Weed-free Period Research Articles

Competitive Ability of Santa-Maria (Parthenium hysterophorus L.) with Spring Maize

ABSTRACT: In recent decades, allelopathy has gained considerable attention in sustainable agricultural systems. In the current situation, an attempt has been made to investigate the optimal competition period for weed control in maize. Experiments were designed in randomized complete block design and Parthenium hysterophorus was naturally infested in experimental plots during the spring season 2014 to determine the critical competition period for weed control in maize. The relations between grain yield and different competition durations were determined via regression analyses. The experiment comprised seven treatments viz, control treatment, competition for 30, 45, 60, 75, 90 and 105 days after emergence. The results suggested that a weed-free period (control treatment) of maize was enough to provide acceptable grain yield. Whole season different competition periods and weed-free plots were included in the experiment for yield comparison. The highest grain yield was obtained from plot kept weed-free (control treatment). The results clearly indicated that P. hysterophorus was competitively more aggressive and its control in maize crop provide maximum grain yield.

INFLUENCE OF WEED REMOVAL AND INFESTATION PERIODS ON MAIN INSECTS INFESTING COMMON BEAN AND THEIR NATURAL ENEMIES AND PRODACTIVITY AT SOHAG GOVERNORAT

Two field experiments were conducted at the farm of Shandweel Agricultural Research Station, Sohag Governorate during seasons of 2016 and 2017 in summer plantation to investigate the effect of weed removal and infestation periods on major insect pests, their associated predators, and productivity of common bean. Eight weed removal and infestation periods were studied in complete randomized block design. In the plots of weed-free, the weed removed for 3, 6, 9, and 12 weeks after emergence, then weeds were left to the end of the growing season. For weed-infested plots, the weed was left to compete with the crop for 3, 6, 9, and 12 weeks after emergence, and then removed to the end of the growing season. Data revealed that the weeding systems affected significantly the populations of Bemisia tabaci (Genn.), Empoasca decipiens (Paoli), Aphis craccivora Koch., Thrips tabaci Lind., Coccinella undecimpunctata L. and Chrysoperla carnea (Steph.) in both seasons of the study. While no significant effect was found for Liriomyza trifolii Burg. in both seasons. It could be included that B. tabaci infestation increased with decreasing weeding frequency. In contrast, E. decipiens and A. craccivora infestation decreased with decreasing weeding frequency. C. undecimpunctata and Ch. carnea increased in weedy common bean plots than free plots. Fresh weight and dry matter of total annual weeds reduced significantly by increased weed-free period, but the previous traits increasing by increasing weed competition period. In contrast, with 100 seeds weight and seed yield per feddan, the yield increased significantly by increasing weed-free period, but decreased by increasing weed competition period.

Assessment of Weed Competition Critical Period in Sugar Beet

Weed control constitutes the most essential issue in cropping systems. The critical periods should especially be determined for each crop. Field experiments were conducted during the seasonal growing periods of sugar beet in 2012 and 2013 in Kayseri, Turkey to assess the critical weed control period (CPWC). A log-logistic model having four parameters was used to assist in monitoring and analyzing two sets of related, relative crop yield. Data was obtained during the periods of increased weed interference and as to compare, during the weed-free periods. In both years, the relative root yield of sugar beet decreased with a longer period of weed-interference and increased where there was a longer weed-free period. In 2012, the CPWC varied between 122-595 GDD (growing degree days) corresponding to 12 to 46 days after crop emergence (DAE). The following year, CPWC were found to be between 82-735 GDD, (8-54 DAE) based on 5% acceptable yield loss. Weed-free conditions are needed to be arranged as early as the first week after crop emergence and maintained up to and including nine weeks thereafter to avoid more than a 5% loss in sugar beet root yield. Those results could assist sugar beet producers through reducing the expenses significantly, as well as improving the efficacy of their weed management programs.

Weed presence altered biotic stress and light signaling in maize even when weeds were removed early in the critical weed-free period.

Weed presence early in the life cycle of maize (typically, from emergence through the 8 to 12 leaf growth stage) can reduce crop growth and yield and is known as the critical weed‐free period (CWFP). Even if weeds are removed during or just after the CWFP, crop growth and yield often are not recoverable. We compared transcriptome responses of field‐grown hybrid maize at V8 in two consecutive years among plants grown under weed‐free and two weed‐stressed conditions (weeds removed at V4 or present through V8) using RNAseq analysis techniques. Compared with weed‐free plant responses, physiological differences at V8 were identified in all weed‐stressed plants and were most often associated with altered photosynthetic processes, hormone signaling, nitrogen use and transport, and biotic stress responses. Even when weeds were removed at V4 and tissues sampled at V8, carbon: nitrogen supply imbalance, salicylic acid signals, and growth responses differed between the weed‐stressed and weed‐free plants. These underlying processes and a small number of developmentally important genes are potential targets for decreasing the maize response to weed pressure. Expression differences of several novel, long noncoding RNAs resulting from exposure of maize to weeds during the CWFP were also observed and could open new avenues for investigation into the function of these transcription units.

Mathematical Models for Determination of the Critical Period of Weed Competition in Maize (Zea mays L.)

Two field experiments were conducted on maize during 2014 and 2015 summer seasons at EL-Serw Station, Damietta Governorate, Egypt. Each experiment included ten weed competition and weed removal treatments either at early or late times after sowing which were: - weed competition for the whole season, weed competition for 2, 4, 6 and 8 weeks from sowing, weed free for 2, 4, 6 and 8 weeks from sowing and weed free for the whole season, to determine when a natural infestation of weeds start to reduce maize yield and when to control without yield losses in maize. The most appropriate study of the critical period of weed competition is on the weed community as a whole than single weed specie. Dominant major weeds in experimental fields were Portulaca oleraceae, L.; Corchorus olitorius, L.; Amaranthus caudatus, L.; Echinochloa crus-gali, L. and Echinochloa colonum, L. The obtained results revealed that which weed infestation rate under maize field was 5.834 and 6.043-ton fresh weight / fed.,which reduced drastically grain yield of maize /fed by 56.8 to 57.2 % under weed competition treatment of maize for the whole season in 2014 and 2015 summer seasons, respectively as compared with weed free for whole season treatment. The use of response curves with weed free or weed competition periods showed that grain yield of maize / fed were the highest with the field free from weeds until seven weeks after sowing and the critical period of weed / maize competition was between 3 to 7 weeks after sowing. In regression approach for maize grain yield and weed free or weed competition period the polynomials (linear and quadratic) and logistic functions were tested and quadratic function was fitted to estimate the expected yields which had the high significant with the data recorded and have the highest values of R2 than the other models (linear or logistic model.) for this reason it is used to estimate predict yield losses due to weed competition periods with maize. Also, results showed that the quadratic equations were significant and had the highest R2 (0.990 and 0.985) for weed free period, and (0.994 and 0.993) for weed competition durations in 2014 and 2015 summer seasons, respectively. The important stage of weed competition to maize is the 1-9 weeks period from sowing was required to be weed free showed that to maintain 95% of maximum grain yield of maize and one week of weeds infestation can be allowed after maize sowing without grain yield maize reduction. This information should be taken in consideration for maize growers to plan their strategies of integrated weed management for this important crop. It could be concluded that weeds should be removed within the period of first week and nine weeks in maize crop to pass up 5 and 10% grain yield reduction, respectively.

The Effect of Various Weedy Periods on Growth and Yield of Soybean in Agroforestry System with Kayu Putih

This study was aimed to detect the effect of various treatment periods of weedy on the growth and yield of soybean; and to determine the most appropriate time periods of weedy for soybean in agroforestry systems with kayu putih. The experiment had been conducted in Menggoran, BDH Playen, KPH Yogyakarta, Gunungkidul Regency, Special Province of Yogyakarta from February 28 to May 9, 2015. Randomized Complete Block Design (RCBD) with three blocks as replications waas applied in this experiment. The treatments were weedy periods on 0, 14, 28, 42, and 56 days after planting (dap) and weed-free period on 0, 14, 28, 42, and 56 dap. The results showed that there were significant differences in the soil moisture content, root surface area, root length, chlorophyll content, root dry weight, shoot dry weight, dry weight of soybean seeds, and weed dry weight. The highest seed weight per hectare was found in weed-free until harvest treatment but it was not significantly different from weedy periods after 56 dap and weed-free after 14 dap. The effective periods of weedy time for soybean in agroforestry systems with kayu putih began at 28 dap - 56 dap.

Determination of Critical Period of Weed-Crop Competition in Rice ( Oryza sativa L.) in Bench Maji and Kaffa Zone, South Western Ethiopia

Accurate assessment of crop weed interference period is an essential part for planning an effective weed management for cropping systems and, hence, can be considered the first step to design weed control strategy. The experiment was conducted at Guraferda and Gojeb, Southern Ethiopia during the 2016 main cropping season to assess the critical period of weed-crop competition and yield loss in rice. There were 14 treatments comprising: two series i.e. early (weedy up to 10, 20, 30, 40, 50, 60 and 70 days after crop emergence) and late (weed free up to 10, 20, 30, 40, 50, 60 and 70 days after crop emergence) competition periods were compared with two checks namely complete weed free and weedy check. The treatments were arranged in randomized complete block design with three replications for each set. The major weed species competing vigorously with rice were Cyperus assimilis, Setaria pumila, Phalaris paradoxa and Xanthium spinosum. With increasing duration of weed interference, weed dry weight, and the number of days of rice plant required to reach physiological maturity were increased whereas number of tillers per plant, panicle length, thousand seed weight, grain yield, aboveground biomass, and harvest index of the rice crop were reduced. Uncontrolled weed growth significantly reduced rice grain yield by 68% compared to the grain yield obtained from the weed-free check plots. The beginning and the end of critical period of weed crop competition were based on 5 and 10% acceptable yield loss levels, which were determined by fitting logistic and Gompertz equations to relative yield data, representing increasing duration of weed-interference and weed-free periods. In conclusion, the results of the study revealed that, to reduce the loss in the grain yield of rice by more than 10% and higher economic return, it is important to keep the crop weed-free between 30 to 70 days after crop emergence at Guraferda, in Bench Maji Zone and 28 to 47 days after crop emergence at Gojeb in Keffa Zone.

High-Residue Cultivation Timing Impact on Organic No-Till Soybean Weed Management

A cereal rye cover crop mulch can suppress summer annual weeds early in the soybean growing season. However, a multi-tactic weed management approach is required when annual weed seedbanks are large or perennial weeds are present. In such situations, the weed suppression from a cereal rye mulch can be supplemented with the use of high-residue cultivators which can prolong the weed-free period during soybean growth. Research trials were conducted to determine the optimum timing of high-residue cultivation for weed control in rolled-crimped cereal rye mulches. Treatments included three cultivation timings with a high-residue cultivator: early (3-4 wk after soybean planting (WAP)), intermediate (5-6 WAP), and late (7-8 WAP), a weed-free and no-cultivation control. Crop and weed measurement included cereal rye biomass, weed biomass, soybean population and biomass, and yield. Cereal rye biomass was 50% lower and weed biomass was three times greater in 2011 than in 2010 and 2012 due to 2011 being a dry year. There was no significant effect of cultivation timing on soybean population when compared to no-cultivation or hand-weeded treatments. While cultivation reduced weed biomass by 67% compared to no-cultivation, soybean yield was only improved by 12% in early and late cultivation treatments and 22% in intermediate cultivation treatment when compared to no-cultivation. Effective strategies for improving weed management by integrating the use of a high-residue cultivator in no-till organic systems could help existing organic field crop producers to reduce tillage while also encourage adoption of organic crop production by conventional growers who prefer reduced-tillage systems. Unlike traditional organic cultivation equipment, therefore, optimal timing of cultivation should be delayed several weeks in organic cover crop-based no-till planted soybean production as compared to the typical tillage-based approach to ensure both weed control and optimal yield.

Critical period of weed control in radish (<i>Raphinus sativus</i> L.)

A field study was carried out at Crop Farm of the Eastern University of Sri Lanka to determine the critical period for weed control and the effects of weed interference in radish in the regosols of the Batticaloa District. An increasing series of the duration of weed interference and length of the weed-free periods were imposed weekly from 0 to 40 days after emergence (DAE). The beginning and the end of CPWC were based on 5, 10 and 20% acceptable yield loss levels (AYL), which were determined by fitting logistic and Gompertz equations to relative yield data, representing increasing duration of weed-interference and weed-free periods. The critical period for weed control increased to 31 days, starting at 7 days and ending at 38 days, at 5% yield loss level. At 10% and 20% yield loss levels, the CPCW decreased to 12 and 8 days respectively, starting at 18 DAE and ending at 30 DAE, and beginning at 13 DAE and ending at 21 DAE, respectively.

Interference of a weed community in the soybean crop in functions of sowing spacing

The use of double row spacing on soybean crop is recent and consists of a new soybean production technology in Brazil and worldwide, so weed interference may be different in relation to single crop row. Thus, the objective of this work was to evaluate the weed interference in soybean crop in function of sowing spacing. The experimental design was randomized blocks and treatments were arranged in a factorial (2x16): two spacings, a double row spacing [(0.20 x 0.20m) x 0.67 m] and a conventional single row spacing (0.45 x 0.45m) and sixteen periods of weed management, being eight control periods in which soybean was kept free of weeds by increasing periods of 7; 14; 21; 28; 35; 42; 49 and 145 days after emergence and eight coexistence periods, when the crop was maintained in the presence of the weed community for the same periods. The Critical Timing of Weed Removal (CTWR) was lower in the double row spacing, with eight days, compared with single spacing that was of 18 days. The Critical Weed-Free Period (CWFP) was of 36 and 31 days for double and single row spacing, respectively, and the emergence of weeds after this period did not cause damage to the productivity, but the Critical Period of Weed Control (CPWC) was higher in double row spacing (28 days) compared to the single row, which was of 13 days. The spacing in double row had a higher grain yield than the spacing in single row.

Crop-Weed Relationships in Okra [<i>Abelmoschus esculentus</i> (L.) Moench], Soybean (<i>Glycine max</i> L.) and Maize (<i>Zea mays</i> L.) in the Middleveld of Swaziland

The effects of okra [Abelmoschus esculentus (L.) Moench], soybean (Glycine max L.) and maize (Zea mays L.) on weed growth were examined in a field experiment carried out at Luyengo (26°34’S; 31°12’E) in the Middleveld of Swaziland. The aim was to check consistency of competitiveness of crops under different weed removal regimes. Two series of weed removal treatments were included. In the first series, treatments of increasing duration of weed control were maintained weed-free until 3, 7 or 11 weeks after emergence of the crops. The weeds were subsequently allowed to develop till crop harvest. In the second series, weeds were allowed to develop with the crops from emergence until 3, 7 or 11 weeks after crop emergence; then the plots were kept weed-free till harvest. The weed species Oxalis latifolia, Cyperus esculentus, Amaranthus hybridus, Ipomoea purpurea and Nicandra physaloides occurred throughout the different weed-infested and weed-free interference durations. Commelina benghalensis and Acanthospermum hispidum were particularly predominant under increasing weed infestation treatments. The similarity matrix based on Jaccard’s coefficient showed that the composition of weeds under weed-free treatments in soybean was not identical to that of maize and okra, respectively. Further, the weed flora was not homogenous under different lengths of weed-free period showing the combined influence of weed removal and crop on the composition of weed infestation. There were no significant differences in the prevalence of weeds with either C3 or C4 photosynthetic pathways associated with the three crops. A longer equality point of weed control and interference, and lower regression coefficient between weed biomass and yield for soybean compared to maize and okra suggested decreased sensitivity of soybean to weed interference. The results indicate potential for competitive crop genotypes such as soybean for use in intentionally designed cropping systems to augment weed control practices.

Light Quality Effect on Corn Growth as Influenced by Weed Species and Nitrogen Rate

<p>Corn-weed competition has often been characterized as the competition for limited resources such as light quantity, water, and nutrients. However, growing evidence suggests that light quality, specifically the red:far red ratio (R:FR), is a crucial component to corn-weed interactions. Additionally, a reduction in the R:FR has shown to down-regulate plant genes similarly to a nitrogen (N) deficient environment. A greenhouse study was conducted to evaluate the effect of N stress and R:FR from common waterhemp, velvetleaf, and volunteer corn on corn growth and development. The R:FR for all three weed species tended to be similar but lower than a weed-free treatment. However, observations from the spectral response curves demonstrated significant changes in the patterns of light reflected from each weed species. In the N-sufficient environment, early-season (V5 corn growth stage) R:FR from all three weed species reduced corn height, leaf chlorophyll content, and shoot biomass while increasing fibrous root biomass. However, in the N-deficient environment, no effects were observed on corn growth from changes in light quality, indicating N stress was a greater limiting factor. These results highlight the importance of the critical weed-free period and the need for proper early-season weed management.</p>

Periods of weed interference in maize crops cultivated in the first and second cycles

The interference of weeds in maize production may be reflected in grain yield losses that vary as a function of the density, stage and degree of aggressiveness of the species present. In the agricultural ecosystem, crops and weeds demand light, water, nutrients and space, which are frequently not available in sufficient quantities, leading to competition. The aim of this work was to determine the period of interference of weed plants, in particular of naked crabgrass (Digitaria nuda) on maize crop in the first and second harvest. The treatments were defined as increasing periods of coexistence and increasing control of weed community (7, 14, 21, 28, 35, 42, 49, and 56 days), two more controls, a control including one with weed control until the end of the culture cycle and another with coexistence until the harvest. For each period, were evaluated the stand of maize plants, length of ear, number of grains per row, number of rows per ear, cob, 100-grain weight, and grain productivity. The data obtained were subjected to analysis of variance using the F test, with average treatments compared using Tukey’s test at 5% probability. Crop productivity was evaluated by means of regressions, the critical periods of interference were estimated. The critical timing of weed removal was 25 days for both harvests. The critical weed free period was 54 and 27 days for the first and second harvest respectively. For the conditions of the first and second harvest, the critical period of weed control was of 29 and 2 days respectively.

The critical period for weed control in three corn (Zea mays L.) types

Knowledge of the crop-weed competition period is vital for designing effective weed management strategies in crop production systems. Field studies were conducted at the Agricultural Research Institute, Kahramanmaras, Turkey in 2013 and 2014 to determine the critical period for weed control (CPWC) in three corn (Zea mays L.) types (field corn, popcorn, and sweet corn). A four parameter log-logistic model described the relationship between relative crop yield to both increasing duration of weed interference and length of weed-free periods. The relative yield of corn was influenced by duration of weed-infested or weed-free period, regardless of corn types. Increasing periods of weed interference significantly reduced corn yields in both years. In field corn, the CPWC ranged from 175 to 788 growing degree days (GDD) in 2013 which corresponded to V2–V12 growth stages, and 165–655 GDD (V1–V10 growth stages) in 2014 based on the 5% acceptable yield loss (AYL) level. In popcorn, the CPWC ranged from 92 to 615 GDD (VE–V10 growth stages) in 2013 and 110–678 GDD (V1–V10 growth stages) in 2014. In sweet corn, the CPWC ranged from 203 to 611 GDD in 2013 (V2–V10 growth stages) and 182–632 GDD (V2–V10 growth stages) in 2014. The practical implication of this study is that weed management should be initiated around V1 stage and maintained weed-free up to V12 stage in all corn types to prevent yield losses greater than 5%. These findings could help corn producers improve the cost effectiveness and efficacy of their weed management programs.

Optimal planting date, row width, and critical weed-free period for grain amaranth and quinoa grown in Ontario, Canada

The popularity of grain amaranth and quinoa is growing in Ontario, increasing the interest in their cultivation. Two experiments were conducted in southern Ontario in 2013 and 2014 to evaluate optimal planting date (every two weeks from early May to late July), row width (38 or 75 cm), and critical weed-free period (the component of the critical period of weed control that defines the number of days that the crop must remain weed-free to prevent yield loss) in each crop. Grain amaranth and quinoa both reached physiological maturity and produced yields when planting dates ranged from mid-May to late-June. When either crop was seeded in July, yields decreased by more than 50% and the crop did not always reach maturity before the first frost. While row width did not have an impact on yield, it is advisable to grow the crops in wider rows (75 cm) to facilitate weed control early in the growing season (up to 30 d after emergence (DAE)). The critical weed-free period was 24 and 16 DAE for grain amaranth and quinoa, respectively, after which yields were maintained at 95% of the weed-free control. Based on these data, both crops could easily be integrated into the normal cropping rotations found in southern Ontario.

Weed Competition Periods Affect Grain Yield and Nutrient Uptake of Black Seed (Nigella Sativa L.)

The combination of the relatively open plant canopy and slow growth, especially at early growth stages, results in lower competitiveness of black seed (Nigella sativa L.) than weeds. Thus, weed interference is known as an important factor affecting black seed yield. Therefore, to determine the critical period of weed control and its effects on nutrients uptake of black seed field experiments were conducted in 2011 and 2012. Two quantitative series of weed removal treatments including weed-infested and weed-free treatments were implemented from black seed emergence to maturity harvest. To determine critical period of weed control, plots were weed-infested or weed-free for 0, 14, 28, 42, 56, 70 days after emergence, in weed-infested and weed-free treatments, respectively. The results revealed that N, P and K contents in weed tissues significantly increased with increasing weed-infested periods during both years of the experiment. The reduction in grain and oil yield due to longer periods of weed-infested conditions or shorter periods of weed-free conditions were accompanied by simultaneous reduction of N, P and K uptake in black seed grains and tissues. Overall, N, P and K contents in weed tissue were found to be 1.8 to 2 times higher than that of black seed. It was concluded that 58 or 49 days weed-free periods are required to avoid yield loss (above 5% or 10%) in black seed.

The Critical Period for Weed Control (CPWC) in Potato (<i>Solanum tuberosum</i> L.)

Accurate assessment of crop-weed control period is an essential part for planning an effective weed management for cropping systems. Field experiments were conducted during the seasonal growing periods of potato in 2012 and 2013 in Kayseri, Turkey to assess critical period for weed control (CPWC) in potato. A four parameter log-logistic model was used to assist in monitoring and analysing two sets of related, relative crop yield. Data was obtained during the periods of increased weed interference and as a comparison, during weed-free periods. In both years, the relative yield of potato decreased with a longer period of weed-interference whereas increased with increasing length of weed free period. In 2012, the CPWC ranged from 112 to 1014 GDD (Growing Degree Days) which corresponded to 8 to 66 days after crop emergence (DAE) and between 135-958 GDD (10 to 63 DAE) in the following year based on a 5% acceptable yield loss. Weed-free conditions needed to be established as early as the first week after crop emergence and maintained as late as ten weeks after crop emergence to avoid more than 5% yield loss in the potato. The results suggest that CPWC could well assist potato producers to significantly reduce the expense of their weed management programs as well as improving its efficacy.

Row spacing impacts the critical period for weed control in cotton (Gossypium hirsutum)

The knowledge on the critical crop-weed competition period is important for designing an efficient weed management program. Field studies were conducted in 2012 and 2013 at the Agricultural Research Institute, Kahramanmaras, Turkey to determine the effects of three row spacing (50, 70 and 90 cm) on the critical period for weed control (CPWC) in cotton. A four parameter logistic equation was fit to data relating relative crop yield to both increasing duration of weed interference and length of weed-free period. The relative yield of cotton was influenced by the duration of weed-infested or weed-free period, regardless of row spacing. In cotton grown at 50 cm row spacing, the CPWC ranged from 117–526 growing degree days (GDD) (V2–V11 growth stages) in 2012 and 124–508 GDD (V2–V10) in 2013 based on the 5% acceptable yield loss level. At 70 cm row spacing, the CPWC ranged from 98–661 GDD in 2012 (V2–V13) and 144–616 GDD (V2–V12) in 2013. At 90 cm row spacing, the CPWC ranged from 80–771 GDD in 2012 (V1–V14) and 83–755 GDD (V1–V14) in 2013. In order to obtain a 95% weed-free yield, the weed management should start at 16 days after crop emergence (DAE) and continued until 52 DAE (V2–V11) for crops grown in 50 cm row spacing, 15 and 60 DAE (V2–V13) for 70 cm row spacing and 11 and 67 DAE (V1–V14) for crops grown in 90 cm row spacing. This suggests that cotton grown in narrow row spacing (50 cm) had greater competiveness against weeds compared with wider row spacing (70 and 90 cm). Cotton growers can benefit from these results by improving cost of weed control through better timing of weed management.

Competitive Ability, Critical Period of Competition, and Density Level of Hydrolea zeylanica (L.) Vahl against Transplanted-Irrigated Lowland Rice

Determining the competitive ability of a certain weed is a valuable guide as to what, where, and when best weed management techniques must be implemented. Likewise, identifying the critical period of weed-crop competition will help crop growers as to when weeds must be controlled or maybe allowed to compete with crops without having significant yield loss and unacceptable economic returns. Pot experiments were conducted to determine the competitive ability, critical period of competition, and density level of Hydrolea zeylanica (L.) Vahl under transplanted-irrigated lowland rice conditions. Treatments for the duration of weed-free period involved keeping rice plants free from H. zeylanica within periods of 10, 20, 30, 40, and 50 DAT; while the duration of tolerable period of competition involved allowing of the weed to compete with rice from 0 DAT, then simultaneously uprooted at 10, 20, 30, 40, and 50 DAT. In the critical density level experiment, H. zeylanica was allowed to compete with rice plants from 0 DAT until harvest at a ratio of 1:1, 1:3, 1:5, 1:7, 1:10, and 1:20 (rice: Hydrolea). Unweeded (control) and weed-free (check) treatments until harvest were included for comparison. Results showed that H. zeylanica was a very competitive weed against transplanted-irrigated lowland rice. When allowed to grow and compete, the weed signifi cantly reduced rice grain yield and its vital agronomic parts. Th e critical period of competition based on classical method suggested that H. zeylanica can be allowed to compete only within 5 to 40 DAT and 10 to 30 DAT to control yield reduction at 5 and 10%, respectively. Th e estimations of Gompertz and logistic models suggested that H. zeylanica should not be allowed to compete within 0 to 700 GDD to achieve 100% relative grain yield. Under unavoidable circumstances in the fi eld, however, the models suggested that the weed can be allowed to compete with transplanted rice within 0 to 670 GDD and 5 to 580 GDD to minimize yield reductions at 5 and 10%. Furthermore, the negative eff ects of the weed on grain yield and yield components intensified when it was allowed to compete in dense population. Yields of rice that competed with H. zeylanica at 1:1, 1:3, 1:5, 1:7, 1:10, and 1:20 rice:weed ratios were reduced by 18.99, 24.68, 23.80, 35.46, 51.72, and 55.90%, in the same order. Keywords - Hydrolea zeylanica, Oryza sativa, weed ecology, weed control, logistic model, Gompertz model, Philippines

Mathematical Models for Determination of The Critical Period of Weed Competition In Sunflower (Helianthus annuus L.)

ABSTRACT Two field experiments on sunflower were conducted during 2013 and 2014 summer seasons at EL-Serw Station, Damietta Governorate, Egypt. Each experiment included ten weed competition and weed removal treatments either at early or late times after sowing which were: - weed competition for the whole season, weed competition for 2, 4, 6 and 8 weeks from sowing, weed free for 2, 4, 6 and 8 weeks from sowing and weed free for the whole season, to determine when a natural infestation of weeds start to reduce sunflower yield and when to control without yield losses in sunflower. Dominant major weeds in experimental fields were Portulaca oleraceae, L.; Corchorus olitorius, L.; Amaranthus caudatus, L.; Echinochloa colonum, L. and Chenopodium album, L. The obtained results revealed that which weed infestation rate under sunflower field was 5.885 and 6.527 ton fresh weight / fad.,which reduced drastically seed yield of sunflower per faddan by 55.3 to 55.2% under weed competition treatment of sunflower for the whole season in 2013 and 2014 summer season, respectively, as compared with weed free for whole season treatment. The use of response curves with weed free or weed competition period showed that seed yield and oil yield of sunflower were the highest with the field free from weeds until 6 weeks after sowing , and the critical period of weed / sunflower competition was between lie 2– 6 weeks after sowing. In regression approach for sunflower seed yield, oil yield and weed free or weed competition period the polynomials (linear and quadratic) and logistic functions were tested and quadratic function was fitted to estimate the expected yields which had the high significant with the data recorded and have the highest values of R2 than the other models (linear or logistic model.) for this reason it is used to estimate critical periods of weed competition with sunflower. Also, results showed that the quadratic equations were significant and had the highest R2 (0.989, 0.982) and (0.989, 0.984) for weed free period, and (0.899, 0.992) and (0.919, 0.994) for weed competition durations in 2013 and 2014 summer seasons, respectively. According to these results, the critical period of weed competition to sunflower is the first six weeks period from sowing was required to be weed free showed that to maintain 95% of maximum seed and oil yield of sunflower and one week of weeds infestation can be allowed after sunflower sowing without seed and oil yield sunflower reduction. The information should be taken in consideration for sunflower growers to plan their strategies of integrated weed management for this important crop.