Chemical weed control in irrigated direct-seeded rice in the Sudan Gezira

Unrestricted weed growth resulted in serious yield losses (60–75%) in rice (Oryza saliva L.). Hand-weeding once at 2 weeks after crop emergence was ineffective, but weeding at 2 and 2 weeks after crop emergence brought yield losses down to 20%. Pre-emergence application of bifenox and oxadiazon (12 l) and post-emergence application of molinate + propanil gave crop yields comparable to that of the clean weeded control (91–129%). Grain yields obtained with pre-emergence application of oxadiazon (25 EC) and post-emergence applications of propanil, bentazone + propanil, and bifenox alone or as a tank mix with propanil or bentazone were considerably less than those obtained from the clean weeded plots. Bifenox, oxadiazon (12 l) and propanil + molinate gave adequate and persistent weed control (72–97%), and did not damage rice. Oxadiazon (25 EC) gave good weed control but resulted in a serious stand reduction. Post-emergence application of propanil, propanil + bentazone bifenox alone or as a tank mix with propanil or bentazone had no adverse effect on crop stand, but gave poor control of grasses (0–40%).     

Competition between Ridolfia segetum and sunflower

Studies on competition between Ridolfia segetum Maris, and sunflower (Helianthemum annuus L.) were conducted at eight locations in southern Spain in 1990 and 1991. in order to define competition models and to estimate from these economic thresholds as affected by crop inputs and potential yields. Competition losses in sunflower crops ranged from 19% to 56% of weed–free yields. There were slightly better correlations between percentage sunflower reduction and weed density than with weed dry weight, (?0.66 and ?0.59, respectively). The weed competitive index, or sunflower crop dry weight reduction per unit dry weight of R. segetum, was 1.09. The percentage yield losses due to weed density (NPR_t) were fitted to multiple linear, quadratic, exponential and hyperbolic models. The hyperbolic equation, %RSY=100 (1+1/b*NPR_t)^?1, where b=0.14 and is the R. segetum competitive ability index, had the lowest error sum of squares (SSE), and gave the best biological explanation for the competition response. Early emergence (before mid–March) made weeds about 1.5 times more competitive than late emergence. The economic threshold to offset the cost of a shallow post–emergence tillage, assuming 70% control efficiency, ranged from about 2.5 plants m ^?2 for low–yielding crops(1200kgha^?1) to less than one plant m^?2 for higher–yielding crops (2800 kg ha^?1).     

Weed competition and control in sugarcane

Losses of about 40% in cane yields due to natural stands of weeds were found in experiments conducted in sugarcane var. Co 527 in the year of planting at Guneid Sugarcane Research Station, Sudan. Weed competition lowered millable stalks per metre row by 32%, stalk height by 24%, stalk thickness by 15% and number of nodes per stalk by 14%. Tillering was the growth phase most affected by weed competition. Cane yields were increased as number of hand weedings increased, but four weedings were not markedly better than three. The average yield (67·04 t ha^?1) obtained from four weedings was not significantly (P= 0·05) better than that of three weedings carried out at 3, 6 and 9 weeks after cane planting. Juice analysis components were also affected by weeds and a 15% reduction in sucrose recovery was recorded. Reductions in the other components were only 4–7%. Atrazine and diuron (3·3 kg ha^?1), metribuzin (2·4 kg ha^?1) and metribuzin (1·3 kg ha^?1) in tank mixture with diuron (1·5 kg ha^?1) gave excellent residual weed control of the dominant weed species, Ipomoea cordofana Choisy., Brachiaria eruciformis (Sm.) Griseb., Corchorus fascicularis Lam., Ocimum basilicum L. and Dinebra retroflexa (Vahl) Panz., for most of the first growing season. Excellent control of weeds achieved by the herbicide treatments resulted in comparable yields to frequently-weeded cane. These herbicides were not phytotoxic to sugarcane var. Co 527.     

Influence of intra-row cruciferous surrogate weed growth on crop yield in organic spring cereals

In Northern Europe, inter-row hoeing has become a popular tactic for controlling weeds in organic cereals. Hoeing is highly effective and can be implemented from crop emergence until stem elongation to maintain a nearly weed-free inter-row zone. However, hoeing has a lesser effect on weeds growing in the intra-row zone, where crop–weed proximity results in heightened competition. In the hoed cereal system, it is investigated whether tall-growing, competitive, cruciferous weeds in the intra-row zone affect crop biomass, yield and thousand kernel weight (TKW). An additive experimental design is employed to enable the fitting of rectangular hyperbolas, describing and quantifying the effects of increasing intra-row surrogate weed density on crop growth parameters. Regressions were studied under the influence of crop (spring barley and spring wheat), row spacing (narrow [12.5 or 15.0 cm] and wide [25.0 cm]) and nitrogen rate (50 and 100 kg NH₄-N/ha). Cruciferous surrogate weeds were found to impact crop yield and quality severely. For example, ten intra-row plants/m² of surrogate weed Sinapis alba reduced grains yields by 7%–14% in spring barley and by 7%–32% in spring wheat with yield losses becoming markedly greater in wheat compared to barley as weed density increases. Compared to wheat, barley limited yield and quality losses and suppressed intra-row weed growth more. Row spacing did not have a consistent effect on crop or weed parameters; in one of six experiments, the 25 cm row spacing reduced yields and increased intra-row weed biomass in wheat. Nitrogen rate did not affect crop or weed parameters. Results warrant the implementation of additional tactics to control intra-row weeds and limit crop losses.     

Critical period of weed competition in three vegetable crops in relation to management practices

The critical period of weed competition was determined in three vegetable crops: early cabbage (Brassica oleracea var. capitata L.), pickling cucumbers (Cucumis sativus L.), and field-seeded processing tomatoes (Lycopersicon esculentum L.). There were significant interactions between weed-removal treatments, year, and row width. Cabbage yields were reduced if plots were not kept weed-free for at least 3 weeks after transplanting or if weeds which emerged with the crop were allowed to remain longer than 4–5 weeks, Cucumber yields were reduced if plots were not kept weed-free for up to 4 weeks after seeding or if plots remained weed-infested longer than 3–4 weeks. Higher crop population densities (narrower row widths) in cabbage and cucumbers resulted in smaller plants, earlier competition from weeds, and therefore a shorter period that the crop could remain weed-infested without suffering reduced yields. Yields of direct-seeded tomatoes were reduced if plots were not kept weed-free for up to 9 weeks after seeding or if weeds which emerged with the crop were allowed to remain longer than 5 weeks. In each crop the timing of the critical period of competition was verified by weed removal only during this interval. There was a true critical period in direct-seeded tomatoes, but not in cabbage or cucumbers where a single weeding was sufficient to prevent yield losses.     

Chemical weed control in rainfed upland rice in Nigeria

Abstract

Two field experiments on chemical weed control in Faro 41 upland rice (Oryza sativa L.) variety were conducted at a rainforest site near the National Cereals Research Institute, Amakama Substation in 1989 and 1990. The herbicides tested were a coformulated mixture of pretilachlor and dimethametryne at 1.5, 2.0 and 2.5 kg a.i./ha, a co‐formulated mixture of piperophos and propanil applied at 3.16 and 3.95 kg a.i./ha, and Oxadiazon at 1.0 kg a.i./ha. Two hand weedings and a ‘no weeding’ treatment were included. The herbicides were applied 5 days after planting. All were safely selective to the crop. Most provided adequate weed control through 8–12 weeks after planting. Weeds controlled included Cynodon dactylon, Commelina benghalensis, Dlgitaria horizontalis, Eleusine indica, Panicum maximum and Pennisetum purpureum as grass weeds. The broadleaf weeds were Emilia sonchifolia, Ageratum conyzoides, Portulaca oleraceae, Richardia brasiliensis and Ipomoea Involuncrata. The sedges Mariscus alternifolius and Cyperus esculentus were encountered in the plots. Pretilachlordimethametryne at 2.5 kg a.i./ha had the best weeding score. Twoyear average grain yields of 1.7–2.6 t/ha were obtained from the herbicide treatments. With only 0.4 t/ha from the unweeded treatment, yield losses of more than 80% were recorded.     

Competition from weeds and their control in direct-seeded rice

In direct-seeded upland rice (Oryza sativa L.) yield reductions due to weed competition ranged from 42 to 65% in field experiments conducted in eastern Utter Pradash, India. The most critical period, when crop losses due to weed competition were most severe, ranged from 10 to 20 days after emergence. Yields generally continued to increase, however, as the length of the weed-free period increased. The weed flora consisted of various grasses, sedges and broadleaved species. The most effective herbicide treatment evaluated was a pre-emergence application of alachlor followed by a post-emergence application of propanil or one mechanical weeding.     

Soybean yield and yield components as influenced by canopy heights and duration of competition of velvetleaf (Abutilon theophrasti Medik.)*

Soybean yields, pod and branch numbers were inversely related to the height of velvetleaf (Abutilon theophrasti). This effect was attributed to increasing interception of light with increasing height of A. theophrasti. Reduction; in soybean yield and yield components were greater if A. theophrasti was maintained at various heights for only 3 weeks rather than 6 weeks following soybean emergence. This effect was attributed to a longer duration of light competition by the weed. Even when the A. theophrasti canopy was kept 25% below that of soybean for 4 weeks, soybean yields, pod and branch numbers decreased when the weed was allowed to regrow during the remainder of the season, but not if the weed was removed after the initial 4-week clipping period. No reduction in seed yields, pod and branch numbers was noted if A. theophrasti plants 25% shorter than soybeans were removed or allowed to resume growth after the initial 6 or 8 weeks of clipping. Soybean seed size was slightly affected by A. theophrasti canopy height and duration of competition.     

A simple model of crop loss by weed competition from early observations on relative leaf area of the weeds

A new simple empirical model for early prediction of crop losses by weed competition was introduced. This model relates yield loss to relative leaf area of the weeds shortly after crop emergence using the relative damage coefficient q as the single model parameter. The model is derived from the hyperbolic yield density relationship and therefore accounts for the effects of weed density. It is shown that the model also accounts for the effect of different relative times of weed emergence. A strong advantage of the approach is that it can be used when weeds emerge in separate flushes. The regression model described experimental data on sugar-beet – lambsquarters (Beta vulgaris L. –Chenopodium album L.) and maize-barnyard grass (Zea mays L. –Echinochloa crus-galli L.) competition precisely. The model describes a single relationship between crop yield loss and relative leaf area of the weeds over a wide range of weed densities and relative times of weed emergence. Possibilities for scientific and practical application of the model are discussed.     

Survey and Assessment of the Natural Enemies of Water Hyacinth,Eichhornia crassipes

Abstract

In northern Queensland, the addition of 2,4,5‐T butyl ester was found to be unnecessary to maintain the control of Echinochloa colona (L.) Link and Cyperus Iria L. In dry seeded rice when propanil rates were reduced below the registered rate of 4 kg a.i. ha^?1. Adequate weed control was obtained with 1.3 kg a.i. ha^?1 propanil alone. No adverse effects on rice yield were found with any of the propanil × 2,4,5‐T treatments. Low rates of propanil, 1.3 and 0.72 kg a.i. ha^?1, compared favourably with pre‐emergence treatments of thiobencarb, butachlor, oxyfluorfen and pretilachlor (plus a safener) when weed yields were low. Where water management was poor and Ischaemum rugosum Salisb. was the dominant weed, oxyfluorfen applied pre‐emergence at 0.96 kg a.i. ha^?1 produced a higher rice and a lower weed yield than the low rates of propanil. In three of the five experiments, weed growth was insufficient to depress rice yields significantly.     

Integrated pest-management approaches in orchard,cereal and potato protection in Poland1

With its total area of 312,683 km² and a population of about 39 million people, the area of agricultural land per person in Poland is about 0.47 ha. The yields of basic crops are rather low and the losses due to pathogens, animal pests and weeds are high. The use of pesticides is about 1.0–1.3 kg a.i. per ha and this gives a good starting point for integrated pest management (IPM) in the production system of various crops. IPM in glasshouse crops reached the level of 500 ha per year. An IPM approach against codling moth (Cydia pomonella) in orchards allowed a reduction by 70% in the number of insecticide treatments. In the case of apple scab (Venturia haequalis), the IPM approach allowed a reduction in the number of fungicide treatments by 30% on an area of 100,000 ha. In respect to small grains, the IPM approach concerns rational use of fungicides and herbicides in winter wheat and barley. In potato protection, the IPM approach mainly concerns Colorado beetle (Leptinotarsa decemlineata), which at present is controlled by chemical insecticides on 77% of the potato area. Special research and demonstration IPM programmes sponsored by USDA/AID in Central and Eastern Europe will be operational during 1992/1995 and will concern orchards (apple, pear), small grains (wheat, barley) and potatoes.     

Dissemination of weeds with rice seedlings

Abstract

Dissemination of weed seeds, weed seedlings, and shoots of perennial weeds along with transplanted rice (Oryza sativa L.) seedlings was observed in farmers’ fields in Guimba, Nueva Ecija, Philippines. The percentage of weed seedlings and shoots in bundles of rice seedlings to be transplanted into the field ranged from 4–5 to 14.2% in the wet season and from 3.0 to 8.7% in the dry season. Grasses comprised 63% of the weed seedlings in the wet season and 81% in the dry season. The number of weed seeds disseminated with the rice seedlings ranged from 271 to 683 per bundle. Mainly sedges and broadleaf weeds were disseminated in this manner. Possible ways of reducing dissemination of weeds with rice seedlings are suggested.     

Radar Studies of Insects

Abstract

Yield and weed growth in dry-seeded rice (Oryza sativa L.) were unaffected by method of seeding. Weeds depressed rice yields by 95%. Fourteen weed species were found growing in association with dry-seeded rice at maximum flowering of the weeds. Echinochloa colona (L.) Link and Leptochloa chinensis (L.) Nees were the major weeds. A dramatic change in the weed flora was observed in transplanted rice planted immediately after the harvest of the dry-seeded crop. Only five species grew in association with the transplanted crop. Monochoria vaginalis (Burm.) Presl. dominated. E. colona and L. chinensis were present in transplanted rice but were only minor weeds. The number of weeds in the transplanted crop was less than 50%, and weighed only 5% as much as those in the dry-seeded crop. There was no carryover of the weed control treatments from the dry-seeded crop to the transplanted crop.     

Crop residue retention suppresses seedling emergence and biomass of winter and summer Australian weed species

Crop residue retention could affect the emergence and biomass of weeds in different ways. A summer and winter pot study was conducted to evaluate the effect of different amounts of sorghum and wheat residue on the emergence and biomass of 12 summer and winter Australian weeds. The equivalent amount of sorghum residue to 0, 1, 2, 4 and 6 t/ha was used in the summer study and winter weed seeds were covered with wheat residue equivalent to the amount of 0, 1, 2, 4 and 8 t/ha in the winter study. The emergence and biomass of Amaranthus retroflexus and Echinochloa colona was not affected by sorghum residue treatment. For other summer weeds, the use of the 6 t/ha sorghum residue treatment resulted in 59–94% reductions in biomass compared to no‐sorghum residue retention. Similarly, the application of 8 t/ha wheat residue in the winter study resulted in a reduced biomass of 15–100% compared to no‐crop residue treatment. The results demonstrated the high potential of using crop residues in eco‐friendly weed management strategies, such as harvest weed seed control tactics.     

The influence of rice plant type on the effect of weed competition on Oryza sativa and Oryza glaberrima

Detailed studies were conducted on three rice cultivars – IG10, an Oryza glaberrima Steudel; Moroberekan, a traditional Oryza sativa L.; and IDSA6, an improved O. sativa L. – in conditions with and without competition from weeds. IG10 suffered less from competition with weeds and suppressed weeds better than the O. sativa cultivars. IG10 accumulated more biomass, produced more tillers, established a higher leaf area index, had higher specific leaf area and, in the earlier stages of growth, partitioned more of its biomass to leaves than the O. sativa cultivars. Without competition from weeds, grain yields did not significantly differ between the cultivars, but IG10 produced higher yields in competition with weeds. The implications of these findings are discussed in relation to the development of rice plants with good weed suppression characteristics.     

Critical periods of weed competition in cotton in Greece

Four experiments were conducted in central Greece during 1997 and 1998 to determine the late-season presence of weeds in cotton (Gossypium hirsutum L.) and the critical times for removing weeds. Experiments were conducted in natural, heavily infested cropland. The presence of weeds for more than 3 weeks after crop emergence caused significant reductions in crop growth and lint yields. However, weeds that emerged 11 weeks or more after crop emergence did not adversely impact yields. Total weed biomass increased with increasing time prior to weed removal. A weed-free period of 11 weeks after crop emergence was needed to prevent significant reductions in cotton height, biomass, number of squares, and yield. These results indicated that postemergence herbicides or other control measures should be initiated within 2 weeks after crop emergence to avoid significant yield reduction. For greater efficiency, soil-applied herbicides in cotton should provide effective weed control for at least 11 weeks. Curvilinear regression equations were derived to describe the relationship between critical periods of weed presence and cotton growth and fruit development.     

Germination ecology of Leptochloa chinensis: a new weed in the Italian rice agro-environment

Leptochloa chinensis is a new weed that has been found with increasing frequency in Italian rice paddies. The germination ecology of L. chinensis seeds was studied in order to investigate the development mechanisms and survival strategy of this weed in rice paddies of northern Italy. Leptochloa chinensis seeds showed no dormancy and exhibited germination even in anoxic conditions. Germination was strongly influenced by temperature (minimum around 15°C; optimal 25–35°C) and light (phytochrome dependent). Temperature fluctuation caused an increase of seed germination in the dark. Seed burial also strongly inhibited germination and emergence of this species. At 5 cm seed burial only 5% of seedlings emerged in flooded conditions, while at the same depth, but with no flooding, no seedling emergence was observed. This phenomenon was not due to oxygen depletion, as germination was not inhibited by complete anoxia, as demonstrated by the fact that some seedlings did emerge in flooding conditions when water was no deeper than 6 cm. Seed burial and concomitant flooding induced an unusual germination: first coleoptile emergence and subsequently emergence of the radicle was observed. The possible exploitation of this knowledge for weed management is discussed.     

A mathematical model to predict the population dynamics of Oryza sativa var. sylvatica

A mathematical model has been developed for the prediction of the population dynamics of Oryza sativa L. var. sylvatica . The input variables included in the model were the seedbank composition; the number of panicles per plant and seeds per panicle; the rate of shattering; the seed longevity and the number of weed seeds contained in the sown grain; the type of soil tillage (ploughing, minimum and No tillage); the weed control efficacy; and the predation. The output variables were the number of seedlings that emerged from different depths and the seedbank evolution. The model relies on probability matrices that predict the vertical movement of the seeds after different soil tillage practices. Sensitivity analysis showed that the weed control efficacy and number of grains per panicle were the parameters that had the highest influence on the development of the weed population. The model evaluation was carried out by comparing the predicted with the observed seedling emergence at different seedbank values and under different soil tillage conditions. The model performance showed a tendency to overestimate seedling densities. The agreement between the estimated and experimental data was closely related to the accuracy of the input values of the seed distribution along the soil profile.     

The influence of the nematode Meloidogyne incognita on competition between Solanum nigrum and tomato

The effect of Meloidogyne incognita (Kofoid & White) Chitwood, 1949, on the competitive relationship between tomato (Lycopersicon esculentum Mill.) and black nightshade (Solanum nigrum L.) was investigated under glasshouse conditions. Two intraspecific competition treatments were set up for the crop and the weed, and five interspecific treatments where the emergence of S. nigrum plants was progressively delayed in relation to tomato. Nematodes reproduced in all inoculated plants, their multiplication rates being much higher in tomato than in S. nigrum plants. Under nematode-free conditions, intraspecific competition of tomato was more severe than the interspecific competition shown by the weed S. nigrum. Tomato was as tronger competition with S. nigrum than the weed was with itself. However, when infested by M. incognita, both species displayed a similar competitive ability. Tomato yield losses increased with prolonged weed competition but were greater under nematode-infested conditions.     

Prediction of Heteranthera reniformis competition with flooded rice using day–degrees

In field studies carried out in 1992 and 1993, several durations of growth of Heteranthera reniformis Ruitz et Pavon were established in flooded rice (Oryza sativa L.) to evaluate weed influence on crop yield using day-degree predictive models for weed growth. H. reniformis was allowed to emerge at 7-day intervals from rice emergence until 49 days later. Weeds that emerged with the rice accumulated 403 day-degrees during the first 49 days. Weeds allowed to utilize 308 day-degrees had a dry weight reduction of 20% compared with plants that emerged with the rice. With the loss of all 403 day-degrees dry weight was reduced by about 95%. Maximum leaf area index (LAI) was 2.8, reached at 308 day-degrees. Weed density ranged from 48 to 5 plants m^-2 when all 403 day-degrees were accumulated or lost by the plant respectively. When the weed lost only 95 day-degrees out of the possible 403 (1 week's delay in emergence) rice yield was 34% and 39% lower in 1992 and 1993 respectively. When the weed was allowed to accumulate ail 403 day-degrees yield reduction reached 62% in 1992 and 68% in 1993, very similar to the control plot, which were maintained weedy throughout (64% and 70.5%). After an accumulation of 403 day-degrees the weed reduced the number of rice panicles by 45% in 1992 and 38% in 1993.