Use of density to predict crop yield loss between variable seasons

Summary Density:yield loss models rely on fixed coefficients, parameterized from a particular site and season to predict the impact of weeds on crop yields. However, the empiricism of this approach and failure to incorporate environmental effects, has major biological and economic implications. In this study, seasonal variability in wheat yield loss and associated economic costs from Avena spp. were quantitated. A competition experiment at Wagga Wagga, NSW, showed large seasonal differences in wheat yield loss from densities of Avena spp. across 2 years. Gross margins, simulated over a 51-year period, decreased as Avena spp. density increased and were more variable at low crop densities and higher weed densities. For example, at a density of 200 Avena spp. plants m⁻², coefficient of variation in crop gross margin ( CV ) was $AUS 47 ha⁻¹ for a crop density of 200 wheat plants m⁻² compared with a CV of $AUS 75 ha⁻¹ for a crop density of 50 wheat plants m⁻². The value of yield loss predictions will be vastly improved by making parameter values in yield loss models a function of seasonal factors such as rainfall.     

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 for Avena 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.     

Interference of redroot pigweed (Amaranthus retroflexus) in green bean (Phaseolus vulgaris)

Weeds that emerge along with or immediately after crop plants usually can reduce the yield of those crops. Two randomized complete block design experiments were conducted during 2006 and 2007 in Tabriz, Iran to determine the critical period of redroot pigweed control in the green bean hybrid “Cantander”. The treatments were weed‐infested and weed‐free plots at 2, 4, 6, 8, 10, and 14 weeks after bean emergence (WABE). The green bean biomass was affected by the early emergence of redroot pigweed, but it was not reduced when redroot pigweed emerged at 10 weeks after crop emergence, along with crop emergence, and grew with green bean until 4 WABE. The redroot pigweed biomass decreased by 2.7 g m^?2 per day when weed emergence was delayed. Each 100 g m^?2 of weed biomass that was produced resulted in a 1.4 kg ha^?1 loss in the green bean yield. When redroot pigweed interference lasted for ≥4 weeks after green bean emergence, the green bean yield was reduced significantly. Weeds, which emerged 2 weeks after green bean and thereafter were controlled, did not decrease crop productivity significantly. The highest crop yield was obtained when the weed emerged at 14 WABE. The critical period of redroot pigweed control, considering a 10% yield loss, was between 19 and 55 days after green bean emergence. Thus, weed control practises should be begun no later than 3 WABE and should continue until at least 8 WABE in order to obtain the maximum green bean yield.     

Influence of sowing density and spatial pattern of spring wheat (Triticum aestivum) on the suppression of different weed species

To better understand the potential for improving weed management in cereal crops with increased crop density and spatial uniformity, we conducted field experiments over two years with spring wheat ( Triticum aestivum ) and four weed species: lambsquarters ( Chenopodium album ) , Italian ryegrass ( Lolium multiflorum ), white mustard ( Sinapis alba ), and chickweed ( Stellaria media ). The crops were sown at three densities (204, 449, and 721 seeds m⁻²) and in two spatial patterns (normal rows and a highly uniform pattern), and the weeds were sown in a random pattern at a high density. In most cases, the sown weeds dominated the weed community but, in other cases, naturally occurring weeds were also important. There were strong and significant effects regarding the weed species sown, the crop density, and the spatial distribution on the weed biomass in both years. The weed biomass decreased with increased crop density in 29 out of 30 cases. On average, the weed biomass was lower and the grain yield was higher in the uniform compared to the row pattern in both 2001 and 2002. Despite the differences in weed biomass, the responses of L. multiflorum , S. media , and C. album populations to crop density and spatial uniformity were very similar, as were their effects on the grain yield. Sinapis alba was by far the strongest competitor and it responded somewhat differently. Our results suggest that a combination of increased crop density and a more uniform spatial pattern can contribute to a reduction in weed biomass and yield loss, but the effects are smaller if the weeds are taller than the crop when crop–weed competition becomes intense.     

Model of crop:weed competition applied to maize:Abutilon theophrasti interactions. I. Model description and evaluation

A complex set of interactions among crops, weeds and their environment determines the impact of weed interference on crop productivity. These interactions can be simulated with dynamic crop:weed competition models, such as ALMANAC. In this study, ALMANAC was modified to simulate maize: Abutilon theophrasti competition. In the modified ALMANAC model, daily increases in leaf area index (LAI), height and rooting depth are attenuated on the basis of accumulated above-ground biomass and by environmental stress. Also, a simple, flexible method is adopted to partition radiation in a mixed canopy. A maize: A. theophrasti competition study conducted near Aurora, NY, in which a range of weed densities (0–16 plants m⁻²) were established in a maize crop, was used to evaluate the model. The modified ALMANAC proved to be a useful tool for segregating the maize response to competition in 1991 (simulated loss of 35% at the highest weed density) from those in 1992–94 (simulated losses not greater than 16%). Based on these findings, the modified ALMANAC model is judged to be capable of distinguishing between environmental conditions that facilitate large yield losses and those that allow maize to outcompete A. theophrasti .     

Absorption and translocation of glyphosate with conventional and organosilicone adjuvants

This study was conducted to determine the effects of three organosilicone-based and six conventional organic adjuvants on the absorption and translocation of ¹⁴C-glyphosate in guineagrass and redroot pigweed. The organosilicone adjuvants produced rapid absorption of the ¹⁴C-glyphosate into the redroot pigweed leaves, reaching maximum absorption within 0.5–1.0 h after application. The conventional adjuvants produced slower absorption of the ¹⁴C-glyphosate, as the maximum absorption was not achieved until at least 24 h after application in redroot pigweed, remaining similar until 72 h. In guineagrass, the maximum absorption of the glyphosate was earlier than 24 h with the organosilicone-based adjuvants, compared with longer times for the conventional adjuvants. The organosilicone-based adjuvants also increased the glyphosate translocation in redroot pigweed, but not in guineagrass. Organosilicone adjuvants have the potential to provide greater rainfastness to glyphosate on redroot pigweed and, to a lesser extent, on guineagrass.     

The influence of interspecific interference on the seed production of Stellaria media and Hordeum vulgare (volunteer barley)

The effects of the density and proportion of both volunteer barley ( Hordeum vulgare L.) and Stellaria media (L.) Vill. (common chickweed) on the seed yield of both species was investigated in linseed ( Linum usitatissimum L.) and autumn-sown field bean ( Vicia faba L.). A model was created to estimate these effects. It was a combination of two models. The first was a simple linear model relating weed seed number m² to weed dry weight m². The second was an inverse linear model relating weed dry weight per plant to weed density. A very good relationship existed between barley dry weight and number of seeds m² and between S. media dry weight and number of seed capsules m². This relationship was relatively consistent between experiments. The inverse linear model provided a good estimation of the relationship between weed density and weed dry weight per plant for both barley and S. media . Model variables, however, differed between experiments. Using the combined model it was estimated that, in the absence of other weed species and at a density of 800 plants m², S. media would produce between 4000 and 48 000 seeds m², whereas volunteer barley, at a density of 400 plants m², would produce between 2000 and 8000 seeds m². The presence of barley always reduced S. media seed yield and a barley population of 100 plants m² could reduce S. media seed yield by up to 79%. The presence of S. media reduced barley seed yield in only one of three experiments, in which a S. media density of 800 plants m² reduced barley seed yield by up to 68%. It was concluded that interspecific weed competition should not be ignored as a factor in models of weed population dynamics     

THE NATURE AND CONSEQUENCE OF INTERFERENCE BY ALOPECURUS MYOSUROIDES HUDS. ON THE GROWTH OF WINTER WHEAT

Summary. In a field experiment with different densities of Alopecurus myosuroides , 10 plants/m² caused no significant reduction in total dry weight of winter wheat at harvest. When autumn- or spring-germinated plants at 100/m² remained until harvest, crop weight was reduced by about 30%. A study of root-zone competition between A. myosuroides and winter wheat in the glasshouse showed that suppression of the weed by the crop was greater at high nitrogen levels; the practical implications of this are discussed.
La nature de l'interférence d' Alopecurus myosuroïdes Huds. sur la croissance du blé d'hiver, et ses conséquences     

Weed:spring barley competition for applied nitrogen in pig slurry

Summary The experiments were carried out in the two spring barley fields of the organic six-course cattle:crop rotation at Foulumgaard, Denmark. The weed density was 300 and 1800 plants m⁻² respectively. Pig slurry was applied by hand in microplots by four methods: broadcasting followed by incorporation, or injected in bands to depths of 5, 10 or 15 cm. Spring barley and weeds were sampled separately six times during the tillering and elongation phase of the spring barley. The effect of application method on dry-matter (d.m.) production, nitrogen uptake and recovery of applied nitrogen in the spring barley and the weeds is reported. Slurry banding halved the weed d.m. and weed N uptake compared with broadcasting, irrespective of weed density. Weeds recovered up to 12% of the applied nitrogen, which made them a significant competitor when the slurry was broadcast and incorporated. Banding by direct injection reduced the slurry:soil contact and the weed:crop competition balance for applied nitrogen moved in favour of the crop. Thus, the crop recovery of applied nitrogen at the end of the sampling period was increased from approximately 45% for broadcast and incorporated to approximately 50% for injected slurry, and coincidental weed recovery was reduced to a maximum of 5%. As the nitrogen supply normally affects plant d.m. production, banding of slurry might well improve crop competitiveness and its tolerance to mechanical weed control.     

Effect of weed interference on Zea mays: Growth analysis

In order to investigate the effect of weed competition on corn, growth trials were conducted in Shushtar, Iran, using a comparative growth analysis. In this study, two sets of treatments were imposed, based on the phenological stages of corn development, using a randomized complete block design with three replicates. The results showed that barnyardgrass and redroot pigweed were the most dominant weeds in these trials. Increasing the duration of weed interference reduced the corn leaf area index gradually. The reduction in the corn leaf area index led to a decline in the crop growth rate as a result of a reduction in the ability of corn to capture light and its photosynthetic ability. The total dry matter and relative growth rate of corn were decreased when the weeds were removed later as a result of a reduction in the leaf area index. According to these results, the leaf area index has a critical role in weed–crop competition.     

Metabolism of 14C-buthidazole in corn (Zea mays L.) and redroot pigweed (Amaranthus retroflexus L.)*

The pattern of buthidazole {3-[5-(1,1-dimethyl ethyl)-1,3,4- thiadiazol -2-y1]-4-hydroxy-1-methyl-2-imidazolidinone} metabolism and its potential contribution to crop selectivity were studied in tolerant corn (Zea mays L., ‘Pioneer 3780’) and susceptible redroot pigweed (Amaranthus retroflexus L). Thin-Layer Chromatographic (TLC) analysis of methanol soluble extracts revealed that both corn and redroot pigweed seedlings metabolized buthidazole in a similar manner but at different rates. At comparable time periods redroot pigweed contained a greater percentage of unmetabolized ¹⁴C-buthidazole than did corn. A major unidentified metabolite with polar properties was formed faster in corn that in redroot pigweed and appeared to be important for the observed crop selectivity of buthidazole. Additional metabolites of buthidazole formed in both species had TLC properties similar to those of the amine, urea, and dihydroxy derivatives of buthidazole.     

Interference between Avena sterilis, Phalaris minor and five barley cultivars

Field experiments were carried out in Northern Greece from 1994 to 1997 to study interference between Avena sterilis L. or Phalaris minor Retz. and five autumn-sown barley cultivars. Weed:crop interference began in early April. Avena sterilis at 120 plants m⁻² showed greater interference against barley than P. minor at 400 plants m⁻². The greatest grain yield and ear number reduction due to interference by either weed was recorded for cvs Klipper and Plaisant, and the least for cv. Athinaida; with cvs Carina and Thermi intermediate. Yield reduction due to A. sterilis for cvs Athinaida, Carina, Thermi, Klipper and Plaisant was 8, 16, 27, 61 and 67%, respectively, while corresponding losses to P. minor were 1, 8, 14, 45 and 55%. These results clearly indicate that growth and consequently yield components of cv. Athinaida were unaffected by the presence of either weed species, while those of cv. Carina were affected by A. sterilis , but not by P. minor . However, dry weight and panicle number of both weed species were severely reduced by interference with cvs Carina, Athinaida and Thermi compared with cvs Klipper and Plaisant. The order of interference of the five barley cultivars tested against A. sterilis and P. minor was Athinaida > Carina > Thermi > Klipper ≥ Plaisant.     

Quantitative evaluation of wheat against volunteer rye in Iran

Rye infestations have increased in frequency and severity in the wheat fields of Iran and have caused yield reductions in wheat production. In order to study the competition effects of wheat against rye, an experiment was conducted at the research centers of Karaj and Varamin, Iran, during 2001 and 2002. The experimental design was a randomized complete block with 24 treatments and four replications. The treatments included pure stands of wheat at densities of 350, 450, 550, and 650 plants m⁻² and volunteer rye densities of 10, 30, 50, and 70 plants m⁻², and mixed stands of wheat and rye at complete factorial densities. The results indicated that rye was a superior competitor compared to wheat. The biological and economic yields of wheat were mainly affected by interspecific competition in the Karaj region. This was also true during the first year in the Varamin region but, in the second year, the biological and economic yields of wheat were equally affected by interspecific and intraspecific competition because of increasing precipitation. The evaluation of competitive ability, using regression coefficients, showed that in Karaj, the effect of one rye plant on wheat was approximately equivalent to three and two plants of wheat in reducing the grain yield of wheat in the first and second years, respectively, while in Varamin, it was equivalent to three and 1.2 plants of wheat, respectively. In other words, each 0.36 and 0.51 rye plant in Karaj and each 0.36 and 0.84 rye plant in Varamin had the same impact on the reciprocal wheat grain yield as did one wheat plant in the first and second years, respectively.     

Influence de la densité et de la période de concurrence de Solanum nigrum L. sur la tomate de semis direct, en relation avec le désherbage

Effects of density and period of competition by Solanum nigrum L. on direct seeded tomatoes in relation to weed control The effects of density and period of competition from Solanum nigrum L. were measured in direct seeded tomatoes given weed control treatments currently used in south-east France. S. nigrum emerging after a diquat treatment at the 2–3 leaf stage of the crop and thinned to low densities (<12.8 plants ha^?1) at the 5–6 leaf stage of the crop caused significant yield loss if left to compete with the crop until harvest. Yield reduction was smaller if the same weed densities were present only until the onset of flowering. The regression curves of yield on weed density differed as annual climatic variations affected sowing date and plant growth; a comparison between years was made using the relation ‘crop yield × weed biomass/crop biomass’. Significant interactions between weed density and period of competition were found with yield of both green and red fruit. For late sown crops with low densities of S. nigrum two weed control treatments at the 5–6 leaf stage and at the onset of flowering were sufficient to prevent yield loss.     

Effects of winter wheat cultivars and seed rate on the biological characteristics of naturally occurring weed flora

Summary Differential competitive ability of six winter wheat cultivars and traits that confer such attributes were investigated for a range of seed rates in the presence or absence of weeds for a naturally occurring weed flora in two successive years in split-plot field experiments. Crop height and tillering capacity were considered suitable attributes for weed suppression, although competitiveness is a relative rather than an absolute characteristic. Maris Huntsman and Maris Widgeon were the most competitive cultivars whereas Fresco was the least competitive. Manipulation of seed rate was a more reliable factor than cultivar selection for enhancement of weed suppression, although competitiveness of cultivars Buster, Riband and Maris Widgeon was not enhanced by increased seed rate. Crop densities ranging between 125 and 270 plants m⁻² were found to offer adequate weed suppression. Linear relationships were observed between individual and total weed species dry weight and reproductive structures per unit area.     

Influence of corn density and planting pattern on the growth of common lambsquarters (Chenopodium album L.)

Crop density and spatial arrangement affect the potential of intraspecific and interspecific competition as they increase the canopy light absorption. The effects of the corn density and planting pattern on the growth of common lambsquarters ( Chenopodium album L.) was studied in a randomized, complete block design with three replications and factorial arrangement at University College, Aburaihan, Iran, in 2003. The plant density was the recommended density and 1.5-fold the recommended density. The planting pattern treatment also was performed at two levels: one- and two-row planting. The common lambsquarters density was 0, 5, 10, and 15 plants m⁻¹ per row. The results showed that the leaf area index, biomass production, crop growth rate, and inflorescence biomass of the dry matter of common lambsquarters were decreased by increasing the corn density. But, the leaf area index, biomass production, crop growth rate, and inflorescence biomass of the dry matter of common lambsquarters under the two-row planting pattern were less than under the one-row planting pattern, although these differences were not significant.     

Competition between maize and Datura stramonium in an irrigated field under semi-arid conditions

Crop growth of maize ( Zea mays L.) and Datura stramonium L. in monoculture and competition was studied over 4 years in a flood irrigated field in Zaragoza (Spain). Plant density was 8.33 m^–2 for maize and 16.66 m^–2 (1994 and 1995) and 8.33 m^–2 (1996 and 1997) for D. stramonium . Maize yield was decreased by 14–63% when competing with the weed. Yield reduction increased as the time between crop and weed emergence decreased. The development of leaf area per plant during the exponential growth phase was faster in maize primarily because the leaf area of maize seedlings at emergence time was greater than that of the weed. The faster growth of maize in leaf area and height reduced the photosynthetically active radiation received by the weed. Datura stramonium had a lower radiation use efficiency (RUE) than maize. Competition from the weed slightly decreased the maximum leaf area index (LAI) of the crop, and leaf senescence of maize was accelerated. The weed competed with the crop late in the season reducing crop growth rate, grain number per ear and grain weight. Competitive ability of D. stramonium for light was mainly due to its growth habit, with the leaves concentrated in the upper part of the canopy (more than 75% of LAI in the upper 25% of its height), its higher light extinction coefficient (0.89) and its indeterminate growth habit. The N plant content of maize was not influenced by the presence of the weed. The weed had a higher N plant content than the crop throughout the season and took up more N in monoculture.     

Effects of the relative time of emergence and the density of common lambsquarters (Chenopodium album) on corn (Zea mays) yield

Common lambsquarters (Chenopodium album) is one of the world's worst weeds. In order to study the competitive potential of single‐cross 704 corn (Zea mays) in competition with common lambsquarters at different relative times of emergence and density levels of the weed, an experiment was conducted in 2006 at the farm of the Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran. This experiment was designed as a split plot based on a randomized complete block design with three replications. The emergence time of the weed was considered at three levels (7 days and 14 days earlier than corn and simultaneously with corn) as the main plot, while the density of the weed was considered at six levels (0, 4, 8, 12, 16, and 20 plants per m²) as the subplot. The results showed a decrease in the grain yield and biomass of corn, as the emergence time of corn was delayed in comparison with the weed in a way that the maximum reduction was observed at the earlier emergence of the weed, compared to corn, and also at a high density of the weed. As the weed emerged earlier than corn, the rate of yield loss resulting from the first flush of weeds was not that high. However, with every few days that the weed emerged earlier than corn, the rate of yield loss became higher as the density of the weed increased to its maximum. The maximum reduction in the yield components was observed at 14 days earlier emergence of the weed, compared to corn, and at high densities, as the corn plants were overshadowed by the weed canopy and no ear was produced.     

Wind-mediated spread of Rice yellow mottle virus (RYMV) in irrigated rice crops

A study was carried out to demonstrate that Rice yellow mottle virus (RYMV), a virus known to be transmitted by beetles, can spread between rice plants by direct leaf contact caused by wind. Almost all healthy plants surrounding an infected plant became infected when exposed to a fan blowing for 15 min at a distance of 50 cm. Spread of RYMV by plant contact, mediated by wind, was also demonstrated in field experiments, the extent of spread depending on plant density. Infection was almost 10 times higher in plots with a density of 33 plants m⁻² than in plots with 16 plants m⁻². Less spread was observed in plots protected by 1·5 m high windscreens. It is suggested that wind-mediated spread of RYMV may result from abrasive contact between leaves of plants.     

Comparison of empirical models depicting density of Amaranthus retroflexus L. and relative leaf area as predictors of yield loss in maize (Zea mays L.)

The outcome of crop-weed competition should be predicted as early as possible in order to allow time for weed control measures. Maize grain yield losses caused by interference from Amaranthus retroflexus L. (redroot pigweed) were determined in 1991 and 1992. The performance of three empirical models of crop-weed competition were evaluated. Damage functions were calculated based on the weed density or relative leaf area of the weed. In the yield loss-weed density model, values of I (percentage yield loss at low weed density) were relatively stable for similar emergence dates of A. retroflexus across years and locations. Estimated maximum yield loss (A) was more variable between locations and may reflect environmental variation and its effect on crop-weed competition, at least in 1991. The two-parameter yield loss-relative leaf area model, based on m (maximum yield loss caused by weeds) and q (the relative damage coefficient) gave a better fit than the single-parameter version of the model (which includes only q). In both relative leaf area models, the values of q varied between years and locations. Attempts to stabilize the value of q by using the relative growth rate of the leaves of the crop and weed were successful; however, the practical application of such relative leaf area models may still be limited owing to the lack of a method to estimate leaf area index quickly and accurately.