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.     

Can winter cover crops influence weed density and diversity in a reduced tillage vegetable system?

Weeds are a major constraint for organic crop production. Previous research has found that cover crops in reduced tillage systems can provide weed interference, subsequently reducing inputs and improving crop yield. However, questions remain about effects of cover crop species identity and cover crop biomass on weed suppression and crop yield. This four-year study investigated how winter cover crops grown alone or in mixture influenced weed presence and crop yield in a reduced tillage organic vegetable system. Treatments were barley (Hordeum vulgare L.), crimson clover (Trifolium incarnatum L.), mixed barley + crimson clover, and a no-cover crop control. Plots were flail-mowed and strip-tilled prior to planting main crops (2011 and 2012: broccoli Brassica oleracea L.; 2013 and 2014: crookneck squash Cucurbita pepo L.). We measured density, diversity, and community composition of weeds and viable weed seeds, changes in weed percent cover within growing seasons, and crop yield. We found that the presence of barley, crimson clover, or barley + crimson clover reduced weed density by 50% relative to the control. Cover crop biomass negatively influenced weed density and weed seed diversity, and positively influenced squash yield. Weed percent cover within growing seasons did not respond differentially to cover crop treatment. Cover crop treatment and cover crop biomass had no influence on weed or weed seed community composition. These results suggest that reduced tillage winter cover crops in mixture or monoculture can similarly suppress weeds and improve yield, primarily due to biomass effects.     

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.     

Agronomic performance of no-tillage relay intercropping with maize under smallholder conditions in Central Brazil

In the context of conservation agriculture on small scale farms of the Brazilian Cerrado, we hypothesized that planting a cover crop in relay with a commercial crop improves the efficiency of use of available natural resources, increasing biomass for use as fodder without reducing the grain yield of the main crop. The objective of this study was to measure the performance of two intercropped systems in terms of total above-ground biomass production and maize (Zea mays) grain yield: pigeon pea (Cajanus cajan) and Brachiaria (Brachiaria ruziziensis) sown as cover crops in established maize under a no-tillage management. The cover crops were sown at two different dates and a comparison was made with the three crops sown as a sole crop at the early sowing date. The experiment was conducted during the 2007-2008 and 2008-2009 growing seasons. Maize grain yield was not reduced by the presence of the relay cover crops in comparison with maize as the sole crop, even when the cover crop was sown soon after maize emergence. In contrast, the production of above-ground biomass by the cover crop was significantly lower when grown with maize than it was when grown as a sole crop. In the intercropped systems, when sown early, the cover crop produced higher total biomass than when sown late. Total above-ground biomass production of maize intercropped with a cover crop was much higher than that of any of the crops sown alone: the total biomass (average of the two growing seasons) produced by maize and pigeon pea was more than double that of maize grown alone. The land equivalent ratio (LER) of maize grain yield and biomass production was higher than one whatever the intercropped system used. It was particularly high when maize was intercropped with early sown pigeon pea; grain yield LER and biomass LER reaching, respectively, 1.72 and 1.73 in 2007-2008 and 2.02 and 2.03 in 2008-2009. These high LER values provide evidence for the complementary and the high efficiency of use of available resources by the intercropped plants and thus the advantage of such systems to produce both maize grain and cover crop forage under the conditions of our study.     

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.     

Weed management in maize using crop competition: A review

Weeds are a major constraint to crop production, and are responsible for considerable yield losses in maize production systems throughout the world. Herbicides are widely used for weed control in maize production systems, but can have negative environmental consequences. Researchers have evaluated the use of crop competition and suppression to manage weeds in various crop combinations, including maize-based systems. Crop competition in maize may involve techniques such as reduced row spacing, increased planting density, and the use of competitive cultivars that exhibit weed suppressive potential. In this review, examination of the literature has revealed the considerable value of using crop competition in integrated weed management programs. Research has demonstrated that narrowing row spacing to half the standard distance reduced weed biomass by 39–68%, depending on weed species. Researchers have also demonstrated that increasing maize planting density by up to twice the standard rate achieved a reduction in weed biomass of 26–99%. While little research has been conducted into the use of competitive maize cultivars for weed management, several studies have documented cultivars with potential to suppress weeds. Attributes of weed competitive cultivars include high leaf area index, and other elements of leaf architecture that improve light interception by the crop, so increasing the shading of weeds. Combining crop competition methods with other agronomic practices can increase their effectiveness in controlling weeds. For example, biomass of Setaria italica (L.) Beauv was reduced by 60% when maize planting density was increased by 1.5 times the recommended spacing, and this effect was more pronounced when fertilizer was banded rather than broadcast. In summary, the strategic use of crop competition to control weeds has been a success in many regions, and is an important tool in integrated weed management. The importance of crop competition methods has particular relevance where farmers are unable to afford herbicides, as making use of crop competition is more economical.     

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.     

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.     

Sustainable weed management in conservation agriculture

Sustainable crop production is necessary to ensure global food security and environmental safety. Conservation agriculture (CA) is gaining popularity around the globe due to its sustainable approaches such as permanent soil cover, minimal soil disturbance, planned crop rotations and integrated weed management. Weed control is the biggest challenge to CA adoption. Weed ecology and management is different in CA than in conventional agriculture. In CA, weeds expression, seed bank status, distribution, dispersal mechanisms, diversification, growing patterns and competition trends are complex and differ from conventional systems. It is due to reduced tillage of the soil and the flora that thrives in CA. Reduced tillage systems affect the efficacy of herbicides and mechanical weed control measures. So, it is an important task to find out the differences and to fabricate new management options. In this review, changing weed dynamics have been framed. A novel aspect of this review is the comprehensive account of sustainable weed management strategies in relation to CA. Modified tillage operations, improved cultural practices, bioherbicides, chemical herbicides, allelopathy, and crop nutrition have been identified as suitable weed management tools. None of these offers complete control but the integration of these tools in suitable combinations works efficiently. Weeds dominating CA and their responses to CA components are highlighted. For example, small seeded and perennial weeds are more abundant in CA. The role of herbicide resistance in weeds and herbicide tolerant (HT) crops in CA is also highlighted. Allelopathy and crop nutrition are discussed as modern weed management tools for CA. A detailed account of weed responses to fertilizer management options is also given. Integrated weed management compatible to cropping patterns and climatic conditions offers the best results in CA. Future efforts must be directed towards the optimization and integration of these weed management practices.     

The competitive ability of pea-barley intercrops against weeds and the interactions with crop productivity and soil N availability

Grain legumes, such as peas (Pisum sativum L.), are known to be weak competitors against weeds when grown as the sole crop. In this study, the weed-suppression effect of pea-barley (Hordeum vulgare L.) intercropping compared to the respective sole crops was examined in organic field experiments across Western Europe (i.e., Denmark, the United Kingdom, France, Germany and Italy). Spring pea (P) and barley (B) were sown either as the sole crop, at the recommended plant density (P100 and B100, respectively), or in replacement (P50B50) or additive (P100B50) intercropping designs for three seasons (2003-2005). The weed biomass was three times higher under the pea sole crops than under both the intercrops and barley sole crops at maturity. The inclusion of joint experiments in several countries and various growing conditions showed that intercrops maintain a highly asymmetric competition over weeds, regardless of the particular weed infestation (species and productivity), the crop biomass or the soil nitrogen availability. The intercropping weed suppression was highly resilient, whereas the weed suppression in pea sole crops was lower and more variable. The pea-barley intercrops exhibited high levels of weed suppression, even with a low percentage of barley in the total biomass. Despite a reduced leaf area in the case of a low soil N availability, the barley sole crops and intercrops displayed high weed suppression, probably because of their strong competitive capability to absorb soil N. Higher soil N availabilities entailed increased leaf areas and competitive ability for light, which contributed to the overall competitive ability against weeds for all of the treatments. The contribution of the weeds in the total dry matter and soil N acquisition was higher in the pea sole crop than in the other treatments, in spite of the higher leaf areas in the pea crops.     

Weed Allelopathy,Its Ecological Impacts and Future Prospects

Summary

The importance, characteristics, positive and negative impacts, and future role of weeds as an integral part of the natural and agroecosystems are evaluated and discussed. Interference between plants in nature and the importance of differentiating between competition and allelopathy are interpreted. Allelopathy as one component of weed/crop interference, allelochemicals from weed species and their possible mechanism of action are listed and discussed. Weed species with inhibitory action against cultivated crops, other weed species, and plant pathogens, as well as self-inhibitory (autopathic) species are reviewed. Stimulatory or inhibitory allelopathic effects of different crop plants, trapping and catching species, and the potential of allelopathic weeds in inhibiting or stimulating certain parasitic weed species are discussed and evaluated. Allelopathy as a mechanism and future strategy for agricultural pest control and farm management and the potential use and development of some allelochemicals as natural pesticides or plant growth regulators are also considered and discussed.     

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.     

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.     

Comparing weed and crop seedling response to pre-emergence pendimethalin application in Corchorus olitorius and Abelmoschus esculentus

Field experiments and bioassay tests were carried out to evaluate the relative response of the crops, tossa jute (Corchorus olitorius L.) and okra [Abelmoschus esculentus (L.) Moench], and their associated weeds to pre-emergence application of pendimethalin at 0.33, 0.66, 0.99, 1.32 kg ai ha⁻¹ and a pendimethalin+atrazine tank mixture at 1.32+2.05 kg ai ha⁻¹. Bioassay tests were carried out using herbicide solutions of corresponding concentrations and herbicide-treated soil. Field application of pendimethalin at 0.66 kg ai ha⁻¹ and the herbicide mixture in both crops effectively controlled most seedling weeds including Rottboellia cochinchinensis. Euphorbia heterophylla and Calopogonium mucunoides, which persisted from 2 weeks after treatment (WAT) were not controlled. Herbicide application during crop establishment markedly inhibited the growth of both seedling weeds and crops. The mixture caused the highest weed and crop injury. Pendimethalin at 0.33 kg ai ha⁻¹ had minimal effect on these crops. Weed growth, weed tolerance of herbicide treatment and crop seedling injury were higher in tossa jute than that in okra, under the conditions of this study. The use of low pendimethalin doses in an integrated weed management system will ensure effective control of seedling weeds, and prevent crop injury and residue accumulation in edible plant produce.     

覆盖作物对坡耕地的减流减沙效应及玉米产量的影响

在2020-2021年进行两年田间径流试验场玉米大田试验，在玉米进入拔节期时播种覆盖作物，共设置3个播期处理，拔节期播种覆盖作物(T1)、播种覆盖作物延后10 d(T2)、播种覆盖作物延后20 d(T3)，不覆盖作物处理作为对照(CK)，研究不同覆盖作物播期对径流量、侵蚀量及玉米产量和构成因素的影响，分析不同覆盖作物处理对坡耕地的减流减沙效应，为吉林省东部种植玉米的坡耕地区提供覆盖作物的最适播期，结果表明，黑麦草和红三叶能够有效减少水土流失，T1处理的地表径流量较其他两组覆盖处理至少下降18.8%、土壤侵蚀量下降8.8%。覆盖作物处理在中强度降雨下的保水保土作用更为显著，黑麦草和红三叶覆盖对玉米有明显的增产作用，其中T1效果最好。播种覆盖作物对于坡耕地的减流减沙作用、玉米生长发育和玉米增产最为显著。     

Predicting maize yield in a multiple species competition with Xanthium strumarium and Amaranthus retroflexus: Comparing of approaches to modeling herbicide performance

A modeling approach based on weed relative leaf area (RLA) was used to describe the maize yield affected by weed competition and herbicide dose. The change of early RLA with herbicide dose was described using standard dose–response model. The dose response of individual weed species was included in a multivariate rectangular hyperbolic relationship between maize yield and weed relative leaf-area. Final model satisfactorily described the change of the weed relative leaf areas with herbicide dose in a multiple species competition with maize to reach a reasonable estimate of crop yield. Parameter estimates indicated that the maize yield could be about 11.23 t/ha in the absence of weeds. Applying herbicide at 0.29 rate of the recommended dose could decrease the Amaranthus retroflexus leaf area by 50%. For Xanthium strumarium, increasing the dose up to 0.58 rate of the recommended dose caused 50% reduction in early relative leaf area. The relative leaf area of the X. strumarium was 4.2-fold larger than that of the A. retroflexus at the time of herbicide application. Model validation indicated significantly better predictive ability of the leaf area model than that of the density model. Leaf area model permits to monitor the canopy (with considering the relative ground cover of neighboring plants) and decide that if a competitive crop allows reducing the dose. Recommending the precise required amount of herbicide is achieved using models that account for the interactions among crop and weed species, which survive and persist in the competition after herbicide application.     

Crotalaria (C. ochroleuca G. Don.) as a green manure in maize–bean cropping systems in Uganda

Crotalaria (C. ochroleuca G. Don.) used as a green manure may improve the productivity of maize–bean cropping systems in eastern Africa. To test this hypothesis, three field studies were conducted over three consecutive cropping seasons at Kawanda Agricultural Research Institute in Uganda. In the first season, crotalaria biomass was produced in pure stands or in intercrop systems with either common beans (Phaseolus vulgaris L.) or maize (Zea mays L.) as companion crops. Crotalaria was sown at planting and three weeks after planting the food crops. The biomass of early planted crotalaria was mulched, that of late planted crotalaria was incorporated into the soil at planting of the following crop. The first subsequent crop was maize, and the second was either beans (in two seasons) or maize (in one season). In the crotalaria production season, mean yield losses of maize through intercropping with early or late sown crotalaria were 40 and 22%, respectively; the corresponding values for beans were 45 and 14%. In the first cropping season after crotalaria production, the increase in maize grain yield on account of crotalaria averaged 39%; the best response (68% increase) to crotalaria was obtained with early sown sole crotalaria applied as mulch. Major differences in soil mineral nitrogen content among the treatments occurred at the six-leaf stage of maize only. In the second cropping season following crotalaria production, the mean increases in seed yields of beans or maize, due to crotalaria, were 23 and 19%, respectively, indicating a considerable residual effect of crotalaria green manure. The decrease in bulk density, and the increase in water infiltration capacity of the soil suggest that the yield stimulation because of crotalaria not only resulted from the increased nitrogen supply, but also from more favorable soil physical properties. Considering the competitive effect of crotalaria with the food crops and the positive effect on yields of subsequent crops, two options are especially promising: incorporation of crotalaria produced by relay intercropping with beans and mulching of early sown crotalaria produced in pure stands.     

Relations between summer crops and ground cover legumes in a subtropical environment 1. Effects of a Vigna trilobata ground cover on growth and yield of sorghum and sunflower

An annual form of Vigna trilobata (L.) Verdc., a semi-domesticated herbaceous legume, was used to provide additional ground cover under rainfed crops in a subtropical environment. This study reports effects of V. trilobata on growth and yield of sorghum (Sorghum bicolor (L.) Moench) and sunflower (Helianthus annuus L.). Crops were sown on a vertisol in south-east Queensland in October and January at densities of 10 and 5 plants m⁻² for sorghum and 5 and 2.5 plants m⁻² for sunflower, and with or without ground cover legume. Crop rows were 0.8 m apart. V. trilobata seed was broadcast at 150 seeds m⁻² when the crops were sown.While a full profile of soil water at sowing, and 330 to 424 mm rainfall during crop growth, led to vigorous growth of both crops and ground cover, crops from both sowings experienced water stress from about anthesis to the middle of grain fill. At the higher crop density, yields of sorghum grain were 453 and 355 g m⁻² and yields of sunflower seed were 161 and 227 g m⁻² after sowing in October and January, respectively. Yields were decreased to 87%, 70%, 76% and 90% respectively in the presence of up to 330 g m⁻² of dry matter of V. trilobata. Sorghum yields were marginally lower at the lower density and decreased relatively more in the presence of ground cover. Neither the yield of sunflower seed, nor the effect of ground cover on it, was affected by density.Competition from ground cover decreased radiation interception by crops through decreasing their leaf area index. It did not appear to affect the efficiency of conversion of intercepted radiation to dry matter. Tiller number in sorghum was decreased by ground cover, except in the low density stand sown in October. Effects of ground cover on the contribution of tiller heads to yield were more severe, leading to a halving of their contribution at the lower density. Lower grain yields in the presence of ground cover were due to lower numbers of seeds m⁻² in both crops. Seed size was also smaller in sunflower, but the percentage of oil was not affected.We conclude that under well-watered conditions in south-east Queensland the decreases in yields of summer crops in competition with V. trilobata are likely to be outweighed by the anticipated long-term benefits from using ground cover to decrease soil erosion. The need for information on the processes and consequences of competition under drier conditions is emphasized, so that we can evaluate the broader potential for ground cover legumes in the subhumid subtropics.     

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%).     

The implications of land preparation, crop establishment method and weed management on rice yield variation in the rice-wheat system in the Indo-Gangetic plains

The implications of adopting alternative seeding methods for rice and wheat establishment were examined at three geographically separate sites in the rice-wheat system of the Indo-Gangetic plains, across northern India. Rice yields in cultivated plots, established by either wet or dry seeding methods, were evaluated in comparison to yields from zero-tillage plots and under conventional transplanting methods. In the same trials, the effects of crop establishment methods in wheat were assessed both on wheat yields and rice yields. Rice crop establishment methods markedly influenced the emerging weed flora and attainable yields were measured in relation to intensity of weed management. Over four years, average rice grain yields in the absence of weed competition were greatest (6.56 t ha⁻¹) under wet seeding (sowing pre-germinated rice seed on puddled soil), and similar to those from transplanted rice (6.17 t ha⁻¹) into puddled soil, and dry seeded rice after dry soil tillage (6.15 t ha⁻¹). Lowest yields were observed from dry seeded rice sown without tillage (5.44 t ha⁻¹). Rice yield losses due to uncontrolled weed growth were least in transplanted rice (12%) but otherwise large (c. 85%) where rice had been sown to dry cultivated fields or to puddled soil, rising to 98% in dry seeded rice sown without soil tillage. Weed competition reduced multiple rice yield components, and weed biomass in wet seeded rice was six-fold greater that in rice transplanted into puddled soil and twice as much again in dry seeded rice sown either after dry tillage or without tillage. Wheat grain yields were significantly higher from crops sown into tilled soil (3.89 t ha⁻¹) than those sown without tillage (3.51 t ha⁻¹), and also were elevated (5% on average) where the soil had been dry cultivated in preparation for the previous rice crops rather than puddled. The method of wheat cultivation did not influence rice yield. Soil infiltration rates in the wheat season were least where the land had been puddled for rice (1.52 mm h⁻¹), and greater where the soil had been dry-tilled (2.63 mm h⁻¹) and greatest after zero-tillage (3.54 mm h⁻¹).These studies demonstrated at research managed sites across a wide geographic area, and on farmers’ fields, that yields of dry seeded rice sown after dry cultivation of soil were broadly comparable with those of transplanted rice, providing weed competition was absent. These results support the proposition that direct seeding of rice could provide an alternative to the conventional practice of transplanting, and help address rising costs and threats to sustainability in the rice-wheat rotation. Further, analysis of patterns of long-term rainfall data indicated that farmers reliant on monsoon rainfall could prepare fields for dry direct seeded rice some 30 days before they could prepare fields for either transplanting or seeding with pre-germinated seed. Dry, direct seeding of rice contributes a valuable component of an adaptive strategy to address monsoonal variability that also may advance the time of wheat establishment and yield. Whilst the results illustrate the robustness, feasibility and significant potential of direct seeded rice, they also highlight the critical nature of effective weed control in successful implementation of direct seeding systems for rice.