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.     

Effect of Chinese Milk Vetch (Astragalus sinicus L.) as a Cover Crop on Weed Control,Growth and Yield of Wheat under Different Tillage Systems

Abstract

Reduced tillage systems are gaining popularity but weed control is often a limiting factor in the adoption of such systems. Cover crops have become a viable option for sustainable agriculture because of its contribution to soil fertility and improved crop performance. However, the contribution of cover crops to weed management is not clearly defined. We compared minimum tillage (MT) and no-tillage (NT) with conventional tillage (CT) for their effects on wheat growth in an original paddy land clay soil in the presence of Chinese milk vetch as a cover crop. Cover crop biomass, weed emergence, main crop growth and yield and soil penetration resistance were examined. Chinese milk vetch was successfully established under MT and CT but not under NT, which retarded its growth resulting in a significantly large biomass of all weed species. Weed suppression was more effective when the cover crop was broadcasted than row seeded. The presence of milk vetch as a cover crop significantly suppressed weed growth under MT especially at the late stage of growth and resulted in a comparable grain yield to that under CT. Although soil penetration resistance under MT remained high throughout the period of wheat growth, milk vetch could be effectively utilized as a cover crop under MT and wheat grain yield under MT was comparable to that under CT without mulch treatment.     

湖北省冬小麦田杂草种类与群落特征

为明确湖北省冬小麦田杂草的发生危害现状和群落组成特征,运用倒置"W"9点取样法对该省冬小麦主产区麦田杂草群落进行了调查。结果表明,湖北省冬麦田杂草有16科39种,其中禾本科杂草占18%,阔叶杂草占82%。相对多度达15以上的杂草依次为猪殃殃、牛繁缕、日本看麦娘、野老鹳草、野燕麦和大巢菜等6种,其中猪殃殃、牛繁缕和日本看麦娘的相对多度分别为74.19、51.66和30.36,为优势种类。     

Impact of managing cover crop residues on the floristic composition and species diversity of the weed community of pepper crop (Capsicum annuum L.)

The decline of farmland biodiversity is mainly attributed to the intensive use of chemical inputs in agriculture. Cover crop residues may contribute to improve weed management while maintaining a high level of weed diversity. A 2-year field experiment was carried out in central Italy to study the effect of cover crop species and their residue management on weed community composition and weed species diversity in a winter cover crop – pepper sequence. Hairy vetch (Vicia villosa Roth.), oat (Avena sativa L.) and canola (Brassica napus L.) were sown in September 2009 and 2010 and grew undisturbed during the winter season until spring when they were suppressed one week before pepper transplanting. Cover crop residues were: (i) green manured at 30 cm depth (conventional tillage, CT), (ii) green manured at 10 cm depth (minimum tillage, MT), and (iii) left on the soil surface as mulch strips covering 50% of the ground area in no-tilled soil (NT). A winter weedy fallow and a bare soil without cover crop in NT, MT and CT were also included as controls. Weed plant density data in pepper were used for calculating weed species richness. Compared to weedy fallow, oat, hairy vetch and canola consistently reduced the weed density and weed aboveground biomass by the time of their suppression (on average 3.6, 21.5, and 41.3 plants m⁻² and 11.0, 49.2, and 161.8 g m⁻² of DM, respectively). In pepper, oat residues generally determined a higher reduction of weed density and species richness compared to hairy vetch and canola regardless the residue management treatments. Converting cover crop aboveground biomass into mulch strips greatly reduced weed species density but did not always imply a reduction of weed species diversity in pepper compared to MT and CT. The weed species richness was reduced inside the mulch strips, while a richer and more diverse weed community was found outside the mulch strips in NT. Weed community in pepper was mainly composed of annual dicot weeds such as Amaranthus retroflexus, Chenopodium album, Solanum nigrum, Polygonum aviculare which were mostly associated with MT and CT tillage systems, while in NT an increase of perennial species such as Rumex crispus was observed. These results suggest that it is possible to manage cover crop residues in NT in order to obtain a lower weed density and consequently a higher yield in pepper compared to MT and CT while maintaining a high level of weed diversity.     

Yield losses of soybean and maize by competition with interseeded cover crops and weeds in organic-based cropping systems

Weed management is a major issue in organic farming systems. Although interseeding cover crops is one alternative to herbicides, cover crops often suppress not only weeds but also main crops. Therefore, using cover crops for weed control without adverse effects on main crop growth is important. To verify the effect of cover crops on competition between main crops, cover crops and weeds in a snowy-cold region, main crops soybean (Glycine max Merr.) in 2005 and maize (Zea mays L.) in 2006 were grown with cover crops winter rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth), respectively. The cover crops were sown on three sowing dates: before main crop planting (Pre-MC), on the same date of main crop planting (Syn-MC) and after main crop planting (Post-MC). A plot without cover crops (Sole-MC) was used as a control. The dry weight (DW), vegetation cover ratio (VCR), vertical community structure and chlorophyll content were measured to estimate the competition between main crops, cover crops and weeds. Weed DW was suppressed significantly by sowing cover crops in both soybean and maize. This weed suppression was associated with the increase of VCR of main crops plus cover crops at the early growth stage of main crops. Soil seed banks of dominant weed also became lower by sowing cover crops, implying the importance of proper weed management for suppressing weeds successively. In addition, the sowing dates of cover crops had large effects on main crops DW especially in maize, i.e., it was significantly lower in Pre-MC and Syn-MC than in Post-MC. Although the cover crop height was markedly shorter than the main crop height, the chlorophyll content of the main crops was significantly lower when cover crops were sown earlier. These results suggest that the growth inhibition of main crops by cover crops was partly caused by competition for nutrients between main crops and cover crops, and this growth inhibition was alleviated when cover crops were sown after the establishment of main crops. Consequently, soybean yield was the highest in Post-MC and decreased by 29%, 18% and 7% in Sole-MC, Pre-MC and Syn-MC, respectively, and maize yield was also the highest in Post-MC and decreased by 68%, 100% and 24% in Sole-MC, Pre-MC and Syn-MC, respectively. It was concluded that weeds could be controlled effectively by sowing cover crops after planting main crops in organic farming systems in a snowy-cold region.     

The effects of cover cropping on yield and weed control of potato in a transitional system

Cover cropping can have various beneficial effects to the cropping system such us the increase of soil nutrient content and weed suppression. In this respect, the species used for covering is of great importance. This paper reports results on the yield and weed control effects in potato crops preceded by different cover crops over a 2-year period (2003 and 2004) in Central Italy (Viterbo). Results were obtained in the frame of a more complex study set up in 2002 where in a 3-year chick-pea/potato/tomato rotation, each crop was preceded by 7 different soil managements: 5 cover crops (rapeseed, Italian ryegrass, hairy vetch, snail medick and subclover) + 1 unfertilised weedy fallow (cover crop absent) + 1 control (weedy fallow fertilised with mineral N at a rate of 170 kg ha⁻¹ for potato). Two different weed control regimes in potato were also applied [weed-free crop (1 inter-row hoeing + 1 hilling up + manual weeding on the row); mechanical control (1 inter-row hoeing + 1 hilling up)]. Cover crops were sown in September and cut and ploughed just before potato planting in March. The potato crops following the cover crops were only fertilised with green manure. Averaged over years, all the cover crops produced more above-ground dry biomass than the weedy fallow (4.79 t ha⁻¹ on average vs 2.36 t ha⁻¹). Hairy vetch and subclover accumulated the highest N in the incorporated biomass (169 and 147 kg ha⁻¹), followed by snail medick (108), rapeseed (99), ryegrass (88) and weedy fallow (47). Rapeseed and ryegrass were the most efficient weed suppressors and had the least proportion of weed biomass (<1%) of the total produced by the cover, while they also reduced weed emergence in the following potato crops (8.8 plants m⁻²vs 25.5 plants m⁻² with all other cover crops). Following subclover and hairy vetch the potato crop yield was similar to that obtained by mineral N-P-K fertilisation (48.5 t ha⁻¹ of fresh marketable tubers). Mechanical weed control compared to weed free crop always reduced potato yield and the reduction, averaged over years, was greater in N-P-K mineral fertilised control (−23.6%) and smaller in ryegrass (−7.9%).     

Winter cover crops influence Amaranthus palmeri establishment

Winter cover crops were evaluated for their effect on Amaranthus palmeri establishment and growth in cotton production. Cover crops examined included rye and four winter legumes: narrow-leaf lupine, crimson clover, Austrian winter pea, and cahaba vetch. Each legume was evaluated alone and in a mixture with rye. Cover crop biomass in monoculture was greatest for rye and lupine (>6750 kg ha⁻¹), while clover, pea, and vetch were less and ranged from 2810 to 4610 kg ha⁻¹. Cover crop biomass was more than doubled when rye was mixed with clover or vetch relative to the legume monoculture. In early-June, A. palmeri densities were 46 seedlings m⁻² in the non-disturbed areas between cotton rows in the fallow, while populations were <4 seedlings m⁻² with rolled vetch or pea and 18 and 29 seedlings m⁻² in rolled clover and lupine. Rye and legume mixtures reduced A. palmeri densities to <3 seedlings m⁻², while rye monocultures had 8 seedlings m⁻². There were no differences in A. palmeri densities (≥144 plants m⁻²) in the cotton row among cover crop treatments. By late-June, rye and winter pea controlled A. palmeri in the row middle >80% relative to the non-cover crop fallow treatment, while control from clover, vetch and lupine ranged from 64 to 70%. The relationship between A. palmeri control in between cotton rows and cover crop biomass was described by a log-logistic regression model with 4530 kg ha⁻¹ providing median weed control (Bio₅₀); predicted A. palmeri control was 25, 50, and 75% from 2950, 4900, and 8600 kg ha⁻¹ cover crop biomass, respectively. However, A. palmeri plants in the cotton rows prevented yield production in the absence of herbicides. Where A. palmeri was controlled with herbicides, the highest yields occurred following rye, with lower yields following lupin/rye mixture and treatments including pea. Management of herbicide resistant weed species requires diverse management tactics; this may include high-biomass cover crops to reduce weed establishment between crop rows. However, greater research effort is needed to devise weed management options for the crop row that do not rely exclusively on the diminishing array of herbicide tools.     

The suitability of non-legume cover crops for inorganic soil nitrogen immobilisation in the transition period to an organic no-till system

The aim of the study was to evaluate non-legume cover crops for growing no-till grain legumes in organic farming systems. Evaluated cover crops should be able to suppress weed growth, reduce plant available nitrogen in the soil and produce large amounts of biomass with slow N mineralisation. Six non-legume species; spring rye (Secale cereale L.), black oat (Avena sativa L.), sunflower (Helianthus annuus L.), white mustard (Sinapis alba L.), buckwheat (Fagopyrum esculentum Moench) and hemp (Cannabis sativa L.) were tested. Plots with organic fertiliser (50 kg N ha^?1) and without fertiliser incorporation at three locations in south-east Germany were trialled and the cover crops’ ability to produce biomass and accumulate N in plant compartments was evaluated. The N mineralisation from stem and leaf material was simulated using the STICS model. The biomass production ranged from 0.95 to 7.73 Mg ha^?1, with fertiliser increasing the total biomass at locations with low-N status. Sunflower consistently displayed large biomass and N accumulation at all locations and fertiliser variations, although not always significantly more than other species. Most N was stored in sunflower leaf material, which can be easily mineralised making it less suited as cover crop before no-till sown spring grain legumes. Rye, which produced slightly less biomass, but accumulated more N in the stem biomass, would be better suited than sunflower in this type of system. The N mineralisation simulation from rye biomass indicated long N immobilisation periods potentially improving weed suppression within no-till sown legume cash crops.     

Vicia villosa ssp. villosa Roth field emergence model in a semiarid agroecosystem

Hairy vetch (Vicia villosa Roth) is a winter annual legume cultivated for pasture and hay with the capability for natural reseeding. Vicia villosa increases N concentrations in the soil, thus contributing to the sustainability of semiarid regions. However, under rotations of 1–2 years of pasture followed by 1 year of crop (1:1–2:1), hairy vetch could become a problematic volunteer weed in the winter cereal crop phase. This study aimed to develop a mechanistic model for hairy vetch seedling emergence in order to (i) estimate the natural reseeding of hairy vetch in the pasture phase of the field rotation, or (ii) develop control strategies considering hairy vetch as a volunteer weed in the winter cereal phase. The proposed model simulates the pattern of field emergence of hairy vetch after natural seed dispersal by integrating four submodels: (i) physical (PY) dormancy release dynamics, (ii) physiological dormancy (PD) release and germination thermal requirements, (iii) hydro‐time requirements for germination, and (iv) pre‐emergence growth respectively. The developed field emergence model was validated with independent field emergence data during 2013, 2014 and 2015. The model adequately predicted the timing and magnitude of field emergence flushes (RMSE < 10.1) despite the environmental variability among years. The additive effect of each submodel clearly improved the explanatory capacity of the field emergence patterns. The alleviation of PD synchronizes the timing for hairy vetch germination, while the PY determines the seedbank persistence. These outcomes suggest the potential applicability of the proposed modelling approach within management decision support systems.     

Growth and weed suppression ability of common and new cover crops in Germany

Cover crops have a wide-ranging influence on the agroecosystem and create multiple benefits for farmers. A major benefit of cover crops is the suppression of weeds during fall and winter, which can help to reduce soil tillage and herbicide use. However, only a small number of cover crop species are currently grown in Germany. To enlarge this number, four new cover crop species including tartary buckwheat, forage radish, red oat and grain amaranth were tested in comparison with common cover crop species such as white mustard, oilseed radish and phacelia. Emergence, soil coverage, dry matter production and weed suppression ability was assessed for all cover crop species. White mustard emerged faster than all other cover crops and produced the highest amount of shoot dry matter at both locations in southwest Germany twelve weeks after planting (WAP). Oilseed radish was the only cover crop that reduced the weed dry matter in all experiments eight WAP. Phacelia was able to reduce weed density by 77% at Meiereihof twelve WAP. Tartary buckwheat offered the highest soil coverage four WAP, produced the greatest shoot dry matter eight WAP and reduced weed dry matter by more than 96% at Meiereihof and Ihinger Hof twelve WAP. Forage radish produced the highest root dry matter and reduced spring weed density by more than 81% in all experiments. Red oat and grain amaranth emerged slowly, produced less biomass than other cover crops and did not suppress weed growth. The results show that tartary buckwheat and forage radish are well suited as new cover crops in Germany due to their fast growth and good weed suppression ability.     

Competition indices of common vetch and cereal intercrops in two seeding ratio

A 2-year field study was conducted using common vetch (Vicia sativa L.), wheat (Triticum aestivum L.), triticale (xTriticosecale Wittmack), barley (Hordeum vulgare L.), and oat (Avena sativa L.) pure stands as well as two common vetch–cereal mixtures for silage with each of the above four cereals in two seeding ratios (i.e., vetch:cereal 55:45 and 65:35, based on seed numbers) to determine the competition among the different species and the economics of each intercropping system. Intercropping systems were assessed on the basis of several existing intercropping indices such as land equivalent ratio (LER), relative crowding coefficient (RCC or K), aggressivity (A), competitive ratio (CR), actual yield loss (AYL), monetary advantage index (MAI), and intercropping advantage (IA). The LER and K values were greater for the common vetch–wheat (55:45) and the common vetch–oat (65:35) mixtures indicating that in these systems, there was an advantage of intercropping for exploiting the resources of the environment. A similar trend to that of LER and K was also observed for AYL. The aggressivity, CR and partial AYL values were greater for barley and oat than for wheat and triticale, whereas the corresponding values for common vetch were lower in mixtures with barley and oat than in mixtures with wheat and triticale. These findings indicate that barley and oat were more competitive partners than wheat and triticale as common vetch was affected more in mixtures with these two crops. The highest MAI and IA values were recorded for the common vetch–wheat (55:45) and the common vetch–oat (65:35) mixtures indicating that these intercropping systems were the most profitable.     

Interseeding a Cover Crop as a Weed Management Tool is More Compatible with Soybean than with Maize in Organic Farming Systems

Abstract

Weeds are the most serious threat to crop production in organic farming systems. Information on the spatial distribution of weeds is important for effective weed management. The objective of this study was to evaluate the effect of the ground cover of the main crops (soybean and maize) and cover crop on the spatial distribution of weeds for two row sections, ‘within the row (IR)’ and ‘between the rows (BR)’. The cover crop was interseeded in BR 3 – 4 weeks after sowing the main crops in two years, and weed density and vegetation cover ratio (VCR, an index of ground cover) of the main crops and cover crop were measured. There was a significant difference in the spatial distribution of weeds between the two main crops. In IR, weed density was higher in maize than in soybean, while in BR, it was higher in soybean. This means that weed suppression in IR was more important for maize, while the suppression in BR was more important for soybean. The negative relationship between VCR of the main crop plus cover crop and weed density in each row section suggests that the difference in ground cover was one of the reasons for the difference in weed density between the two main crops. The cover crop markedly increased the VCR in BR, but only slightly increased VCR in IR in both main crops. These results suggest that the cover crop was more compatible with soybean than with maize, because the high weed density in IR of maize could not be decreased sufficiently by the slight increase of VCR in BR by the cover crop.     

Colonization of rotation crops and weeds by the potato black dot pathogenColletotrichum coccodes

The rotation crops wheat, barley, oat, maize, soybean, rye, yellow mustard, alfalfa, and spring canola and weeds eastern black nightshade, velvetleaf, timothy grass, orchard grass, and Giant foxtail common to potato-growing areas in North America were used to study the host range ofColletotrichum coccodes, the causal agent of potato black dot. The fungus was isolated from nine of 14 rotation crops and weeds that were inoculated: yellow mustard, soybean, spring canola, alfalfa, oat, eastern black nightshade, velvetleaf, giant foxtail, and timothy grass. In all, colonization was highest in black nightshades (87%) and velvetleaf (80%). Among the rotation crops, colonization was highest on yellow mustard (59%) followed by spring canola (33%) and soybean (30%).Colletotrichum coccodes was not isolated from wheat, barley, rye, maize, or orchard grass. The results indicated that crops used for rotation with potato should be selected carefully to prevent the increase ofC. coccodes inoculum in the soil and that weeds may help maintain viable inoculum ofC. coccodes in the absence of potato. Based on these results we recommend that wheat, barley, maize, or rye be used in rotation with potato in areas whereC. coccodes is present in high levels in the soil.     

Crop rotation and N fertilization effects on growth,yield, and disease incidence in potato

Crop rotation can be an effective mechanism for reducing disease incidence and contributing nitrogen (N) to succeeding crops. Interactions of plant pathogen suppression and soil nutrient availability may also exist, adding to the cropping system complexity. This study examined the impact of crop rotation, N fertilization, and their interaction on growth, yield, andRhizoctonia solani incidence in potato (Solanum tuberosum L. Norwis). Potato was grown continuously and in two-year rotations with annual alfalfa (Medicago sativa L. Nitro), hairy vetch (Vicia villosa Roth), white lupin (Lupinus albus L. Ultra), and oat (Avena sativa Astro). Fertilizer was banded at potato planting with 0, 45, 90, 135, 180, or 225 kg N ha^?1 as (NH₄)₂SO₄. Approximately 58% of continuous potato possessed stem lesions caused byR. solani, but only 12 to 22% of potato stems from other rotations possessed lesions. Tuber dry weight was affected by crop rotation in 1989, a dry year, but not in 1990. Apparent N fertilizer replacement values for hairy vetch, Nitro alfalfa, white lupin, and oat were 65, 43, 26, and 11 kg N ha^?1, respectively. All crop rotations studied appeared to enhance potato production by reducing stem infection byR. solani. Vetch and alfalfa provide additional benefits through their N contributions.     

Relationship between canopy structure and weed biomass in different winter crops

Weed biomass was evaluated 67 days after sowing in five crops (wheat at two densities, rye, lucerne, and rye-lucerne mixtures) and in a sole-weed stand. Canopy structure was described using the inclined point quadrat method.Crop canopy structure and weed responses differed according to the crop considered. A simple exponential model relating crop gap frequency, at half the maximum height of the weeds, with weed biomass provided a good estimate (P<0.001) of the aerial weed biomass in all crops.     

不同作物茬口对冬油菜养分积累和产量的影响

为探究关中平原地区不同作物茬口对冬油菜养分积累和产量的影响,通过两年田间试验,研究分析不同茬口,包括休闲茬口(FW)、大豆茬口(SW)、毛苕子茬口(HW)和玉米茬口(MW)对冬油菜土壤养分含量、地上部干物质积累、分配和氮磷养分吸收积累、产量构成因素以及产量的影响,筛选冬油菜生产适宜的前茬作物,为加强冬油菜生产,提高土地...     

Discrimination of sterile oat (Avena sterilis) in winter barley (Hordeum vulgare) using QuickBird satellite images

Site-specific weed management implies detecting the location of weeds in order to generate maps of their spatial distribution. This information facilitates a more accurate application of herbicides, spraying them in the exact areas of weed growth and in the required doses. In order to explore the potential of commercial satellites to discriminate and map weeds, we used the information contained in high spatial resolution images acquired by the QuickBird satellite to assess the density of sterile oat (Avena sterilis) present in a winter barley field at two different dates (March and June). Our results confirmed the potential of using satellite images in the spectral discrimination of weed patches in infested fields. The results of binary logistic regressions showed that the best matches in the classification of three categories (low, medium, or high sterile oat densities) corresponded to the March image. QuickBird’s March image provided reliable estimates of sterile oat patches in barley crops when weed density was relatively high (between 86% and 94% of agreement between predicted and observed densities). However, when weed densities were lower than 10 plants/m² there were serious difficulties to distinguish them from weed-free zones (between 72 and 75% of global agreement in the classification) with large underestimation of medium density weed patches (10 plants/m²). This is a potential limitation considering than the thresholds used for herbicide application decisions are generally close to this density. However, the information obtained may still be useful for producing field maps to describe the spatial distribution of this weed. Moreover, these studies have provided valuable information on the best spectral regions and/or vegetation indices for approaching discrimination between sterile oat and cereal crops and the most suitable period for it.     

Evaluation of post-emergence herbicides for the control of wild oat (Avena fatua L.) in wheat and barley in Argentina

Wild oat (Avena fatua L.) is the most troublesome weed in cereal crops in Argentina. With the aim of studying the effects of different herbicides, doses, and wild oat growth stage at application on weed control and crop yield, field experiments were conducted in wheat and barley crops during three growing seasons in the south of Buenos Aires Province, Argentina. Treatments were post-emergence applications of new herbicide, pinoxaden + cloquintocet mexyl (5%-1.25%), at doses that ranged from 20 g to 60 g a.i. pinoxaden ha⁻¹, applied at two to three leaves and the beginning of tillering of wild oat. In addition, standard treatments were included and applied at the same wild oat growth stages. Diclofop methyl at 511 g a.i. ha⁻¹ and fenoxaprop-p-ethyl at 55 g a.i. ha⁻¹ were applied in barley. In wheat, diclofop methyl was replaced by clodinafop-propargyl + cloquintocet mexyl (24%-6%) at 36 g a.i. clodinafop-propargyl + 9 g cloquintocet mexyl ha⁻¹ and in 2008/09 wheat experiments, iodosulfuron plus metsulfuron methyl (5%-60%) at 3.75 g a.i. ha⁻¹ + 3 g a.i. ha⁻¹ also was included. In both crops, pinoxaden at 30 g a.i. ha⁻¹ and at higher rates, fenoxaprop-p-ethyl and clodinafop-propargyl gave the best control of wild oat. In 2006/07 wheat crops, treatments applied at tiller initiation provided better control than the early timing averaged across herbicides. However, wheat yield generally was greater with early application. In barley, wild oat control and crop yield were similar regarding time of application. Variations in crop yield were correlated with grain number m⁻² both in wheat and barley, but relationships between both grain number and spikes m⁻² and with grains per spike were identified only in wheat.     

Selectivity of fomesafen based systems for preemergence weed control in cucurbit crops

Experiments at two sites during two years evaluated the selectivity of preemergence fomesafen in cucurbit crops of winter and summer squash, zucchini, cantaloupe, cucumber, and pumpkin. Cucumbers were the most sensitive of the cucurbit crops to fomesafen and produced little or no fruit in two out of three experiments when applied at 0.28 kg ai ha⁻¹. Fomesafen also reduced cantaloupe yield. Visual damage was noted on the other crops tested, but crop yield was not impacted by fomesafen at 0.28–0.35 kg ai ha⁻¹. With the exception of cucumbers, injury caused by fomesafen to cucurbit crops was transitory even when fomesafen-treated soil splashed onto the leaves of emerging cucurbits during a powerful thunderstorm at one of the test sites. Control of redroot pigweed (Amaranthus retroflexus), Powell amaranth (Amaranthus powellii) and other Amaranthus spp., lambsquarters (Chenopodium album), hairy nightshade (Solanum physafolium), common purslane (Portulaca oleraceae), and velvetleaf (Abutilon theophrastii) ranged from 92 to 100% with fomesafen applied at 0.28 kg ai ha⁻¹. The excellent efficacy on these difficult to control weed species suggests that lower rates of fomesafen may be appropriate and improve crop tolerance, particularly if fomesafen is tankmix-applied with other preemergence herbicides such as s-metolachlor or dimethenamid-p. Weed control with these combinations was excellent for all weed species in these experiments.     

Weed emergence as affected by maize (Zea mays L.)-cover crop rotations in contrasting arable soils of Zimbabwe under conservation agriculture

Weed control in smallholder farming systems of sub-Saharan Africa is labour intensive or costly. Many researchers have therefore advocated for the use of cover crops in weed management as an affordable alternative for smallholders. Cover crops may be grown in rotations to suppress weeds and reduce the reliance on herbicides. The use of cover crops creates microenvironments that are either conducive or inhibitive to the emergence of certain weed species. A study, initiated in 2008 in contrasting soils at four different locations of Zimbabwe, investigated the effect of maize (Zea mays L.)-cover crop rotations on the emergence of weeds that showed dominance in those soils. Weed assessments were however, carried out from 2011 to 2014. The weed species Galinsoga parviflora Cav., Commelina benghalensis L., and Richardia scabra L. showed dominance in all four locations with weed densities as high as 500 plants m⁻² being recorded for R. scabra L. in a sandy soil. Maize-cover crop rotations resulted in higher densities of Bidens pilosa compared with maize monocropping (control treatment) due to its high nitrogen (N) requirement to produce more seeds. On the other hand, the integration of cover crops such as pigeon pea [Cajanus cajan (L.) Millsp.] that had poor shading qualities, due to large gaps or spaces and slower initial growth, had limited effects on competitive weeds such as Cyperus esculentus L. which tend to dominate exhausted soils. The density of C. esculentus was 38% greater in maize–pigeon pea rotations compared with the control treatment. Variability between seasons and sites affected emergence of all weeds in the present study, which masked long-term trends. The results suggest that there is need to identify the germination and emergence requirements of specific weeds and select cover crops best suitable for their control. The study provides useful information for farmers and advisors on the best cover crops for control of certain problematic weeds in different soil types of Zimbabwe.