Increasing Crop Density Research Articles

Response of Yield Formation of Maize Hybrids to Different Planting Densities

Crop density is a means that controls grain yield establishment. This study was focused on evaluating the effect of four crop densities (CD1—50,125 plants ha−1, CD2—59,523 plants ha−1, CD3—69,686 plants ha−1, and CD4—79,365 plants ha−1) on yield components, grain yield, sustainable yield index (SYI), and rain use efficiency (RUE) of the maize hybrids ZP 500, NS 5010, and AS 534 during 2016–2018. In 2017, due to unfavorable meteorological conditions, ear length, number of grains per ear, grain weight per ear, 1000-grain weight, grain yield, SYI, and RUE were low compared to 2016 and 2018. The hybrid NS 5010 had the lowest ear length, number of grains per ear, grain weight per ear, 1000-grain weight, grain yield, and SYI. Increasing crop density significantly decreased yield components and increased grain yield, SYI, and RUE. The lowest ear length was recorded in treatments CD3 and CD4, the lowest number of grains per ear, grain weight per ear, and 1000-grain weight in treatment CD4. However, the highest grain yield, SYI, and RUE were recorded in the CD4 treatment because the number of plants per unit area is an essential determinant of the final grain yield.

Improving upon the interrow hoed cereal system: the effects of crop density and row spacing on intrarow weeds and crop parameters in spring barley

AbstractAutomated guidance systems have advanced precise interrow hoeing in narrowly spaced cereals. Compared with other direct mechanical strategies, hoeing provides superior weed control and improved yields. However, weeds in the uncultivated intrarow zone may survive and compete intensely with the crop, causing yield loss. Therefore, improved intrarow weed management strategies in hoed cereals must be investigated. In spring barley (Hordeum vulgare L.), the effect of crop density was assessed at four levels (200, 300, 400, and 500 plants m−2); interrow spacing at two levels (15 and 20 cm), relevant to the abilities of current automated equipment to hoe between narrowly spaced rows; and weed management treatment at three levels (no additional controls, herbicide, and preemergence tine harrowing). All treatments received interrow hoeing, and a surrogate weed (white mustard, Sinapis alba L.) was sown and monitored throughout experiments. The manipulation of crop density was a reliable method for suppressing the growth of intrarow weeds. As barley density increased from the target 200 to 500 plants m−2, percent reduction in intrarow surrogate and ambient weed biomass (g m−2) increased from 49% to 82% and 53% to 99%, respectively. Increasing crop density caused a decrease in grain bulk density (kg hl−1) both years, and grain protein (%) and 1,000-kernel weight (g) in one year; whether these changes constitute a loss in grain quality depends upon end use. While row spacing had no effect on intrarow weeds, crop yields were 7% to 8% lower at 20 cm compared with 15 cm, incentivizing narrow row sowing. Barley yields were unaffected by increasing crop density, and the effect of preemergence tine harrowing was inconsistent. In one year, harrowing reduced surrogate and ambient weed biomass and increased barley yield; however, in another year, ambient weed biomass increased, and harrowing did not affect yield or surrogate weed biomass.

Does weed suppression by high crop density depend on crop spatial pattern and soil water availability?

Size-asymmetric competition, in which larger plants obtain a disproportionally larger share of contested resources, can be applied in agriculture to suppress weeds by increasing crop density and spatial uniformity, as these practices enhance the initial size-asymmetric competitive advantages of crop seedlings over weed seedlings early in the growing season. We do not yet know how agronomic factors influence weed suppression at high crop density. We performed a field experiment to ask how crop density, spatial pattern and irrigation interact to influence weed suppression and grain yield in semi-arid croplands. The experimental was a factorial design with 4 factors: wheat cultivar (Ningchun4, Xihan2), irrigation level (control, irrigated), sowing density (low, 196 seeds m−2; moderate, 400 seeds m-2; high, 625 seeds m−2), and spatial sowing pattern (rows, uniform). Weed growth was effectively suppressed by increased crop density and spatial uniformity. Effects of crop density on weed suppression and grain yield were more pronounced in the uniform pattern than in crop rows. Weed biomass was 55.7% lower and grain yield increased 29.7% higher in the high density uniform pattern compared to the low density and row pattern. Crop density interacted with cultivar in determining both weed biomass and grain yield, potentially reflecting different traits regulating crop competitive ability. Irrigation and crop density had additive effects on weed biomass but interacted to influence grain yield. Our findings support the idea that increased crop density and spatial uniformity can make a valuable and environmentally friendly contribution to weed control in wheat, reducing the need for chemical or mechanical weed control. We need a better understanding of the interactions among climate, agricultural management and crop genotype to improve our ability to effectively suppress weeds with high crop density and spatial uniformity.

Differences in Weed Suppression between Two Modern and Two Old Wheat Cultivars at Different Sowing Densities

Crop losses to weeds can be exacerbated in modern agricultural systems because modern crop cultivars have high population yields but low individual competitiveness. High density cropping systems offer the possibility of effectively suppressing weeds by increasing the initial size-asymmetric advantage of crops over the weeds. We tested this hypothesis in an outdoor mesocosm experiment with two old (Cultivar Heshangtou (HST) and Jinbaoyin (JBY)) and two modern (Xihan2 (XH2) and Xihan3 (XH3)) cultivars of spring wheat (Triticum aestivum L.), grown in a uniform pattern at four sowing densities under high weed pressure. Two annuals (Brassica napus and Linum usitatissimum) were used as model weeds sown at the same density in all treatments. Weed growth decreased and wheat yield increased with increasing crop density for all the cultivars, although yield levelled off at the highest densities. The old cultivars suppressed weeds better than the new cultivars at low density, reflecting the decline in individual competitiveness in modern cultivars. At high crop density, however, the modern XH3 suppressed the weeds as well as the old cultivars, supporting the hypothesis that traits that promote weed suppression are different at low vs. high density. Increasing crop density can be an effective way to suppress weed growth in many agricultural systems, and there is great potential for developing genotypes that can do this and produce high yields much better than the cultivars currently available.

Rattail fescue (Vulpia myuros) interference and seed production as affected by sowing time and crop density in winter wheat

AbstractField experiments were conducted in the growing seasons of 2017 to 2018 and 2018 to 2019 to evaluate the competitive effects of rattail fescue [Vulpia myuros (L.) C.C. Gmel.] in winter wheat (Triticum aestivum L.) and to assess whether delayed crop sowing and increased crop density influence the emergence, competitiveness, and fecundity of V. myuros. Cumulative emergence showed the potential of V. myuros to emerge rapidly and under a wide range of climatic conditions with no effect of crop density and variable effects of sowing time between the two experiments. Grain yield and yield components were negatively affected by increasing V. myuros density. The relationship between grain yield and V. myuros density was not influenced by sowing time or by crop density, but crop–weed competition was strongly influenced by growing conditions. Due to very different weather conditions, grain yield reductions were lower in the growing season of 2017 to 2018 than in 2018 to 2019, with maximum grain yield losses of 22% and 50% in the two growing seasons, respectively. The yield components, number of crop ears per square meter, and 1,000-kernel weight were affected almost equally, reflecting that V. myuros’s competition with winter wheat occurred both early and late in the growing season. Seed production of V. myuros was suppressed by delaying sowing and increasing crop density. The impacts of delayed sowing and increasing crop density on seed production of V. myuros highlight the potential of these cultural weed control tactics in the long-term management programs of this species.

UPAYA PENINGKATAN HASIL MENTIMUN SECARA ORGANIK DENGAN SISTEM TASALAMPOT

Utilization of organic material in crop farming is one of the methods to exploit the local wisdom in agricultural practice. The research was conducted from June to September 2012 in the research farm, State Islamic University of Sultan Syarif Kasim Riau. The objective of the research was to investigate the effects of organic materials and crop density ? in growth and yield on cucumber. Randomized block design with 2 factors and 4 replications was employed in this study. The first factor consists of three levels organic fertilizer treatments i.e. no fertilizer, poultry manure, and compost of oil palm empty bunch. Second factor comprises of three levels of crop density i.e. 1, 2, and 3 plants per pot).. The results showed that there was significant increase on growth and yield on cucumber with poultry manure treatment compare to that of with compost of oil palm empty bunch. Increasing crop density ? improves the number and weight of cucumber yield per pot, even though, some parameters, i.e. number of leaves, stem diameter, crop dry weight, productive number of branches, cucumber size and cucumber weight per plant were significantly decreased.

The effect of crop density on maize grain yield

The aim of this investigation was to estimate the effects of crop density on the plant height (PH), ear height (EH), leaf number per plant (LN), ear length (EL), number of rows per ear (NRE), number of grain per row (NGR), number of grain per ear (NGE), grain weight per ear (GWE), cob weight (CW), 1000-grain weight (1000-GW), ear diameter (ED) and grain yield (GY) in staygreen maize hybrid Dijamant 6 (FAO maturity group 600). Three crop densities (51020 plants ha-1, 59524 plants ha-1 and 71429 plants ha-1) were tested. The field experiment was carried out during 2006 and 2007 at Srem region (Putinci: latitude 44? 59? 19" N; longitude 19? 58? 11" E). Plots were organized as completely randomized block system design in four replications. PH (284.3 cm), EH (119.9 cm), LN (13.9), EL (20.8 cm), GWE (232.5 g), CW (56.4 g), 1000-GW (378.4 g) and GY (13.56 t ha-1) were significantly higher in 2006 (favorable climatic conditions) than in 2007 (258.5 cm, 112.8 cm, 13.2, 17.9 cm, 192.9 g, 46.9 g, 232.7 g and 11.50 t ha-1, respectively). Increasing crop density significantly increased the PH, EH and GY, and significantly decreases the EL, NGR, NGE, GWE, CW and 100-GW. The crop density of 71429 plants ha-1 is the optimal for growing this hybrid in Srem region. On that crop density hybrid more efficiently used available resources and achieved the highest grain yield.

Xanthium strumarium L. impact on corn yield and yield components

Xanthium strumarium L. is a major weed affecting flour corn in Khyber Pakhtunkhwa Province of Pakistan. Studies conducted in 2006 and 2007 evaluated corn yield and yield component responses to competition from X. strumarium over a range of corn populations (5, 7.5, 10, and 12.5 plants m-2) and X. strumarium densities (0, 2, 4, 6, 8, 10, and 12 plants m-2). Flour corn yield and yield components (grains ear-1, 1000-grain weight, harvest index, and days to silking) were significantly correlated with and affected by corn population, weed density, and the interaction between them. The percent reduction in grain yield was 5%-40% and fit a quadratic relationship. The number of grains ear-1 declined in a linear fashion with increasing X. strumarium density, except at the highest corn density, with an average decrease of 5%-6% in grains ear-1 for every X. strumarium plant m-2. As the X. strumarium density increased, 1000-grain weight and harvest index declined, while the number of days to silking increased slightly. Increasing densities of either crop or weed generally delayed silking and decreased yield and yield components due to inter- and intraspecific competition, suggesting that increasing crop density will likely not be effective in suppressing X. strumarium and making up for possible yield loss in corn.

Integrating Cultural and Mechanical Methods for Additive Weed Control in Organic Systems

Effective weed management strategies are limited in organic cropping systems because herbicide use is prohibited. Enhancing crop competitive ability by integrating both cultural and mechanical weed control methods is a key strategy in such instances, but the relative efficacy of different cultural and mechanical strategies and their interactions and additive effects when combined is not well known. The objective of this study was to determine the individual and additive effects of cultural and mechanical methods on weed suppression and crop yield under organic conditions. A study was performed in two organically managed oat (Avena sativa L.) cropping systems in Saskatoon, SK, Canada, in 2008 and 2009. Three cultural practices, two crop genotypes (competitive and less competitive), high and standard crop densities (500 and 250 plants m–2), narrow and standard row spacings (11.5 and 23 cm), and two post‐emergence tillage levels (harrowing and nonharrowed control) were factorially applied in a randomized block design. Increasing crop density and harrowing increased grain yield by 11 and 13%, respectively. Competitive genotype and high crop density reduced weed biomass by 22 and 52%, respectively. Combining high crop density with post‐emergence harrowing increased the grain yield by 25%. The combined treatment effects on weed biomass were more profound, as the competitive genotype, increased seeding rate, and post‐emergence harrowing decreased weed biomass by 71% compared with standard practices. Integrating cultural and mechanical weed management practices was superior to the use of individual practices because they additively control weeds in an organic cropping system.

The relationship between root biomass and productivity of spring oilseed rape (Brassica napus L.) as influenced by crop density and fertilization

A field experiment was conducted from 2008 to 2010 at the Experimental Station of the Lithuanian University of Agriculture (currently – Aleksandras Stulginskis University) on a Calc(ar)i-Endohypogleyic Luvisol (LVg-n-wcc). The objective of this study was to establish the relationship between root biomass and productivity of spring oilseed rape (Brassica napus L.) as influenced by crop density and fertilization level. Treatments of the experiment: factor A – fertilization: 1) without fertilizers, 2) with fertilizers (N 134 P 64 K 94 ); factor B – crop density: 1) 2 kg ha -1 (50.1–100 plants m -2 ), 2) 4 kg ha -1 (100.1–150 plants m -2 ), 3) 6 kg ha -1 (150.1–200 plants m -2 ), 4) 8 kg ha -1 (200.1– 250 plants m -2 ), 5) 10 kg ha -1 (250.1–300 plants m -2 ), 6) 12 kg ha -1 (300.1–350 plants m -2 ), 7) 14 kg ha -1 (350.1–400 plants m -2 ), 8) 16 kg ha -1 (400.1–450 plants m -2 ). It was established that without fertilizers, the highest root dry biomass (2.48 t ha -1 ) of spring oilseed rape at the 0–10 cm soil layer was formed at a plant density of 150.1–200 plants m -2 , while in the crop with fertilizers at a plant density of 100.1–150 plants m -2 (2.64 t ha -1 ). Plant root biomass in the 0–10 cm soil layer at a very high crop density (400–450 plants m -2 ) with and without fertilizers was significantly (P < 0.05) lower (by 35.0% to 37.9% and by 32.2% to 40.9%, respectively) as compared with those densities, where root biomass was the highest. In the treatment without fertilizers parabolic statistically significant (P < 0.05) relationships were established between rape crop density and root dry biomass at the 0–10 and 10–20 cm soil layers. Mineral fertilization, compared with no fertilization, significantly (P < 0.05) increased (by 34.7% to 44.6%) spring rape root biomass at the 0–10 cm soil layer and did not have any significant effect on root biomass at the 10–20 cm soil layer. Parabolic and statistically significant (P < 0.01) relationships exist between the above-ground dry biomass of oilseed rape and plant density. In the crop without fertilizers, the highest spring rape seed yield (2.31–2.38 t ha -1 ) was obtained at a crop density of 250.1–350 plants m -2 and in the crop with fertilizers – at a crop density of 100.1–200 plants m -2 (3.28–3.32 t ha -1 ). A trend of spring oilseed yield decrease with increasing crop density at investigated levels of fertilization was established. Mineral fertilization, compared with no fertilization, at all spring rape plant densities significantly (P < 0.05) (from 30.3% to 81.4%) increased rape seed yield. In the crop without fertilizers, spring rape seed yield significantly depended on the crop density (r 2 = 0.90, P < 0.05) and the above-ground dry biomass of rape at the flowering stage (r 2 = 0.67, P < 0.05). In the crop with fertilizers, rape seed yield was significantly influenced by the crop density (r 2 = 0.66, P < 0.05) and dry biomass of rape roots in the 0–10 cm soil layer at the flowering stage (r 2 = 0.86, P < 0.01).

Potential Synergistic Effects of Cereal Rye Biomass and Soybean Planting Density on Weed Suppression

Increasing crop density is a cultural weed management practice that can compliment the use of cover crops for weed suppression. In this research, we created a range of cover crop biomass and soybean densities to assess their weed-suppressive ability alone and in combination. The experiment was conducted in 2008 and 2009 in Maryland and Pennsylvania using five levels of cereal rye residue, representing 0, 0.5, 1.0, 1.5, and 2.0 times the ambient level, and five soybean densities ranging from 0 to 74 seeds m−2. Weed biomass decreased with increasing rye residue and weeds were completely suppressed at levels above 1,500 g m−2. Weed biomass also decreased with increasing soybean density in 2 of 4 site–years. We evaluated weed suppression by fitting an exponential decay model of weed biomass as a function of rye biomass and a hyperbolic model of weed biomass as a function of soybean density at each of the five tactic levels. We multiplied these individual tactic models and included an interaction term to test for tactic interactions. In two of the four site-years, the combination of these tactics produced a synergistic interaction that resulted in greater weed suppression than would be predicted by the efficacy of each tactic alone. Our results indicate that increasing soybean planting rate can compensate for lower cereal rye biomass levels when these tactics are combined.

Persian Darnel (Lolium persicum) Fecundity Response to Spring Wheat, Canola, and Sunflower Interference

Persian darnel control options are limited and unmanaged populations can cause substantial crop yield loss. Integrating crop diversification and higher crop seeding rates into a cropping system might improve Persian darnel management. Field experiments were conducted to determine the effect of different crops and increased crop seeding rates on Persian darnel fecundity. Persian darnel produced up to 2,800 seeds per plant and 53,000 seeds/m2when grown without competition. Increasing crop density reduced Persian darnel tillers per plant, seed weight, and fecundity. Increasing crop density reduced Persian darnel fecundity 0.4 to 0.2% per spring wheat plant, 0.6 to 0.1% per canola plant, and 16 to 8% per sunflower plant. Persian darnel fecundity was impacted the greatest by reduced weed seedling establishment, which was caused by crop competition and seeding sunflower late in the spring preceded by a nonselective herbicide application. Results indicated delaying the seeding of spring crops or including a late-seeded warm season crop, like sunflower or safflower, in the cropping system is an effective weed management tool for reducing Persian darnel fecundity.

Barley (Hordeum vulgare) and Wild Oat (Avena fatua) Competition Is Affected by Crop and Weed Density1

Wild oat is the most serious grass weed in Argentine barley crops and its control has concentrated on herbicide strategies. Increasing crop density could be an effective strategy to reduce the effect of wild oat on barley yield. However, limited research has been conducted to evaluate the effect of crop density on the competitive balance between barley and spontaneous populations of wild oat. A field experiment was conducted in 1992, 1993, and 1999, to study the effect of spontaneous populations of wild oat on barley sown at densities of 160, 220, and 280 plants/m2. Wild oat density averaged 84 plants/m2. Wild oat biomass increased linearly with weed density in all treatments but was reduced by increasing barley seeding rates. Barley biomass and yield were not affected by wild oat at high crop sowing densities, but for the low and medium barley densities, yield loss was almost 25% when 70 wild oat plants/m2 were established. Barley yield loss was mostly related to competition from the early emerged wild oat. The relationship between yield losses and wild oat density was equally significant when the whole population or only early emerged individuals of the weed were considered.Nomenclature: Wild oat, Avena fatua L. #3 AVEFA; barley, Hordeum vulgare L.Additional index words: Wild oat interference in barley, integrated wild oat management, seeding rates of barley.

Effect of soybean spatial arrangement and glyphosate dose on Anoda cristata demography

The annual weed Anoda cristata is recognised as a problem of increasing importance in soybean crops and can significantly reduce crop yields. The effect of soybean density, row spacings and glyphosate doses in reduced and no-tillage systems on demography and phenology of A. cristata was examined.The seedbank of A. cristata did not differ between tillage systems and was initially similar for all treatments. In the fourth year, it was higher in plots without glyphosate application than in plots treated with the recommended and half the recommended glyphosate dose. The highest seedbank was observed in wide-row soybean without glyphosate. Total emergence was not affected by the planting pattern with any glyphosate dose, but emergence was higher in the untreated plots than in those sprayed with the recommended glyphosate dose. Total weed mortality without glyphosate was independent of the planting pattern in 2001 but was lower in wide-row soybean in 2002. The application of half of the recommended dose killed almost all A. cristata plants in narrow-row soybean. Mortality was 100% with the recommended glyphosate dose in all years and in all planting patterns.Without herbicides, biomass and seed production per unit area was lower in narrow-row compared to wide-row soybean but no differences in biomass production of the weed were observed between narrow spacings with high and low densities. In all planting patterns, weed biomass and seed production decreased progressively from the first to the third weed emergence. The use of half of the recommended glyphosate dose relative to treatments without glyphosate application reduced biomass and seed production in all planting patterns, but this reduction was greatest in narrow-row soybean. Without glyphosate, in both the row spacings, flowering and seed production occurred earlier for the first relative to the second weed emergence. With the application of half of the recommended dose, flowering occurred only in the first weed emergence and both flowering and seed production were observed first in wide-row relative to narrow-row soybean. The recommended glyphosate dose controlled both the first and the second emergence, but the third emergence was not affected and remained in the vegetative stage till crop harvest. Without glyphosate and using half of the recommended glyphosate dose, reducing soybean row spacing is a more important factor than increasing crop density in reducing A. cristata populations.

Spring Wheat, Canola, and Sunflower Response to Persian Darnel (Lolium persicum) Interference

Integrated weed management practices, such as crop rotation and increased seeding rates, potentially improve weed management. Yet, few studies compare competitive interactions of weeds with different crops. This research quantified the impact of Persian darnel on spring wheat, canola, and sunflower yield across different seeding rates. Increasing crop density increased yield when Persian darnel affected crop yield early in physiological development. Crop yield loss was estimated to reach 83, 70, and 57% for spring wheat, canola, and sunflower, respectively, at high Persian darnel densities. Persian darnel reduced spring wheat yield by limiting the number of tillers per plant and seed per tiller; reduced canola yield by limiting the number of branches per plant, pods per branch, and seed per pod; and reduced sunflower yield by limiting the number of seed per plant. Persian darnel affected crop growth early in physiological development, indicating that interspecific interference occurred early in the growing season. Cultural and resource management aimed at reducing Persian darnel impact on resource availability and crop yield components will reduce Persian darnel impact on crop yield.

The Interrelationships of Winter Wheat Cultivars, Crop Density and Competition of Naturally Occurring Weed Flora

ABSTRACT The effects of intraspecific and interspecific competition on a wide range of winter wheat cultivars were investigated in two consecutive split plot field experiments. Significant reductions of grain yield at greatly reduced seed rates were observed in the first experiment, whereas increasing crop density up to 380 plants m−2 in the second experiment failed to produce a significant yield response due to compensation through increased ears and grains per plant at lower crop densities. Appreciable weed suppression and acceptable grain yield can be achieved at crop densities between 150 and 270 plants m−2. Reductions in final yield due to weed competition occurred in both experiments; 11.7 and 13.6% for the first and second experiment, respectively, with the onset of weed competition occurring from tillering in the first experiment and from stem elongation in the second. The possibility of enhancing crop competitiveness for weed suppression and improved grain yield is discussed.

Improved management of Avena ludoviciana and Phalaris paradoxa with more densely sown wheat and less herbicide

Summary The effectiveness of crop competition for better weed control and reducing herbicide rates was determined forAvena ludoviciana and Phalaris paradoxa. Four experiments, previously broadcast with seeds of the two weeds in separate plots, were sown with three wheat densities, and emerged weeds were treated with four herbicide doses (0–100% of recommended rate). The measured crop and weed traits were first analysed across experiments for treatment effects. Grain yield and weed seed production data were then analysed using cubic smoothing splines to model the response surfaces. Although herbicide rate for both weeds and crop density for P. paradoxa had significant linear effects on yield, there was a significant non‐linearity of the response surface. Similarly, herbicide rate and crop density had significant linear effects on weed seed production, and there was significant non‐linearity of the response surface that differed for the weed species. Maximum crop yield and reduction in seed production of P. paradoxa was achieved with approximately 80 wheat plants m−2 and weeds treated with 100% herbicide rate. For A. ludoviciana, this was 130 wheat plants m−2 applied with 75% herbicide rate. Alternatively, these benefits were achieved by increasing crop density to 150 plants m−2 applied with 50% herbicide rate. At high crop density, application of the 100% herbicide rate tended to reduce yield, particularly with the A. ludoviciana herbicide, and this impacted adversely on the suppression of weed seed production. Thus, more competitive wheat crops have the potential for improving weed control and reducing herbicide rates.

Predicting the competitive effects of weed and crop density on weed biomass, weed seed production and crop yield in wheat

SummaryCompetition between winter‐sown wheat and Viola arvensis Murray or Papaver rhoeas L. was studied in two experiments in two successive years. The effects of varying crop and weed density were modelled in terms of weed biomass over time, weed seed production and crop yield. Biomass model parameters, representing maximum weed biomass and intra‐ and interspecific competition, were obtained for different assessment dates, enabling biomass levels to be predicted during the two growing seasons. Weed biomass declined, and its maximum level was reached earlier, with increasing crop density. Intraspecific competition was higher in the absence than in the presence of crop, increasing with time and with weed density. Halving the wheat population increased June biomass of V. arvensis by 74% and of P. rhoeas by 63%. Crop yield losses with increasing weed density were greater with low than with medium and high crop populations. P. rhoeas was significantly more competitive than V. arvensis in both years. Weed biomass in 1989 responded more to reductions in crop density following the milder winter of 1988/89 than in the previous year; however crop yields were less affected in 1989 due to summer drought, restricting late weed growth and competition. Weed seed production was related to weed biomass; the progressive lowering of crop density increased seed production, and both species were very prolific in the absence of crop. By combining models, seed production could be derived for a given competitive effect on the crop. Threshold weed populations, based on low weed levels that are not economic to control, could then be equated with the accompanying weed seed production.

The effects of population density and intercropping with cowpea on the water use and growth of sorghum in semi-arid conditions in Botswana

Soil moisture was measured below sorghum crops of differing density in rainfed trials in increasingly droughty conditions from 1981/2 to 1983/4. Additional measurements were made in sorghum/cowpea intercrops and cowpea monocrops during the drought seasons of 1982/3 and 1983/4. Increasing crop density, by addition of either sorghum or cowpea, resulted in increased effective root activity, which was accompanied by small increases in actual/potential evapotranspiration ( E a/ E p) early in each season. These small increases were followed by small decreases later in the season, but these effects were rarely statistically significant. Both sorghum density and intercropping had little effect on total seasonal water use. Dry matter production increased linearly with increasing seasonal water use. Water use efficiencies of dry weight production were increased by increasing sorghum density. Intercropping in dry conditions did not have significant effects on water use efficiencies. Estimates of soil evaporation suggested that the effects of density on water use efficiency could be attributed to reductions in soil evaporation. The effects of density on reproductive yields were governed by two opposing processes: increasing crop density increased water use efficiencies of dry weight production, but decreased the proportions of dry weight allocated to grain.

Competition between annual weeds and vining peas grown at a range of population densities: effects on the weeds

SummaryLinear regression of dry weight of weeds against crop density, together with the use of diversity indices and principal component analysis were used to derive information about changes in the behaviour of annual weeds over the growing season and in response to a wide range of crop densities in vining peas Pisum sativum L. Using linear regression it was possible to quantify reductions in weed dry weights per unit increase in crop plant density The ‘weed losse’ factor was acceptably consistent between experiments. Indices examining richness and evenness showed that numbers of weed species declined with increasing crop density and as the season progressed, but although species evenness became less at successive sampling dates the presence of a pea crop, whatever its density, did not radically alter the composition of the weed flora. Principal component analysis demonstrated that although there was competition within the weed flora, the crop did not replace the dominant weed species on high density plots, but reduced growth of all weed species alike.