Relative Yield Data Research Articles

Анализ на зависимостта между общия добив и напоителната норма при царевицата

The purpose of this work is to study the influence of reduced irrigation rates on the productivity of corn (FAO group 600) grown for grain in the Plovdiv region and to establish the parameters of the “Total yieldirrigation rate” relationship. The experiment was carried out in the period 2004-2018 in Agricultural University – Plovdiv, with the late hybrid “Knezha-613”. The variants of the experiment are: 1) without irrigation; 5) optimal irrigation (pre-irrigation soil moisture 75% of FC for the layer 0-80 cm); 2, 3, and 4) irrigation with respectively 25, 50 and 75 % of the rate for variant 5. The relationship is determined based on three types of formulas – regression equation, quadratic formula, and power formula. The highest accuracy is achieved by power relationship at power exponent n=1.3 and r=0.725. The quadratic formula is recommended when relative yield data are only under non-irrigated conditions. These two variants of relationship give a sufficiently accurate forecast for the yield when irrigation rates are reduced by up to 25%. The regression is also usable, but it is inferior to the power and quadratic variants.

Critical Period of Weed Control in Cotton (Gossypium hirsutum L.) at Three Nitrogen Levels*

The critical period for weed control is that part of the cropgrowth cycle during which the crop must be kept weed-free to preventunacceptable yield losses. Field studies were conducted for twoconsecutive years, 2004 and 2005, in the Thessaly plain in central Greeceto determine the critical period for weed control in cotton at three nitrogenlevels (50,100, and 150 kg N ha-1 ).Treatments of increasing duration ofweed interference and weed-free period were imposed at biweeklyintervals from 0 to 10 weeks after crop emergence (WAE). Based on anarbitrarily acceptable yield loss level of 5% and 10%, the beginning andend of the critical period were determined by fitting the polynomialregression to relative yield data representing increasing duration of weedinterference and weed-free period. Plant height, shoot and root dryweights and yield of cotton were reduced by prolonged delays in weedremoval at all nitrogen levels in 2004 and 2005. The results showed thatweeds reduced cotton shoot and root dry weights, respectively, by 55%and 38 % at 50 kg N ha-1 , 50% and 42 % at 100 kg N ha-1 and 40% and 46% at 150 kg N ha -1 , when allowed to compete for two weeks. At 10%acceptable yield loss level, the critical period was 8.2 weeks, starting 0.4WAE and ending 8.6 WAE at 50 kg N ha -1 , and was decreased to 7weeks, starting 1 WAE and ending 8 WAE at 100 kg N ha-1 . The criticalperiod decreased to 6.1 weeks, at 150 kg N ha -1 , starting 1.3 WAE andending 7.4 WAE. These findings showed that the supply of nitrogen to thecrop and weeds significantly influenced the crop-weed interferenceduration. This information will be useful for providing weed controlrecommendation to cotton producers.

Influence of nitrogen fertilizer and duration of Purple Nutsedge (<em>Cyperus rotundus</em>) interference on Onion (<em>Allium cepa</em>) growth and yield in minor seasons in Batticaloa, Sri Lanka

Purple Nutsedge (Cyperus rotundus) is difficult to manage and persists in onion (Allium cepa) fields of the tropics. Critical period of weed control can be highly influenced by Nitrogen fertilizer (N) application. Thus, determine the critical period of purple nutsedge control in onion for different levels of nitrogen fertilizer is useful in making decisions on the need for and timing of purple nutsedge control in both biological and economic perspectives. A field study was conducted from June – September, 2010 in Sri Lanka to evaluate the influence of nitrogen application on the critical period of purple nutsedge control in onion. Nitrogen fertilizer was applied at rates equivalent to 100, 150, and 200 kgha-1. A quantitativeseries of treatments of both increasing duration of purple nutsedge interference and length of weed-free period were imposed within the different rates of N. The beginning and end of the critical period of purple nutsedge control based on 5% acceptable yield loss level were determined by fitting logistic and Gompertz equations to relative yield data representing increasing durations of purple nutsedge interference and weed-free period, respectively. The beginning of the critical period of the purple nutsedge control for onion varied with N application rate. The addition of N hastens the end of the critical period of the purple nutsedge control. Critical period of purple nutsedge control for onion reduced with increasing N. Plant height, dry weight and bulb diameter were significantly (P<0.05) influenced by interaction effect of purple nutsedge infestation period and Nitrogen application rate. In addition, Nitrogen application rates and duration of weed free period significantly (P<0.05) interacted with onion height, dry weight and bulb diameter. Overall, this study revealed that reductions in nitrogen use may create the need for more intensive purple nutsedge management in onion fields in the minor seasons of the Batticaloa.

Critical timing of Palmer amaranth (Amaranthus palmeri) removal in sweetpotato

AbstractPalmer amaranth is the most common and troublesome weed in North Carolina sweetpotato. Field studies were conducted in Clinton, NC, in 2016 and 2017 to determine the critical timing of Palmer amaranth removal in ‘Covington’ sweetpotato. Palmer amaranth was grown with sweetpotato from transplanting to 2, 3, 4, 5, 6, 7, 8, and 9 wk after transplanting (WAP) and maintained weed-free for the remainder of the season. Palmer amaranth height and shoot dry biomass increased as Palmer amaranth removal was delayed. Season-long competition by Palmer amaranth interference reduced marketable yields by 85% and 95% in 2016 and 2017, respectively. Sweetpotato yield loss displayed a strong inverse linear relationship with Palmer amaranth height. A 0.6% and 0.4% decrease in yield was observed for every centimeter of Palmer amaranth growth in 2016 and 2017, respectively. The critical timing for Palmer amaranth removal, based on 5% loss of marketable yield, was determined by fitting a log-logistic model to the relative yield data and was determined to be 2 WAP. These results show that Palmer amaranth is highly competitive with sweetpotato and should be managed as early as possible in the season. The requirement of an early critical timing of weed removal to prevent yield loss emphasizes the importance of early-season scouting and Palmer amaranth removal in sweetpotato fields. Any delay in removal can result in substantial yield reductions and fewer premium quality roots.

Characteristics of drought vulnerability for maize in the eastern part of Northwest China

Based on information distribution and diffusion method theory and combined with the standardized precipitation index and relative meteorological yield data, meteorological factors and social factors were comprehensively considered to assess the vulnerability of maize (Zea mays) to drought. The probability distribution curve of meteorological drought degree (MDD) and relative meteorological yield in the eastern part of Northwest China (Gansu, Ningxia and Shaanxi) from 1978 to 2016 were obtained, using a two-dimensional normal information diffusion method to construct the vulnerability relationship between MDD and relative meteorological yield. The drought vulnerability curve of maize in the study area was obtained. The probability distribution of MDD was multiplied by the fragility curve and summed to obtain the multi-year average risk. The MDD probability distribution curve showed that the probability of moderate drought in Shaanxi was relatively high, followed by Gansu and Ningxia. The probability distribution of Gansu was more discrete. The probability of strong meteorological drought in Ningxia was high, followed by Shaanxi and Gansu. Probability distribution of relative meteorological yield for maize in Gansu Province was highly discrete, with thick tailings, large uncertainties, and more extreme values, which were strongly affected by meteorological conditions, followed by Shaanxi and Ningxia. Taking meteorological drought as the cause and maize damage as the result, the vulnerability relationship between MDD and drought damage was obtained. With an increased MDD, the relative meteorological yield of maize gradually declined. From the average value, when MDD was less than −2.60, the relative meteorological yield of maize was reduced within 15%; when MDD was greater than −2.60, the relative meteorological yield of maize increased within 10%. When the degree of meteorological drought exceeded −2.2, maize was most vulnerable to drought in Shaanxi followed by Ningxia and Gansu. When meteorological drought was less than −2.2, maize was most vulnerable to drought in Shaanxi followed by Gansu and Ningxia. The expected values of relative meteorological production in Gansu, Ningxia, and Shaanxi were 1.36%, 2.48%, and −1.76%, respectively; therefore, Shaanxi had the highest maize drought risk, followed by Gansu and Ningxia. This research had a clear physical background and clear risk connotations. The results provide a data foundation and a theoretical basis for drought disaster reduction for maize in the study area.

Phosphorus requirement for irrigated bush beans production on coastal plain soils

Economic agricultural production of coastal plain sandy soils requires intensive nutrient and irrigation management. Optimization of nitrogen and phosphorus applications will bring environmental sustainability also to the production. A greenhouse study was conducted on irrigated sandy soils with bush beans comparing six different phosphorus (P) rates set in completely randomized blocks with four replications during 2016 and 2017 to determine optimum P requirement. Soil and tissue nutrients, plant height, dry matter, and relative yield (RY) data were collected. Agronomic Use Efficiency (AUE) was calculated. Highest RY and AUE were recorded at 53 or 59 kg ha−1 of P application rates, above which there was no response. A negative P: zinc (Zn) impact was also documented. A maximum soil application rate of 59 kg P ha−1 can be suggested as adequate to optimize the yield of bush beans produced on coastal plain sandy soils of the southeastern United States.

Critical Period for Palmer Amaranth (Amaranthus palmeri) Control in Pickling Cucumber

AbstractField studies were conducted in North Carolina to determine the critical period for Palmer amaranth control (CPPAC) in pickling cucumber. In removal treatments (REM), emerged Palmer amaranth were allowed to compete with cucumber for 14, 21, 28, or 35 d after sowing (DAS) in 2014 and 14, 21, 35, or 42 DAS in 2015, and cucumber was kept weed-free for the remainder of the season. In the establishment treatments (EST), cucumber was maintained free of Palmer amaranth by hand removal until 14, 21, 28, or 35 DAS in 2014 and until 14, 21, 35, or 42 DAS in 2015; after this, Palmer amaranth was allowed to establish and compete with the cucumber for the remainder of the season. The beginning and end of the CPPAC, based on 5% loss of marketable yield, was determined by fitting log-logistic and Gompertz equations to the relative yield data representing REM and EST, respectively. Season-long competition by Palmer amaranth reduced pickling cucumber yield by 45% to 98% and 88% to 98% during 2014 and 2015, respectively. When cucumber was planted on April 25, 2015, the CPPAC ranged from 570 to 1,002 heat units (HU), which corresponded to 32 to 49 DAS. However, when cucumber planting was delayed 2 to 4 wk (May 7 and May 21, 2014 and May 4, 2015), the CPPAC lasted from 100 to 918 HU (7 to 44 DAS). This research suggested that planting pickling cucumber as early as possible during the season may help to reduce competition by Palmer amaranth and delay the beginning of the CPPAC.

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.

Critical period of weed control in radish (&lt;i&gt;Raphinus sativus&lt;/i&gt; 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.

Critical Period for Weed Control in Grafted and Nongrafted Fresh Market Tomato

Field experiments were conducted to determine the critical period for weed control (CPWC) in nongrafted ‘Amelia’ and Amelia grafted onto ‘Maxifort’ tomato rootstock grown in plasticulture. The establishment treatments (EST) consisted of two seedlings each of common purslane, large crabgrass, and yellow nutsedge transplanted at 1, 2, 3, 4, 5, 6, and 12 wk after tomato transplanting (WAT) and remained until tomato harvest to simulate weeds emerging at different times. The removal treatments (REM) consisted of the same weeds transplanted on the day of tomato transplanting and removed at 2, 3, 4, 5, 6, 8, and 12 WAT to simulate weeds controlled at different times. The beginning and end of the CPWC, based on a 5% yield loss of marketable tomato, was determined by fitting log-logistic and Gompertz models to the relative yield data representing REM and EST, respectively. In both grafted and nongrafted tomato, plant aboveground dry biomass increased as establishment of weeds was delayed and tomato plant biomass decreased when removal of weeds was delayed. For a given time of weed removal and establishment, grafted tomato plants produced higher biomass than nongrafted. The delay in establishment and removal of weeds resulted in weed biomass decrease and increase of the same magnitude, respectively, regardless of transplant type. The predicted CPWC was from 2.2 to 4.5 WAT in grafted tomato and from 3.3 to 5.8 WAT in nongrafted tomato. The length (2.3 or 2.5 wk) of the CPWC in fresh market tomato was not affected by grafting; however, the CPWC management began and ended 1 wk earlier in grafted tomato than in nongrafted tomato.

Determination of critical period for weed control in intensive and non-intensive sugarcane (&lt;i&gt;Saccharum officinarum&lt;/i&gt; L., Poaceae) production systems in center Côte d’Ivoire

Field experiments were conducted in Zuenoula and Yamoussoukro for determining Critical Period for Weed Control (CPWC) in sugarcane. The treatments consisted in two sets of weed interference. In the first set, the crop was kept weed-free until 31, 61, 92, 123 days after planting (DAP) in Zuenoula and until 32, 69, 98 and 162 DAP in Yamoussoukro. In the second set, weeds were permitted to grow within the crop until the above-mentioned DAP. The CPWC was determined for 5, 10, 15 and 20% acceptable yield loss levels by fitting Logistic and Gompertz nonlinear equations to relative yield data. In both locations, increasing the duration of weed interference decreased sugarcane yield significantly. In Zuenoula, the CPWC was from 28 to 117, 30 to 93, 32 to 75 and 34 to 59 DAP to prevent yield losses of 5, 10, 15 and 20%, respectively. In Yamoussoukro, the CPWC ranged from 38 to 163, 39 to 112, 40 to 99 and from 40 to 91 DAP to prevent yield losses of 5, 10, 15 and 20%, respectively. Results suggest weed control between 28 and 117 DAP in Zuenoula and between 38 and 163 DAP in Yamoussoukro to provide maximum yield.Keywords: Weed interference, Gompertz, Logistic, weed control, yield.

Prediction of Dry Matter Yield of Hybrid Forage Corn Grown for Silage

ABSTRACTPredicting yield at an early growth stage will allow producers to optimize the amount and timing of N to maximize crop yield. The objective of this study was to evaluate the usefulness of individual measurements and combinations of normalized difference vegetation index (NDVI) and plant height (PH) as early stage predictors of corn (Zea mays L.) dry matter (DM) yield for silage. Corn was grown during 2 yr in Camden, Australia with different rates of pre‐and post‐sown N fertilizer, with or without irrigation, and harvested for silage at physiological maturity. In both years, NDVI and PH data were collected at 3‐, 5‐, 6‐ and 8‐leaf stages (V) and their relative values were calculated by using the reference treatment (nonlimiting N and water). In contrast to the actual data, the relative DM yield data in both years settled on the same regression line against relative NDVI indicating that expressing DM yield and NDVI in relative terms reduces the variation due to year and may be more useful than the actual values for prediction purposes. The accuracy of prediction was higher for PH than NDVI data. However, the best accuracy in DM yield prediction at V6 stage was obtained with combinations of relative PH × NDVI (r2 ≥ 0.78). As slopes of all indicators were similar in stages from V6 to V8 stages, all indicators may be used to predict DM yield of forage corn grown for silage as early as V6 stage. Plant height and PH × NDVI can be used to predict DM yield of forage corn at V5 stage because slopes of these indicators were similar from V5 to V8 stages.

Determination of the critical period of weed control in potato (Solanum tuberosum L.)

Field studies were conducted in north-eastern Iran in 2010 and 2011 to establish the critical period of weed interference in potato (Solanum tuberosum L.) and to investigate the effects of weed interference on weed biomass. The critical period for weed control in potato based on a 5% acceptable yield loss level was calculated by fitting logistic and Gompertz equations to relative yield data. Total dry biomass and total number of weeds increased as the duration of weed infestation increased. The beginning of the critical period for weed control was 19 days after potato emergence in both years. The end of the critical period for weed control in 2011 was 22 days after potato emergence, whereas in 2010 the beginning and end of the critical period for weed control occurred simultaneously. Tuber yields of potato were reduced by prolonged delays in weed removal in both years. The practical implication of this study is that weeds must be controlled during the first 3 weeks of the crop's growing season. Such an approach would keep yield loss levels below 5%.

Competitive ability of some cover crop species against Asystasia gangetica and Pennisetum polystachion

Asystasia gangetica and Pennisetum polystachion are the most troublesome weeds in oil palm plantations in Malaysia and establishment of cover crops under oil palms is an integral part of sustainable weed management in plantations. Replacement-series experiments were conducted to evaluate the mode and strength of competitiveness of four legume cover crop species, Calopogonium caeruleum, Centrosema pubescens, Mucuna bracteata and Pueraria javanica, and one soft grass species, Axonopus compressus, against Asystasia gangetica and Pennisetum polystachion. Relative yield, relative yield total and relative crowding coefficient were calculated with reference to shoot biomass. Asystasia gangetica was a consistently weaker competitor than Pennisetum polystachion. The legume cover crop species had higher relative yield when grown in mixtures, whereas the yield of Asystasia gangetica was lower in mixtures with legumes than in monoculture. The higher competitiveness of legumes was due to greater canopy height and leaf area index. Centrosema pubescens was a slightly weaker competitor than the other legumes. Although leaf area per plant of Centrosema pubescens did not differ from that of other three legumes, canopy height and blocking photosynthetically active radiation to the soil surface were lower. When grown with a cover crop for 12 weeks, dry weight per plant, leaf area and shoot number of Pennisetum polystachion increased as its proportion decreased in mixtures. Relative yield data indicated that Pennisetum polystachion is a strong competitor. Axonopus compressus was less susceptible to Pennisetum polystachion interference than the legumes. All cover crop species studied proved highly competitive against Asystasia gangetica, but none could compete against Pennisetum polystachion. The study confirms the feasibility of using a cover crop for management of Asystasia gangetica.

The critical period of weed control in corn in Birjand region, Iran

The critical period for weed control (CPWC) is the period in the crop growth cycle during which weeds must be controlled to prevent unacceptable yield losses. Field studies were conducted in 2005 and 2006 in the University of Birjand at the south east of Iran to determine CPWC of corn using a randomized complete block design with 14 treatments and three replications. The treatments consisted of two different periods of weed interference, a critical weed-free period and a critical time of weed removal, were imposed at V3, V6, V9, V12, V15, and R1 (based on phonological stages of corn development) with a weedy check and a weed-free check. The CPWC was determined with the use of 2.5, 5, 10, 15 and 20% acceptable yield loss levels by non-linear regression method and fitting Logistic and Gompertz nonlinear equations to relative yield data. The CPWC of corn was from 5- to 15-leaf stage (19-55 DAE) to prevent yield losses of 5%. This period to prevent yield losses of 2.5, 10 and 20% was 4- to 17-leaf stage (14-59 DAE), 6- to 12-leaf stage (25-47 DAE) and 8- to 9-leaf stage (31-36 DAE) respectively.

Critical period for weed control in direct seeded red pepper (Capsicum annum L.)

Direct seeded red pepper is a cash crop in Kahramanmaraş province of Turkey as well as some other nearby provinces. Weeds are a major constraint in red pepper production. Field studies were conducted to determine critical period for weed control (CPWC) in direct seeded spice pepper in Kahramanmaraş, in 2008 and 2009. The CPWC in red pepper based on a 2.5%, 5% and 10% acceptable yield loss (AYL) was calculated by fitting logistic and Gompertz equations to relative yield data. The CPWC in red pepper was determined from 0 to 1087 growing degree days (GDD) in 2008 and from 109 to 796 GDD in 2009 for 10% AYL after crop emergence in red pepper. For 2.5–5% AYL, the CPWC starts with germination and lasts until harvest. Direct seeded red pepper is very vulnerable to weed competition and weed control programs for direct seeded spice pepper in Turkey should include pre‐emergence and residual herbicides.

Improvement in regression of corn yield with plant height using relative data

The traditional approach of analyzing absolute variable data across multiple locations and/or years has drawbacks in precision agriculture. This study was conducted to evaluate the impacts of using relative yield and plant height data of corn (Zea mays L.) on the regression of yield with plant height using linear and exponential models in a nitrogen (N) rate field trial under four cropping systems. The use of relative yield to replace absolute yield frequently increased the determination coefficient (R(2) ) values in the regression of yield with plant height on datasets combined across cropping systems or/and years. Relative yield and relative plant height sometimes further enhanced the R(2) values compared with relative yield and absolute plant height. All these improvements mostly occurred when the fit of the model was not strong with absolute yield and absolute plant height or relative yield and absolute plant height. The advantages of using relative data of yield or/and plant height were similar for the two regression models. The use of relative yield or relative data of both yield and plant height may be effective in improving the regression of corn yield with plant height across multiple cropping systems/locations and years in precision agriculture.

Critical period of weed control in chickpea under non-irrigated conditions

The present study was conducted during the growing seasons of 2005, 2006, and 2007 to determine the critical period of weed control (CPWC) in chickpea (cv. Aziziye 94). In order to evaluate the beginning of CPWC, weeds were allowed to compete at weekly intervals for 1 to 8 weeks after emergence (WAE) and, at the end of CPWC, plots were kept weed-free at weekly intervals for 1 to 8 WAE by periodic hand hoeing. The beginning and the end of CPWC were based on 5% acceptable yield loss (AYL) levels, which were determined by fitting logistic and Gompertz equations to relative yield data, representing increasing duration of weed-interference and weed-free period, estimated as growing degree days (GDDs). The major weed species were Centaurea depressa Bieb. and Bromus tectorum L. for the 1st year, Salsola ruthenica Iljin., Amaranthus sp. and Heliotropium europaeum L. for the 2nd year and Amaranthus spp., Salsola ruthenica Iljin., Sisymbrium septulatum DC. and Heliotropium europaeum L. for the 3rd year. Overall weed density was 190 plants m^{-2} in 2005, 215 plants m^{-2} in 2006, and 191 plants m^{-2} in 2007. Yield losses were 26.4% in 2005, 31.5% in 2006, and 25.0% in 2007 when the crop was not weeded. It was found that at the 5% AYL level CPWC was 2.32 WAE to harvest in the 1st year, from emergence to harvest in the 2nd year, and from 0.34 WAE to harvest in the 3rd year. The present findings suggest that the determination of CPWC is crucial in chickpea production.

Effect of crop plant density on critical period of weed competition in potato

A field study was conducted in Hamedan western Iran in 2006 to establish the critical period of weed competition (CPWC) in potato ( Solanum tuberosum L.) for commercial and seed production plant densities. A quantitative series of treatments concerned with both increasing duration of interference and length of weed-free period were imposed within each commercial and seed potato production plant density. The beginning and end of the CPWC based on a 10% loss of tuber yield was determined by fitting logistic and Gompertz equations to the relative tuber yield data representing increasing duration of weed interference and weed-free period, respectively. At a 10% tuber yield loss level, the duration of weed interference for 571 and 676 growing degree days (GDDs) from crop emergence, corresponding to 40% and 50% canopy closure, marked out the beginning of the CPWC for commercial and seed production plant densities, respectively. When maintained weed-free for 1163 and 1014 (GDD), corresponding to 100% and 80% canopy closure, weed emerging later caused tuber yield losses of less than 10% for commercial and seed production plant densities, respectively. Practical implications of this study are that post-emergence herbicides or other weed control methods should be used in western Iran to eliminate weeds from 19–24 days post-crop emergence up to 43–51 days. Such an approach would keep yield loss levels below 10%. Interactions between plant density and weed competition durations indicated that weed management in commercial plant density of potato could be less intensive than that of seed production plant density, reducing herbicide use and risk of herbicide carryover to sensitive rotation crops.

Critical Period for Weed Control in Leek (Allium porrum L.)

Field studies were conducted in Mersin, Turkey, in 2002 and 2003 to determine the critical period for weed control in leek and to investigate the effects of weed interference on weed biomass. The critical period for weed control in leek based on a 5% acceptable yield loss level was calculated by fitting logistic and Gompertz equations to relative yield data. Total fresh biomass of weeds increased as the duration of weed infestation increased. The beginning of the critical period for weed control was 7 days after transplanting in 2002 and 13 days after transplanting in 2003. The end of the critical period for weed control was 85 days after transplanting in 2002 and 60 days after transplanting in 2003. Results of this study suggest that leek should be kept weed free between 7 days after transplanting and 85 days after transplanting to avoid yield losses in excess of 5%.