Total Weed Density Research Articles

MANEJO DA PALHA DE AVEIA PRETA SOBRE AS PLANTAS DANINHAS E RENDIMENTO DE SOJA EM SEMEADURA DIRETA

A aveia preta é cultivada durante o inverno numa expressiva área do Rio Grande do Sul, proporcionando uma cobertura densa, a qual exerce efeitos sobre o crescimento das plantas daninhas e no rendimento da cultura da soja. O manejo da palha da aveia é bastante variável, sendo determinado mais em função da disponibilidade de equipamentos do que pelo seu efeito na palha. Objetivou-se, através deste estudo, avaliar os efeitos de diferentes manejos da palha de aveia preta (Avena strigosa Schreb.) sobre as plantas daninhas e no rendimento de grãos de soja no sistema de semeadura direta. Um experimento foi instalado em área da Universidade Federal de Santa Maria, em Santa Maria, RS, Brasil, num solo ARGISSOLO VERMELHO Distrófico arênico. O delineamento experimental foi blocos ao acaso com parcelas subdivididas com cinco repetições. Nas parcelas principais, foram distribuídos os manejos de palha: (i) palha picada e distribuída; (ii) palha em pé; (iii) palha rolada; (iv) palha gradeada ; (v) palha roçada; (vi) sem palha, sem invasoras e (vii) sem palha, com invasoras. Nas subparcelas, foram alocados os tratamentos com e sem controle químico em pós-emergência das plantas daninhas. Determinaram-se densidade e massa seca das plantas daninhas, umidade e rendimento de grãos de soja, e altura de inserção do primeiro legume. Os manejos da palha de aveia preta testados não afetaram a densidade de plantas de papuã e de plantas daninhas em geral emergidas até os 15 dias após a semeadura. O rendimento de grãos nos manejos palha picada e distribuída, palha em pé, palha roçada e sem palha sem invasoras não diferiram da opção de controle das plantas daninhas em pós-emergência, demonstrando que as plantas desses manejos foram mais eficientes em conviver com as plantas daninhas que as plantas dos demais. Os manejos palha rolada, palha gradeada e sem palha com invasoras proporcionaram incremento no rendimento de grãos de soja com a opção de controle das plantas daninhas em 31, 37 e 76%, respectivamente. Os tipos de manejos da palha de aveia preta não afetaram a altura de inserção do primeiro legume nem a umidade dos grãos na colheita. Os manejos da palha de aveia preta não alteraram a densidade de plantas daninhas, mas a presença de palha exerceu efeitos supressivos sobre as mesmas, reduzindo a massa seca de papuã por unidade de área e do número de plantas daninhas, com exceção do manejo palha gradeada.

LONG-TERM EFFECTS OF FERTILIZER APPLICATION AND THREE CROP ROTATIONS ON CHANGES IN THE WEED SPECIES IN THE 68 TH CROPPING (AFTER 26 YEARS)

A field study was made at Tamil Nadu Agricultural University, Coimbatore, India during 1998 in a long-term fertilizer experiment to assess the influence of long-term manure and fertilizer schedules on weed flora changes in anirrigated finger millet–maize– cowpea cropping system. The weed flora of the experimental field, observed in the unmanured plot at 60 days after the transplanting (DAT) of finger millet (the 68 th crop in the sequence), consisted of nearly equal proportions of broad-leaved weeds (46.7%) and grasses (43.5%), while the sedge population was 9.8% of the total weed density. Trianthema portulacastrum (Linn.), Digera arvensis (Forsk.) and Cynodon dactylon (L.) Pers., were the most dominant weed species in fertilized plots, while in unfertilized plots, Digera arvensis (Forsk.), Cynodon dactylon (L.) Pers. and Flaveria australasica Hook dominated. Maximum weed growth was recorded in the plot fertilized with the recommended dose of N alone and the value was statistically at par with N, P and K ...

Separating the effects of crop rotation from weed management on weed density and diversity

Crop rotation is thought to reduce weed density and maintain species diversity, thus preventing the domination of a few problem weeds. Because cropping sequence dictates other agricultural management practices, variations in weed populations between cropping systems may be the direct result of crop rotation, the result of different weed management practices associated with crop rotation, or both. Studies that fail to separate the effects of crop rotation from weed management may generate misleading results. A 10-yr crop rotation study was undertaken to study the dynamics of the standing weed vegetation inZea maysL.,Glycine maxL., andTriticum aestivumL. The present paper compared total weed density and diversity between monocultures and rotations under three levels of weed management. Weed management accounted for 37.9% of the variation in total weed density, whereas crop rotation accounted for only 5.5%. Weed density varied between monocultures and rotations in plots where herbicides were applied. The effectiveness of rotations in reducing weed density was dependent upon the crop. Margalef's species richness index (DMG), a measure of diversity, varied among weed management strategies, with 38.4% of the variance attributed to this factor. In the 10th year, when all plots were sown withZ. mays, few cumulative effects of crop rotation were apparent, with two exceptions. In weedy and herbicide-treated plots, weed density was higher on plots cropped withZ. maysthe previous year. Also, under these weed management treatments, including a cereal in the crop rotation reduced weed density. Crop rotation, when used in combination with herbicides, provides additional weed control and is therefore an effective tool in integrated weed management.

Probability Maps for Risk Asessment in a Patchy Weed Control

Weeds were monitored during April 1996 along a 36 m×36 m grid within a 21 ha winter wheat field to detected whether black bindweed (Fallopia convolvulus L.), cleavers (Galium aparine L.), which are important weeds in cereals, and total weed densities reached economical thresholds for control. The geostatistical method of block kriging with indicator values (1 if below the threshold and 0 otherwise) was applied to estimate probabilities that weed intensity in 324 m2 areas was below the threshold. The probabilities could be used to assess the risk that would occur if those areas were not sprayed with herbicides. Because indicator variograms showed a autocorrelation structure of the data the influence of a reduction of sampling points on the reestimation of the probabilities was tested. The coincidence of the probabilities obtained by the reduced data with those estimated with the complete data set was evaluated by correlation (r) and mean relative error (MRE). The loss of information due to the reduction of sampling points to one half was not substantial. Correlations coefficients were high (black bindweed, 0.78; cleavers, 0.82; total weeds, 0.77) with low MRE's. A satisfactory reestimation with one-fourth of the sample points was not possible.

Weed communities of winter wheat as influenced by input level and rotation

SummaryThe influence of cropping systems constituted by the combination of three input levels (low, intermediate and high) and four 2‐year rotations between a spring summer crop and winter wheat (Triticum aestivum L.) on the density and composition of the winter wheat weed flora was evaluated from 1989 to 1992. Spring summer crops were maize (Zea mays L.). soyabean (Glycine max (L.) Merr.). sugar beet (Beta vulgaris L.) and sunflower (Helianthus and L.) Total weed density at the end of the wheat tillering stage ranged from 2.5 to 128.7 plants m‐2 and generally decreased with cropping intensity, although to a differing extent, depending upon rotation. Differences among weed communities of the 12 cropping systems were mostly dependent upon input level. The effect of the preceding crop on the weed flora composition of winter wheat was evident only with reduced herbicide use (low‐input systems). Weed species could be ascribed to four categories: (a) species sporadically associated with cropping systems (most of the weeds); (b) species associated with a specific input level (e.g. Papaver rhoeas L. to low and intermediate input levels and Veronica persica Poiret to the high input ievel): (c) species associated with a specific rotation (e.g. Anm majus L. to sugar beet winter wheat): and (d) species associated with specitie plots that become important every other year (e.g. Convolvulus arvensis L. and Stellaria media (L.) Vill.).

The influence of seeding rate on weed control in small-red lentil (Lens culinaris)

Experiments were conducted at two sites for 2 yr in the Pacific Northwest dryland cropping region to determine if seeding rate of small-red lentil could enhance weed control with herbicides and increase lentil seed yield. At Pendleton, OR, and LaCrosse, WA, lentil was planted at 22 or 44 kg ha−1in one direction in all plots. In one-half of the plots, lentil was cross-seeded at right angles with an additional 22 kg ha−1to provide seeding rates of 22, 44, 22 + 22, and 44 + 22 kg ha−1. Seeding rate main plots were split into three herbicide treatments and an untreated control. Total weed density was reduced by increasing seeding rate at Pendleton both years when averaged over all herbicide treatments. Seeding rate reduced total weed density to a greater extent when herbicides did not adequately control weeds or when herbicides were not applied at Pendleton in 1992. Increased seeding rate also reduced total weed dry weight at Pendleton in 1992 and 1993 and at LaCrosse in 1993. The suppressive effect of increased seeding rate on weed dry weight was more evident when herbicides were not used or when herbicides gave only partial control. Herbicides generally reduced weed density, but the effectiveness of individual treatments was related to the weed species present and environmental conditions present in each experiment. Lentil aboveground dry weight production increased with seeding rate at both locations; however, only in 1 yr did lentil seed yield increase with seeding rate. The primary benefit from increased seeding rate in this study was to reduce weed density and dry weight.

Weed Control and Sweet Corn (Zea maysvar.rugosa) Response in a No-till System with Cover Crops

Studies were conducted at the Main Agricultural Experiment Station in Fayetteville and the Vegetable Substation in Kibler, Arkansas, in 1992 and 1993 on the same plots to evaluate weed suppression by winter cover crops alone or in combination with reduced herbicide rates in no-till sweet corn and to evaluate cover crop effects on growth and yield of sweet corn. Plots seeded to rye plus hairy vetch, rye, or wheat had at least 50% fewer early season weeds than hairy vetch alone or no cover crop. None of the cover crops reduced population of yellow nutsedge. Without herbicides, hairy vetch did not suppress weeds 8 wk after cover crop desiccation. Half rates of atrazine and metolachlor (1.1 + 1.1 kg ai ha−1) reduced total weed density more effectively in no cover crop than in hairy vetch. Half rates of atrazine and metolachlor controlled redroot pigweed, Palmer amaranth, and goosegrass regardless of cover crop. Full rates of atrazine and metolachlor (2.2 + 2.2 kg ai ha−1) were needed to control large crabgrass in hairy vetch. Control of yellow nutsedge in hairy vetch was marginal even with full herbicide rates. Yellow nutsedge population increased and control with herbicides declined the second year, particularly with half rates of atrazine and metolachlor. All cover crops except hairy vetch alone reduced emergence, height, and yield of sweet corn. Sweet corn yields from half rates of atrazine and metolachlor equalled the full rates regardless of cover crops.

Performance of reduced herbicide doses in spring cereals

The consequences of dose reduction of three new herbicide formulations were studied for the control of annual broad-leaved weeds in fields of spring barley (Hordeum vulgare L.) and spring wheat (Triticum aestivum L.). The herbicide formulations were MCPA/mecoprop-P, MCPA/dichlorprop-P and MCPA/fluroxypyr. The efficacy of the lowest recommended dose and a 30% lower rate were tested and compared with the reference herbicide tribenuron-methyl. Trials were conducted at seven sites for three years. Considerable annual fluctuations in weed infestation were recorded. Although the dose reduction occasionally caused considerable decline in control (on %-scale), suppression of weed biomass was still satisfactory in most of the trials. On average, a 75% reduction of weed biomass in spring barley and an 83% reduction in spring wheat were achieved with reduced herbicide doses. Use of reduced herbicide doses for three years in the same fields caused neither a significant increase in weed infestation nor changes in the species composition of weed populations compared with treatments at recommended rates of application. There was a significant difference in biomass production between weed species. Consequently, the total biomass production of annual dicotyledonous weeds correlated only weakly (r=0.48) with the total weed density. Even in untreated plots the weed biomass at harvest constituted, on average, only 3.1-3.6% of the total vegetative biomass of crop stands. Thus, the crop yield responses to chemical weed control remained low.

Cropping History, Tillage, and Herbicide Effects on Weed Flora Composition in Irrigated Corn

AbstractA change from conventional tillage to a conservation tillage system can lead to shifts in weed species composition. A weed species shift can result in the emergence of weeds tolerant of existing weed management practices. An understanding of crop production effects on weed species shifts can lead to development of improved weed management strategies. Research evaluated the effects of primary tillage (moldboard and chisel plowing), row cultivation, and herbicide input level on weed species changes over a 5‐yr period in three irrigated cropping sequences. The cropping sequences consisted of continuous corn (Zea mays L.) for 5 yr (CN), pinto bean (Phaseolus vulgaris L.) for 3 yr followed by corn for 2 yr (PB), and sugar beet (Beta vulgaris L.) for 2 yr followed by corn for 3 yr (SB). Over the course of the 5‐yr study, total weed density increased from 1 to 245 weeds m−2 in PB, from 100 to 209 in SB, and from 2 to 190 in CN cropping sequences in chisel‐plowed treatments, while weed density increases in moldboard‐plowed treatments were negligible. General observation of each cropping sequence indicated that during the final year, green foxtail [Setaria viridus (L.) P. Beauv.] was most prevalent in the CN sequence, redroot pigweed (Amaranthus retroflexus L.) and hairy nightshade (Solanum sarrachoides Sendtner) in the PB sequence, and redroot pigweed and green foxtail in the SB sequence. However, weed species differences due to cropping sequences were evident only in treatments receiving chisel plow primary tillage. Further, the increase in redroot pigweed density due to cropping sequence and chisel plowing was diminished by high herbicide input levels. Row cultivation also had an influence on the weed species composition when compared to uncultivated plots.

Weed Suppression by Live and Desiccated Hairy Vetch (Vicia villosa)

Hairy vetch was grown as a winter annual cover crop and evaluated for weed suppression when desiccated by paraquat or left alive until natural senescence in a 3-yr field experiment. Total weed density and biomass were variable in the desiccated hairy vetch treatment relative to a bare soil treatment but were consistently lower in the live hairy vetch treatment relative to the desiccated or bare soil treatments. An average of 87% of sites under live hairy vetch compared to 8% of sites under desiccated hairy vetch transmitted less than 1% of unobstructed sunlight. The red (660 nm) to far-red (730 nm) ratio of transmitted light was reduced by 70% under live hairy vetch compared to 17% under desiccated hairy vetch. Daily maximum soil temperature and diurnal soil temperature amplitude were reduced by live hairy vetch > desiccated hairy vetch > bare soil. Soil moisture content was greater under both live and desiccated hairy vetch compared to bare soil during droughty periods. Changes in light extinction, red to far-red ratio, and diurnal soil temperature amplitude were sufficient to explain greater weed suppression by live than desiccated hairy vetch.

Response of Weeds to Tillage and Cover Crop Residue

Total weed density increased after 1 yr of no-tillage and after 2 yr of conventional tillage in a 4-yr experiment with repeated assignment of the same treatment to the same plots. Large crabgrass, goosegrass, and carpetweed densities were higher in the no-tillage compared with the conventional-tillage treatment in at least 1 yr whereas common lambsquarters density was greater in the conventional-tillage treatment the last year of the experiment. Within the no-tillage treatment, rye or hairy vetch residue reduced total weed density an average of 78% compared to the treatment without cover crop when cover crop biomass exceeded 300 g m–2and when residue covered more than 90% of the soil. Goosegrass, stinkgrass, and carpetweed densities were reduced by cover crop residue in at least 1 yr whereas large crabgrass was unaffected. Common lambsquarters density increased where rye was grown as a cover crop prior to conventional tillage. Despite differences in weed density among treatments, weed biomass was equivalent in all treatments during the last 2 yr.

Weed Management Decisions in Corn Based on Bioeconomic Modeling

A fixed (conventional) weed management strategy in corn was compared to three other strategies (two mixed and one flexible) in terms of weed control, grain yield, gross margin (gross income minus herbicide treatment costs), and herbicide use under furrow irrigation for four consecutive years. The fixed strategy prespecified preplanting, preemergence, postemergence, and layby herbicides. The flexible strategy herbicide treatments were specified by a computer bioeconomic model. Model decisions were based on weed seed in soil before planting, weed densities after corn emergence, herbicide costs, expected corn grain yield and selling price, and other parameters. The two mixed strategies were a combination of fixed and flexible strategies and designated either specified soil-applied herbicides (mixed/soil), or no soil-applied herbicide (mixed/no soil); postemergence treatments were determined by the model. Average corn grain yield was 10 280 kg ha–1and gross income was 920 $ ha–1and neither differed among strategies. Total weed density and gross margin were significantly higher for the mixed/no soil and flexible strategies compared to the mixed/soil and fixed strategies. Total weed density averaged 28 720, 28 100, 10 910, and 680 plants ha–1for the mixed/no soil, flexible, mixed/soil, and fixed strategies, respectively. Annual gross margins for the four strategies averaged 885, 875, 845, and 810 $ ha–1, respectively. Herbicide use over the 4-yr period for these four strategies averaged 3.8, 5.3, 20.5, and 26.9 kg ha–1, respectively, and each value differed from the other. Thus, weeds can be managed in corn, gross margins increased, and herbicide use decreased by employing a bioeconomic weed-corn model to make weed management decisions.

Efeito do metribuzin no controle das plantas daninhas e na produção de grãos em Glycine max (L ) merrill

No ano agrícola 1985/ 86, em Viçosa-MG, foi instalado um ensaio de campo em solo Pdzólico Vermelho-Amarelo argiloso e com 2,9% de matéria orgânica, objetivando estudar o efeito das doses de metribuzin (0,0; 0,35; 0,70 e 1,05 kg i.a.ha-1) no controle de plantas daninhas e na produtividade da soja (Geycine max (L.) Merri ll, cv. 'Uber aba'). A maioria das monocotiledôneas que ocorreram na area experimental foi represent ada por Cyperus rotundus L., Brachiaria plantaginea (Link) Hitch. e Cynodon dactylon (L.) Pers., tendo-se verificado somente redução em Brachiaria planta taginea em virtude do aumento das doses de metribuzin, ocorrendo o mesmo com relação às dicotiledôneas que se fizerem presentes no experimento, com exceção de Oxalis Oxyptera Prop., que não foi controlada nas doses utilizadas. A densidade total médias das invasoras, menos Cyperus rotundus , Oxalis oxyptera e Cynodon dactylon, foi de 141; 124; 62 e 59 plantas . m-2, respectivamente, para as doses de 0,0; 0,35 ; 0,70 e 1,05 kg i.a.ha-1 de metribuzin. A dose de 0,35 kg i.a. de metribuzin.ha-1 foi suficiente para promover a redução da matéria seca da parte aérea das plantas daninhas com a mesma eficiência de controle da dose de 1,05 kg i.a .ha-1 Entretanto, a densidade total médil das invasoras foi reduzida sig nificativamente nas doses de 0,70 e 1,05 kg i.a. de metribuzin.ha-l. O efeito do metribuzin na soja foi evidenciado somente na dose de 1,05 kg i.a.ha-1, com injúria foliar (clorose) leve ocorrida até 25 dias, aproximadamente, apôs a emergência das plântulas. Após esse período, houve total recuperação de todas as plantas de soja submetidas a essa dose. A produção de grão se o índice de colheita não foram influencia dos significativamente pelas doses de metribuzin.

Dicotyledonous weeds of spring cereal crops in north‐east Scotland

SummaryA survey of dicotyledonous weeds of spring cereals was conducted prior to herbicide application in 1985 in six districts of north‐east Scotland, which involved a total of 113 fields. Results were similar to those from an earlier survey in 1973. The main species remained the same but in four of the six districts, total weed densities were lower. Stellaria media was the most common species except in Upper Banffshire and Morayshire where Galeopsis spp. and Polygonum aviculare, respectively, were predominant. The main changes apparent were a decline of Spergula arvensis and Polygonum persicaria, and an increase in Matricaria perforata and Viola arvensis in several districts.