectively), had different seed longevity using the controlled aging test. Chrysanthemoides monilifera ssp. monilifera fruits were collected from across five Australian states, and C. monilifera ssp. rotundata from one state, covering their invasive ranges. Seed quality was assessed visually and using tetrazolium staining, and a series of germination tests established appropriate germination conditions. The controlled aging test was run for a subset of collections. Chrysanthemoides monilifera ssp. rotundata seeds died more quickly (time to lose 50% viability, P50 = 16 days) than the C. monilifera ssp. monilifera collections (P50 = 47 days) when aged at 45°C and 60% RH. This difference was significant even considering the large differences in longevity between C. monilifera ssp. monilifera populations (P50 = 35–61 days; probably due to differences in maturity at collection). Based on a published correlation, we predict that mature C. monilifera ssp. monilifera may have a long‐lived (>3 years) seed bank and C. monilifera ssp. rotundata may have a transient (<1 year) seed bank. This suggests the two sub‐species should be considered separately when designing effective control measures.

Weed control, environmental impact and profitability with glyphosate tank mixes in glyphosate-tolerant corn

Eight field trials were conducted over a 3-yr period at Exeter (one site in 2005 and 2006) and Ridgetown (three sites in 2005, two sites in 2006 and one site in 2007), Ontario, to evaluate different postemergence glyphosate tank mixes for weed management in glyphosate-tolerant corn. Treatments included a weedy check, a single application of glyphosate, a sequential application of glyphosate and tank mixes of glyphosate plus either atrazine, dicamba/atrazine, mesotrione, s-metolachlor plus atrazine, s-metolachlor/atrazine, dicamba/diflufenzopyr, and dimethenamid plus atrazine. A single application of glyphosate, the sequential application of glyphosate and tank mixes of glyphosate plus either atrazine, dicamba/atrazine, mesotrione, s-metolachlor plus atrazine, s-metolachlor/atrazine, dicamba/diflufenzopyr, and dimethenamid plus atrazine provided 92-100% control of redroot pigweed, 87-100% control of common ragweed, 74-100% control of common lambsquarters, and 43-100% control of green foxtail. All herbicide treatments had a higher yield than the weedy check. There were no differences in corn yield among the herbicide treatments evaluated. The glyphosate alone or in tank mix combination with mesotrione or dicamba/diflufenzopyr programs had the lowest environmental impact. Glyphosate plus atrazine and dicamba/diflufenzopyr were the most profitable weed management programs in glyphosate-tolerant corn.Key words: Atrazine, dicamba, diflufenzopyr, dimethenamid, glyphosate, mesotrione, pendimethalin, rimsulfuron, s-metolachlor, Zea mays L.

Herbicide resistance in weeds – a threat to effective chemical weed control in South Africa

Apart from the last 40 to 50 years, weed control in crops has always been a labour-intensive and time-consuming mechanical practice. Relief came in the form of chemical compounds that could control weeds, in particular selective herbicides. The development of highly effective selective herbicides has resulted in farmers becoming increasingly reliant on chemical weed control. Continuous application of herbicides with the same mode of action, however, selected for resistant mutations within weed populations to become dominant, rendering the herbicides less effective. Herbicide resistant weeds currently occur in 187 weed species distributed over 60 countries worldwide. South Africa is no exception and resistance in 15 weed species to herbicides in seven modes of action groups has been verified. This situation is compelling farmers to consider alternative weed control strategies and cropping systems. The most effective strategy to manage resistant weeds would most probably be an integrated weed management program that does not rely on any one type of weed control method alone. Farmers will therefore have to adapt weed management systems and cropping systems to delay the onset of herbicide resistance or, if it is already present, to manage it effectively in order to keep future crop production profitable and sustainable.

Weed Control in Dicamba-Resistant Soybeans

Field experiments were conducted in 11 states to evaluate broadleaf weed management programs in dicamba-resistant soybeans which involved the use of preemergence and postemergence dicamba. Preemergence (PRE) dicamba at 0.25 lb ae/acre provided less than 60% control of smooth pigweed, giant ragweed, velvetleaf, palmer amaranth, waterhemp, and morningglory spp., but 97% control of common lambsquarters and horseweed at 3 weeks after treatment (WAT). Preemergence flumioxazin plus chlorimuron or sulfentrazone plus cloransulam provided 66 to 100% control of these weeds. Use of dicamba postemergence (POST) improved uniformity of control of velvetleaf, smooth pigweed, morningglory, and glyphosate-susceptible waterhemp. However, combining dicamba at 0.25 lb/acre with glyphosate resulted in 30% to 65% greater control of glyphosate-resistant palmer amaranth, glyphosate-resistant common waterhemp, glyphosate-resistant horseweed, and glyphosate-resistant giant ragweed compared to sequentially applied glyphosate.

Effect of cover crops and mulches on weed control and nitrogen fertilization in tomato ( Lycopersicon esculentum Mill.)

Cover crops and mulches are a suitable choice for sustainable agriculture because they improve weed control and crop performance. The aim of this research was to investigate weed control and nitrogen supply by using different winter cover crop species which were converted into mulches in spring. We carried out a 2-year field experiment where a tomato crop was transplanted into four different types of mulches coming from winter cover crops [hairy vetch ( Vicia villosa Roth.), subclover ( Trifolium subterraneum L.), oat ( Avena sativa L.), and a mixture of hairy vetch/oat)] and in conventional treatment (tilled soil without mulch). The mixture of hairy vetch/oat cover crop produced the highest aboveground biomass (7.9 t ha −1 of DM), while the hairy vetch accumulated the highest N in the aboveground biomass (258 kg N ha −1). The oat cover crop was the most effective cover crop for suppressing weeds (on average −93% of weed aboveground biomass compared to other cover crops). After mowing the cover crop aboveground biomass was placed in strips as dead mulch into which the tomato was transplanted in paired rows. Weed density and total weed aboveground biomass were assessed at 15 and 30 days after tomato transplanting to evaluate the effect of mulches on weed control. All mulches suppressed weeds in density and aboveground biomass compared to the conventional system (on average −80% and −35%, respectively). The oat was the best mulch for weed control but also had a negative effect on the marketable tomato yield (−15% compared to the conventional treatment). Amaranthus retroflexus L. and Chenopodium album L. were typical weeds associated with the conventional treatment while a more heterogeneous weed composition was found in mulched tomato. Legume mulches, in particular hairy vetch, gave the best marketable tomato yield 28% higher than the conventional system both with and without nitrogen fertilization. This research shows that winter cover crops converted into dead mulch in spring could be used successfully in integrated weed management programs to reduce weed infestation in tomato crops.

Effects of alternative winter cover cropping systems on weed suppression in organically grown tomato (Solanum lycopersicum)

Weed control is a major concern for organic farmers around the world and non-chemical weed control methods are now the subject of many investigations. Field studies were conducted in tomato (Solanum lycopersicum L.) from 2004 to 2006 at the Black Sea Agricultural Research Institute experiment field to determine the weed suppressive effects of winter cover crops. Treatments consisted of ryegrass (Lolium multiflorum L.), oat (Avena sativa L.), rye (Secale cereale L.), wheat (Triticum aestivum L.), gelemen clover (Trifolium meneghinianum Clem.), Egyptian clover (Trifolium alexandrinum L.), common vetch (Vicia sativa L.), hairy vetch (Vicia villosa Roth.) and a control with no cover crop. Treatments were arranged in a randomized complete block design with four replications. To determine the weed suppressive effects of the cover crops, weed density and total weed dry biomass were assessed at 14, 28, and 56 days after termination (DAT) of the cover crops from all plots using a 50 × 50 cm quadrat placed randomly in each plot. After cover crop kill and incorporation into soil, tomato seedlings variety ‘H2274’ were transplanted. Broadleaved weed species were the most prominent species in both years. Total weed biomass measured just prior to cover crop incorporation into the soil was significantly lower in S. cereale plots than in the others. The number of weed species was lowest at 14 DAT and later increased at 28 and 56 DAT, and subsequently remained constant during harvest. This research indicates that cover crops such as L. multiflorum, S. cereale, V. sativa and V. villosa could be used in integrated weed management programs to manage some weeds in the early growth stages of organic tomato.

Glyphosate effects on diseases of plants

Glyphosate, N-(phosphonomethyl)glycine, is the most extensively used herbicide in the history of agriculture. Weed management programs in glyphosate resistant (GR) field crops have provided highly effective weed control, simplified management decisions, and given cleaner harvested products. However, this relatively simple, broad-spectrum, systemic herbicide can have extensive unintended effects on nutrient efficiency and disease severity, thereby threatening its agricultural sustainability. A significant increase in disease severity associated with the wide spread application of the glyphosate herbicide can be the result of direct glyphosate-induced weakening of plant defenses and increased pathogen population and virulence. Indirect effects of glyphosate on disease predisposition result from immobilization of specific micronutrients involved in disease resistance, reduced growth and vigor of the plant from accumulation of glyphosate in meristematic root, shoot, and reproductive tissues, altered physiological efficiency, or modification of the soil microflora affecting the availability of nutrients involved in physiological disease resistance. Strategies to ameliorate the predisposing effects of glyphosate on disease include judicious selection of herbicide application rates, micronutrient amendment, glyphosate detoxification in meristematic tissues and soil, changes in cultural practices to enhance micronutrient availability for plant uptake, and biological amendment with glyphosate-resistant microbes for nitrogen fixation and nutrient availability. Given that recommended doses of glyphosate are often many times higher than needed to control weeds, we believe the most prudent method to reduce the detrimental effects of glyphosate on GR crops will be to use this herbicide in as small a dose as practically needed. Such a frugal approach will not only curtail disease predisposition of GR crops, but will also benefit the grower and the environment.

Seedbank and Emerged Weed Communities Following Adoption of Glyphosate-Resistant Crops in a Long-Term Tillage and Rotation Study

The compositions of the germinable weed seedbank and aboveground weed communities in a long-term tillage and rotation study were characterized 4, 5, and 6 yr (2002 to 2004) after the adoption of glyphosate-tolerant corn and soybean. Averaged across rotation, mean germinable weed seed density and diversity were greatest in the no-tillage treatment as compared to the minimum- and conventional-tillage treatments. Averaged over tillage, density and diversity were greater in the corn–oat–hay (ryegrass + alfalfa) system as compared to the continuous corn and corn–soybean rotations. Similar trends in density and diversity were observed for the aboveground weed communities. Differences in community composition among treatments were quantified with the use of a multiresponse permutation procedure. Results indicated that the weed seedbank community in a corn–oat–hay rotational system differed from the communities associated with the continuous corn and corn–soybean rotational systems. Weed seedbank communities developing under a no-tillage operation differed from those in minimum- and conventional-tillage scenarios. Compositional differences among the aboveground weed communities were less pronounced in response to tillage and rotation. Indicator species analyses indicated that the number of significant indicator weed species was generally higher for no tillage than minimum or conventional tillage for both the seedbank and the aboveground weed communities. The number of significant indicator species for the seedbank and weed communities was generally greater in the three-crop rotation as compared to the continuous corn and corn–soybean rotations. The trends observed in density, diversity, and community composition after the adoption of glyphosate-tolerant corn and soybeans, and a glyphosate-dominated weed management program, were also observed when soil-applied herbicides were included in the study. We suggest that the switch to a POST-glyphosate protocol did not significantly alter weed communities in the short term in this study.