Impact of Different Weed Types on Growth and Yield of Mungbean (Vigna radiata L. Wilczek)

The influence of different weed types (grass, broadleaved and sedge species) on growth and yield parameters of mungbean ( Vigna radiata L. Wilczek) was evaluated under field conditions. The presence of all types of weeks had the most significant effect on growth and yields of mungbean. Broadleaved species alone had a greater adverse effect than grasses, and the influence of sedges was the lowest. The adverse effect of weeds was most prominent on vegetative growth. The influence on yield components reduced with time. However, the adverse impact of weeds on yields was similar to that on vegetative growth. A significant correlation between weed biomass and yield loss was observed. The study indicated the critical competitive period to be in the vegetative phase. Some possible implications for selective weeding are presented.     

Reduced inter-row distance improves yield and competition against weeds in a semi-dwarf durum wheat variety

Weed and nutrient management in cropping systems of semi-arid areas is a major constraint to cereal yield. Where the use of herbicides is banned or discouraged, the competitive ability of a crop is crucial to reduce weed growth and diffusion. Genotypic differences in the competitive abilities of crops are an important trait to reduce weeds, especially for plant height. However, there is contrasting information about the interactions of other management practices and genotypic traits on wheat yield and competitive ability against weeds and weed growth. The present study investigated yield and quality of durum wheat (Triticum durum Desf.) and weed growth and composition for two wheat cultivars with contrasting competitive abilities against weeds. Wheat was grown under three spatial arrangements (5-cm, 15-cm, 25-cm inter-row distance) and three sowing densities, and broadleaf weeds were either removed or not. The sowing rate did not affect the yield of these wheat cultivars or the weed growth. Reduced inter-row distance dramatically reduced weed biomass for both wheat cultivars, and increased wheat yield and nitrogen uptake in the low-competitive, high-yielding, semi-dwarf cv. ‘PR22D89’, when both weed free and with weeds. These results have direct implications for weed and nutrient management in low-input and organic cropping systems.     

Zur Eignung nicht eingesäter Teilflächen in Getreideschlägen für die störungsfreie, bildanalytische Ermittlung von Unkrautschadschwellen

Suitability of not seeded test squares in crop fields for reliable ascertainment of weed threshold levels by quantitative image analysis
The possibility to measure weed ground cover using a computerized image analysis system has been reported in former investigations. In the present studies suitability of not seeded test squares in crop fields for reliable detection of weed ground cover for precise determination of weed threshold levels (beyond that crop yield losses are economically greater than costs for weed control) has been tested.
Test squares in which weed grew without competing crop as well as corresponding squares including both, growing crop and weeds, were examined for weed ground cover by computer assisted quantitative image analysis of color slides taken from the respective squares. In the squares, that includes crop and weeds, the former was removed immediately before taking the slides.
In both seasons from the beginning of weed growth in October to the latest feasible application time of herbicides in April weed growth as measured by ground cover in the not seeded test squares did not differ significantly from that grown under competition of winter barley.
Results indicate, that up to the growth stage of weed where herbicides if ever should be applied at the latest, crop-free test areas in the same field could serve for correct determination of weed growth by image analysis.     

Growth and Development of Six Barley (Hordeum vulgare ssp. vulgare L.) Cultivars in Response to a Model Weed (Sinapis alba L.)

The aim was to study the growth and development of six spring barley (Hordeum vulgare ssp. vulgare L.) cultivars as a response to a model weed population of Sinapis alba L. The development of light interception profiles over time was characterized for each cultivar in weed‐free stands. The cultivars were chosen such that they represent a range of weed‐suppressive abilities based on previously performed trials. One field experiment each was conducted in 1996 and 1997 at a site SE of Uppsala, Sweden. The two cultivars with low competitive ability against weeds, Etna and Blenheim, allowed the highest weed biomass and the lowest crop biomass in competition with the weeds. These two cultivars transmitted more photosynthetic active radiation through the canopy down to 20 and 40 cm height than did the other cultivars. Although the biomass of cv. Etna was low, the grain yield was higher than that of the other cultivars when grown in competition with weeds. In 1997, cv. Svani with good competitive ability against weeds transmitted least light and had greater grain yield than most other cultivars. The absence of a relationship between high grain yield and low weed suppressive ability in the present study indicates that it should be possible for plant breeders to combine high grain yielding capacity with approved weed‐competitive ability.     

麦茬水稻旱种田间杂草综合防除研究初报

在田间条件下,研究了杂草的发生危害规律及防除关键技术等.初步认为:1.杂草对旱种水稻生长的影响主要表现在降低产量,减少成穗数、结实率、千粒重和穗长;2.杂草累计收获总干重或禾本科杂草干重可作为杂草危害程度的指标;3.禾本科杂草危害较严重,是旱种水稻田的优势草类;4.旱种水稻除草临界期为播种后第25～40天.此外,对杂草发生潜力、除草时期等也进行了讨论.     

The influence of organic and conventional fertilisation and crop protection practices,preceding crop,harvest year and weather conditions on yield and quality of potato (Solanum tuberosum) in a long-term management trial

The effects of organic versus conventional crop management practices (fertilisation, crop protection) and preceding crop on potato tuber yield (total, marketable, tuber size grade distribution) and quality (proportion of diseased, green and damaged tubers, tuber macro-nutrient concentrations) parameters were investigated over six years (2004–2009) as part of a long-term factorial field trial in North East England. Inter-year variability (the effects of weather and preceding crop) was observed to have a profound effect on yields and quality parameters, and this variability was greater in organic fertility systems. Total and marketable yields were significantly reduced by the use of both organic crop protection and fertility management. However, the yield gap between organic and conventional fertilisation regimes was greater and more variable than that between crop protection practices. This appears to be attributable mainly to lower and less predictable nitrogen supply in organically fertilised crops. Increased incidence of late blight in organic crop protection systems only occurred when conventional fertilisation was applied. In organically fertilised crops yield was significantly higher following grass/red clover leys than winter wheat, but there was no pre-crop effect in conventionally fertilised crops. The results highlight that nitrogen supply from organic fertilisers rather than inefficient pest and disease control may be the major limiting factor for yields in organic potato production systems.     

Annual Medicago as a Smother Crop in Soybean

Use of conservation tillage and narrow row spacing in soybean [Glycine max (L.) Merr.] production has led to increased use of herbicides for weed control. Some producers are seeking alternative weed control methods, such as smother crops, that would reduce dependence on chemical weed control. A successful smother crop must compete strongly with weeds but minimally with the crop. In four environments, we intercropped three annual Medicago spp. (medics) with soybean to test their utility as a smother crop for weed control. Annual medics were intercropped with soybean at rates of 0, 85, 258, or 775 seeds m^?2, and the intercrops were grown with and without weed control. Increasing medic seeding rate decreased weed yields but also reduced soybean herbage and grain yields. For the weed‐controlled treatment, average soybean grain yields declined 7 kg ha^?1 for every 10 seeds m^?2 increase in medic seeding rate. Soybean grain yield was lower when grown with Medicago scutellata L. cv. Sava than when grown with Medicago polymorpha L. cv. Santiago or Medicago lupilina L. cv. George. Soybean grain yield was negatively related (r=?81) to medic herbage production. In the autumn following soybean harvest, medic residue ranged from 200 to 3700 kg ha^?1 depending on the location and seeding rate. Medics provided residue for soil protection, suppressed weeds, but also reduced soybean yields.     

河套黄灌区麦田杂草绿色防控措施

为解决河套黄灌区麦田杂草绿色防控问题,在河套黄灌区小麦田杂草严重发生的区域,对小麦与杂草竞争关系观测的基础上,试验测定了采用不同小麦品种、种植密度、有机肥、耕翻次数、地膜覆盖方式等绿色控草措施的效果和小麦产量。结果表明,以藜科杂草、籽粒苋和稗草危害为主的麦田,杂草出土滞后于小麦10天以上。杂草主要分布于行（穴）间,株数分别占66.6%（条播）、97.6%（穴播）,且行（穴）中杂草长势较弱,单株鲜重较行（穴）间降低39.3%~41.9%,小麦生长前期生态抑草作用明显;绿色控草措施中除品种和种植密度对控草效果差异不显著外,其余措施控草效果明显。综合比较得知,黑色地膜全覆膜穴播、常规覆膜穴播、增加耕翻次数、无杂草草籽污染有机肥的株数防效分别为82.3%、71.7%、22.0%、8.6%;黑色地膜全覆膜穴播、常规覆膜穴播、增加耕翻次数、无杂草草籽污染有机肥的鲜重防效分别为98.0%、97.1%、23.9%、9.6%;黑色地膜全覆膜穴播、常规覆膜穴播、增加耕翻次数分别增产69.4%、56.4%、21.1%,可实现有机小麦生产机械化绿色控草以及低成本、高产量、规模化生产的目的。     

Incremental crop tolerance to weeds: A measure for selecting competitive ability in Australian wheats

Total reliance on herbicides for weed control is unsustainable with the spread of herbicide resistance and the environmental need to reduce pesticide use. Strongly competitive wheat crops that have high tolerance to weed pressure and therefore maintain high yields in the presence of weeds are a low-cost option for reducing dependence on herbicides. We examined the feasibility of selecting for wheat tolerance to weeds by crossing varieties differing for traits associated with competitiveness. Competitive ability and yield potential must be treated as separate traits for selection. Current measures of crop tolerance to weed competition do not separate the two traits so that selection based on these measures is often synonymous with selection for yield potential rather than pure tolerance. We propose a new measure, termed Incremental Crop Tolerance (ICT) that reflects the incremental yield difference between genotypes associated with tolerance, over and above differences in underlying yield potential.     

Response of Sweet Potato (Ipomoea batatas L.) to Different Planting Times

Agricultural seasons of the tropics are associated with rainfall, which provides the principal limiting resource for crop production. However, as tuber crops are sensitive to temperatures and moisture, the time of planting could have a profound influence on yields. Thus a study was carried out over a period of 12 months to determine the effect of different planting times on establishment, tuber initiation and yields of sweet potato. The trial was planted on similar soils in two agroecological zones as this species is a popular home garden crop in most regions of the tropics and subtropics.
Planting sweet potato with the onset of rains in October produced the highest yields. This is attributed to the receipt of adequate rainfall over the growth cycle, along with the higher diurnal variation in temperatures. Planting in the dry season or later in the wet season, which receives a lower quantity of rainfall with low diurnal variations in temperatures delayed tuber initiation and reduced yields. The study highlights differences in growth patterns of sweet potato when planed at different times. The importance of planting sweet potato in agricultural systems at appropriate times to produce higher yields is presented.     

豫北露地直播棉田杂草的发生及其与棉花的竞争作用

 为了明确棉田自然混生杂草对棉花的危害程度及其关键防除时期,通过田间杂草共生期试验,研究了豫北露地直播棉田杂草的发生及其与棉花的竞争临界期。结果表明,豫北地区露地直播棉田的优势杂草为牛筋草、马齿苋、藜和鳢肠,杂草出土高峰期集中在棉花苗期和花蕾期,且存在3个出土高峰,分别在5月中旬、6月上中旬和7月中下旬。当杂草与棉花竞争持续时间少于4周时,由于杂草与棉花植株均较小,且土壤中养分相对充足,因此杂草对棉花生长和产量的影响不大;当杂草与棉花竞争期达8周以上时,杂草对棉花的竞争作用明显增强,棉花的株数减少,茎秆变细,形成瘦高植株,且单株结铃数、铃重等产量指标明显降低。因此,豫北棉田自然混生杂草群落与棉花的竞争临界期为棉花出苗后4～8周,在此期间应采取相应措施,保证田间无草害。     

湖南省稻-稻-油三熟制油菜田杂草防治研究进展

杂草严重影响油菜的产量和品质,为了减少稻茬田杂草对油菜的危害,本文从种子库、杂草种类和杂草发生特点三方面介绍了湖南省稻-稻-油三熟制油菜田的杂草发生情况以及目前所采取的化学的、物理的、生物的和其它农作措施的杂草防治方法,并对未来油菜田杂草的综合治理进行了探讨和展望。结果表明,杂草种子库密度高达224 550 seed/m2,且三熟制油菜田的杂草主要以看麦娘、日本看麦娘、菵草、硬草、早熟禾等喜湿性的杂草为主,目前采用的防治方法以化学方法为主,其余的措施应用较少,对环境污染严重。因此积极研究推广综合友好型治理体系是湖南省稻-稻-油三熟制杂草的防治最优的选择。     

Sunflower allelopathy for weed control in agriculture systems

Recent developments in weed science and allied aspects have involved several interdisciplinary approaches. In this context, indiscriminate use of herbicides for weed control has become a questionable subject, which besides controlling the weeds, the chemical herbicides are harmful in many ways to soil, crops, other plants and the environment as a whole. Taking into consideration ecologically sound weed management, in modern days the reliance on chemical herbicides has decreased and a shift towards naturally occurring biological herbicides has received great attention throughout the world. Sunflower is an annual dicotyledonous plant, herbaceous, erect, and a native of North America. It is thermo and photo-insensitive, hence it can be grown year round in sub-tropical and tropical countries. Only two spp. Helianthus annuus L. and Helianthus tuberosum are cultivated for food, the remaining spp., are ornamentals weeds and wild plants. However, H. annuus is allelopathic and inhibits the growth and development of other plants thus reducing their productivity. Sunflower is a major oil-yielding crop in India and its cultivation in northwest India started 25 to 30 years ago in areas located in the plains. In this region, rice-wheat rotation became very popular owing to its high yields; however, these crops are highly infested by weeds, thus farmers use herbicides for their control. Hence, this rotation consumes a maximum quantity of herbicides in this region, which has resulted in several problems viz., environmental pollution, human health hazards, and development of herbicide resistance in weeds. Thus, serious ecological questions about the reliance on herbicides for weed control in this rotation have been raised. One of the alternatives to overcome these problems is with the use of allelopathic strategies, including the use of weed-smothering crops for weed management and for the sustainability of agriculture. The field, pot culture, and laboratory studies have shown that inclusion of sunflower crops in rotation and intercropping considerably reduced the weed population in the current and succeeding crops. Rhizosphere soil of sunflower drastically smothered the weed germination, population, and biomass. The residual suppression effect of sunflower also persisted in the next crop up to 75 days. Thus, it is conceptualized that the inclusion of such oilseed crops before the rice crop in the rice-wheat rotation may provide satisfactory weed control in the succeeding rice crops and may minimize the use of herbicides. Likewise, the replacement of sorghum by summer sunflower oilseed crops may also help in the control of summer as well as winter weeds. More studies in this direction may provide avenues for satisfactory weed management in agro-ecosystems and may help to minimize the use of herbicides and thereby pave the way to develop sustainable agricultural practices for biodiversity conservation and enhancing biological integrity.     

冀中南谷田杂草发生与除草剂筛选试验

以冀中南谷田杂草为对象,探讨了谷田杂草的发生规律与除草剂筛选试验。结果表明:不同播期的谷田杂草种类基本一致。春播谷田杂草发生有3个峰期,5月中旬以双子叶杂草为主,6~7月份以单子叶杂草为主;夏播谷田杂草发生仅1个峰期,主要在7月上中旬,以单子叶杂草为主。化学防除以44%谷友WP剂量1800g/hm²为最佳,药效明显高于10%单嘧磺隆WP、50%扑灭津WP与2,4-D丁酯EC;春播谷田施药后79d对单、双子叶杂草的株数防效与鲜重抑制率分别为49.19%、96.76%与96.01%、96.03%。夏播谷田在施药后55d对单、双子叶杂草的株数防效与鲜重抑制率分别为41.13%、92.34%与93.39%、98.15%。     

Legume cover crops as living mulches for winter wheat: Components of biomass and the control of weeds

To gain information about the possible use of legume cover crops as an alternative and sustainable weed-control strategy for winter wheat (Triticum aestivum L.), an experiment was conducted at two sites in the Swiss Midlands in 2001/2002. Under organic farming conditions winter wheat was direct-drilled into living mulches established with four different legume genotypes or into control plots without cover crops. Compared to NAT (control plots without cover crops but with a naturally establishing weed community), white clover (Trifolium repens L.), subclover (Trifolium subterraneum L.), and birdsfoot trefoil (Lotus corniculatus L.) reduced the density of monocotyledonous, dicotyledonous, spring-germinating, and annual weeds by the time of wheat anthesis. Strong-spined medick (Medicago truncatula Gaertner) was less efficient in this regard. While the grain yield was reduced by 60% or more for all legumes when compared to NOWEED (control plots kept weed-free), a significant negative correlation between the dry matter of the cover crop and weeds as well as between the cover crop and the winter wheat was observed by the time of wheat anthesis. The effect of manuring (60 m³ ha⁻¹ liquid farmyard manure) was marginal for weeds and cover crops but the additional nutrients significantly increased total winter wheat dry matter and grain yields. The suppression achieved by some legumes clearly demonstrates their potential for the control of weeds in such cropping systems. However, before living legume cover crops can be considered a viable alternative for integrated weed management under organic farming conditions, management strategies need to be identified which maximise the positive effect in terms of weed control at the same time as they minimise the negative impact on growth and yield of winter wheat.     

几种除草剂对当茬玉米生长发育的影响

除草剂为作物产量提供保障的同时,对作物造成的药害也频繁发生。为此,本文以目前陇东旱塬区主栽玉米品种为试验材料,选用对恶性杂草具有良好防除效果的除草剂,明确了75%二氯吡啶酸SG、90%莠去津WG和56% 2甲4氯钠盐SP 3个单剂及其2个混配配方对玉米主要农艺性状及产量的影响,以期为玉米的安全生产提供依据。结果表明：参试除草剂在药后0~7天及不同生育期对玉米生长均无直观影响。75%二氯吡啶酸SG、90%莠去津WG、75%二氯吡啶酸SG+90%莠去津WG对苗期玉米株高稍有影响,但随着玉米自身防御能力的增强,除草剂对之后各生育期的玉米株高无显著抑制作用。不同除草剂对各生育期玉米茎粗无抑制作用,玉米果穗粗、果穗有效长、穗粒数和百粒重无显著差异。各除草剂处理后玉米产量较空白对照增加21.93%~30.22%,较人工除草对照增加-2.91%~3.69%。可见,供试除草剂对玉米安全性高,可应用于玉米田的杂草防除。     

Tillage, cover crops, and nitrogen fertilization effects on soil nitrogen and cotton and sorghum yields

Sustainable soil and crop management practices that reduce soil erosion and nitrogen (N) leaching, conserve soil organic matter, and optimize cotton and sorghum yields still remain a challenge. We examined the influence of three tillage practices (no-till, strip till and chisel till), four cover crops {legume [hairy vetch (Vicia villosa Roth)], nonlegume [rye (Secaele cereale L.)], vetch/rye biculture and winter weeds or no cover crop}, and three N fertilization rates (0, 60–65 and 120–130 kg N ha⁻¹) on soil inorganic N content at the 0–30 cm depth and yields and N uptake of cotton (Gossypium hirsutum L.) and sorghum [Sorghum bicolor (L.) Moench]. A field experiment was conducted on Dothan sandy loam (fine-loamy, siliceous, thermic, Plinthic Paleudults) from 1999 to 2002 in Georgia, USA. Nitrogen supplied by cover crops was greater with vetch and vetch/rye biculture than with rye and weeds. Soil inorganic N at the 0–10 and 10–30 cm depths increased with increasing N rate and were greater with vetch than with rye and weeds in April 2000 and 2002. Inorganic N at 0–10 cm was also greater with vetch than with rye in no-till, greater with vetch/rye than with rye and weeds in strip till, and greater with vetch than with rye and weeds in chisel till. In 2000, cotton lint yield and N uptake were greater in no-till with rye or 60 kg N ha⁻¹ than in other treatments, but biomass (stems + leaves) yield and N uptake were greater with vetch and vetch/rye than with rye or weeds, and greater with 60 and 120 than with 0 kg N ha⁻¹. In 2001, sorghum grain yield, biomass yield, and N uptake were greater in strip till and chisel till than in no-till, and greater in vetch and vetch/rye with or without N than in rye and weeds with 0 or 65 kg N ha⁻¹. In 2002, cotton lint yield and N uptake were greater in chisel till, rye and weeds with 0 or 60 kg N ha⁻¹ than in other treatments, but biomass N uptake was greater in vetch/rye with 60 kg N ha⁻¹ than in rye and weeds with 0 or 60 kg N ha⁻¹. Increased N supplied by hairy vetch or 120–130 kg N ha⁻¹ increased soil N availability, sorghum grain yield, cotton and sorghum biomass yields, and N uptake but decreased cotton lint yield and lint N uptake compared with rye, weeds or 0 kg N ha⁻¹. Cotton and sorghum yields and N uptake can be optimized and potentials for soil erosion and N leaching can be reduced by using conservation tillage, such as no-till or strip till, with vetch/rye biculture cover crop and 60–65 kg N ha⁻¹. The results can be applied in regions where cover crops can be grown in the winter to reduce soil erosion and N leaching and where tillage intensity and N fertilization rates can be minimized to reduce the costs of energy requirement for tillage and N fertilization while optimizing crop production.     

Allelopathic Potential of Shoot and Root Leachates of Certain Weed Species

The allelopathic potential of shoot and root leachates of seven weed species was assessed using sorghum and blackgram as test crops. Weeds differed significantly among themselves in their allelopathic effect. Shoot leachates of weeds, in general, showed more allelopathic effect than their root leachates. Phenolic content of weed leachates has positive association with the allelopathic potential of the weed species.     

Assessing innovative sowing patterns for integrated weed management with a 3D crop:weed competition model

Weed dynamics models are needed to design innovative weed management strategies. Here, we developed a 3D individual-based model called FlorSys predicting growth and development of annual weeds and crops as a function of daily weather and cropping practices: (1) crop emergence is driven by temperature, and emerged plants are placed onto the 3D field map, depending on sowing pattern, density, and emergence rate; plants are described as cylinders with their leaf area distributed according to height; (2) weed emergence is predicted by an existing submodel, emerged weed seedlings are placed randomly; (3) plant phenology depends on temperature; (4) a previously developed submodel predicts available light in each voxel of the canopy; after emergence, plant growth is driven by temperature; when shaded, biomass accumulation results from the difference between photosynthesis and respiration; shading causes etiolation; (5) frost reduces biomass and destroys plants, (6) at plant maturity, the newly produced seeds are added to the soil seed bank. The model was used to test different sowing scenarios in an oilseed rape/winter wheat/winter barley rotation with sixteen weed annuals, showing that (1) crop yield loss was negatively correlated to weed biomass averaged over the cropping season; (2) weed biomass was decreased by scenarios allowing early and homogenous crop canopy closure (e.g. reduced interrows, increased sowing density, associated or undersown crops), increased summer fatal weed seed germination (e.g. delayed sowing) or, to a lesser degree, cleaner fields at cash crop sowing (e.g. sowing a temporary cover crop for “catching” nitrogen); (3) the scenario effect depended on weed species (e.g. climbing species were little affected by increased crop competition), and the result thus varied with the initial weed community (e.g. communities dominated by small weed species were hindered by the faster emergence of broadcast-sown crops whereas taller species profited by the more frequent gap canopies); (4) the effect on weed biomass of sowing scenarios applied to one year was still visible up to ten years later, and the beneficial effect during the test year could be followed by detrimental effects later (e.g. the changed tillage dates accompanying catch crops reduced weed emergence in the immediately following cash crop but increased seed survival and thus infestation of the subsequent crops). This simulation showed FlorSys to predict realistic potential crop yields, and the simulated impact of crop scenarios was consistent with literature reports.     

Relay-intercropped forage legumes help to control weeds in organic grain production

In organic grain production, weeds are one of the major limiting factors along with crop nitrogen deficiency. Relay intercropping of forage legume cover crops in an established winter cereal crop might be a viable option but is still not well documented, especially under organic conditions.Four species of forage legumes (Medicago lupulina, Medicago sativa, Trifolium pratense and Trifolium repens) were undersown in six organic wheat fields. The density and aerial dry matter of wheat, relay-intercropped legumes and weeds were monitored during wheat-legume relay intercropping and after wheat harvest until late autumn, before the ploughing of cover crops.Our results showed a large diversity of aerial growth of weeds depending on soil, climate and wheat development. The dynamics of the legume cover crops were highly different between species and cropping periods (during relay intercropping and after wheat harvest). For instance, T. repens was two times less developed than the other species during relay intercropping while obtaining the highest aerial dry matter in late autumn. During the relay intercropping period, forage legume cover crops were only efficient in controlling weed density in comparison with wheat sole crop. The control of the aerial dry matter of weeds at the end of the relay intercropping period was better explained considering both legumes and wheat biomasses instead of legumes alone. In late autumn, 24 weeks after wheat harvest, weed biomass was largely reduced by the cover crops. Weed density and biomass reductions were correlated with cover crop biomass at wheat harvest and in late autumn. The presence of a cover crop also exhibited another positive effect by decreasing the density of spring-germinating annual weeds during the relay intercropping period.