To investigate the effect of WEED interference and WEED-free PERIODs on Balangu (Lallemantia royleana Benth. in Wall) and to evaluate the CRITICAL PERIOD of WEED CONTROL (CPWC) in this oilseed crop, an experiment was conducted as a randomized complete block design with three replications in Naghadeh region in 2018. Experimental consisted of two groups of treatments; WEED infestation treatments (infestation for 144.2, 345.9, 574, 848.2, 1147.6 and 1451.3 growing degree day (GDD) after transplanting and then plots were remained WEED-free the rest of the growing season) and WEED-free treatments (WEEDing for 144.2, 345.9, 574, 848.2, 1147.6 and 1451.3 GDD after transplanting and then plots were remained infested until harvesting time). In each block, one WEED free and WEED infested CONTROL was for all the growing season. Results indicated that the density and dry biomass of WEEDs were increased as WEED removal was delayed. In contrast, density and dry biomass of WEEDs were decreased as WEEDs establishment was delayed. Height, tetrachenes fruits, ripened achenes, 1000-seed weight and biological, seed and relative yields of Balangu were decreased and increased as the duration of WEED-infested and WEED-free PERIOD increased, respectively. Also, logistic and Gompertz curves fitted to data showed that the beginning and end of the CPWC based on a 10% yield loss of relative yield, was estimated to be from 11 to 108 days after transplanting (97 days WEED-free PERIOD) in order to prevent a noticeable yield loss.

To evaluate the effect of nitrogen and water withholding in vegetative growth stage of corn on efficacy of nicosulfuron and 2,4-D+ MCPA an experiment was conducted as a factorial arrangement in a randomized complete block design with three replications at Tarbiat Modares University in 2017. The first factor contained four treatments including application of nicosulfuron, 2,4-D+ MCPA, WEED free and WEED infested as CONTROL. The second factor consisted of normal irrigation and water withholding from 4 to 8 leaf stage. Third factor contained either urea application or no fertilizer application. Normal irrigation and nitrogen fertilizer application significantly increased nicosulfuron and 2,4-D+ MCPA efficacy in WEED CONTROL and total WEED dry weight decreased to 341 and 361 g.m-2 respectively compared 1370 g.m-2 as WEED infested. Nicosulfuron and 2,4-D+ MCPA reduced WEED total dry weight significantly under irrigation withholding. Grain yield increased with application of nitrogen and nicosulfuron. Grain yield was higher with application of 2,4-D+ MCPA than nicosulfuron without nitrogen application under irrigation withholding. Application of nicosulfuron with nitrogen application provided satisfactory WEED CONTROL and increased grain yield.

In order to evaluate the CRITICAL PERIOD of WEED CONTROL in Cumin, an experiment with Complete Randomized Block Design and three replications was conducted in experimental field of Faculty of Agriculture, Ferdowsi University of Mashhad, During 2004-2005 growing season. Treatments included different combinations of WEED free and WEED infested PERIODs (20, 30, 40 and 80 days after germination) plus WEEDy check and WEED free check. CRITICAL PERIOD WEED CONTROL was evaluated with Gompertz and Logistic functions. Traits measured were yield and yield components, harvest index of cumin and also number and weight of WEED species. Results showed the CRITICAL PERIOD was between 24-38 days after germination. With increasing interfering PERIOD at early or late growth stages of cumin, the economic yield was reduced. By extending WEEDing PERIODs at early stage of growth or during the growth PERIOD, dry weight of WEEDs were reduced, while extending WEEDing PERIOD at the end of growth stage and also WEED free during growth PERIOD, early or late stages of growth had no significant effects on yield components except on number of umbels per plant. Harvest index was positively affected by early WEEDing. It appears that early WEEDing was somehow more effective on yield components for cumin.

The CRITICAL PERIOD of WEED CONTROL (CPWC) is a PERIOD in the crop growth cycle during which WEEDs must be CONTROLled to prevent yield losses. In order to determine the CRITICAL PERIOD of WEED CONTROL (CPWC) of corn in Hamedan, an experiment was conducted in 2007 at Agricultural Research Station of Bu-Ali Sina University of Hamedan. The experiment was carried out as a randomized complete block design with three replications. The trial included twelve treatments consisted of six initial WEED-free PERIODs in which plots were kept free of WEEDs for 0, 15, 25, 35, 45 and 55 days after crop emergence (DAE), and then WEEDs were allowed to grow until harvest and six initial WEED-infested PERIODs in which, WEEDs were allowed to grow for 0, 15, 25, 35, 45 and 55 DAE, after which the plots were kept free of WEEDs until harvest. Each plat consisted of four rows (six meter per row) with a row spacing of 75 cm. . WEED sampling carried out in first series of treatments at the end of growth season and in second series of treatments at the4 end of infested PERIODs. The results showed that beginning of CPWC ranged from 124 to 204 GDD, at 5% and 10% AYL, which equates to 8 and 16 days after crop emergence (3 and 5 leaf), respectively. The end of the CPWC varied from 395 to 289 GDD, at 5% and 10% AYL, which equates to 31 and 24 days after crop emergence (10 and 7 leaf), respectively. Total dry weight of WEEDs increased as the duration of WEED infested PERIOD increased and it was decreased with increasing duration of the WEED-free PERIOD. Grain yield of corn increased with increasing duration of the WEED-free PERIOD and decreased as the duration of WEED infested PERIOD increased.

A study was conducted to determine the CRITICAL PERIOD in silage corn (Zea mays) during 1998 in tehran (in Aburihan Campus). Treatments included WEED free and WEED infested PERIOD up to 3 leaf; 6 leaf; 10 leaf; 14 leaf; tassseling plus 1 week and to the end of growth season (WEED free and WEEDy check). Treatment were replicated, four times in a complete randomized block design. Results based on Gompertz and logistic equation showed a CRITICAL PERIOD of WEED CONTROL for forage yield from 4-5 to 12 leaf stages (based on 5% of acceptable damage); and for grain yield (5% of acceptable damage); from 4 to 13 - 14 leaf stage of corn development. Correlation between dry matter of WEED and corn was shown that between this two variables there was a negative relation.- Number of WEED in early season increased and then decreased.

In order to determine the CRITICAL PERIOD of WEED CONTROL in Thyme (Thymus vulgaris), an experiment was carried out in a randomized complete blocks design with 18 treatments and three replications in 2008-2009 at the research field of the medicinal plants departments, Shahed University, Tehran, Iran. Two series of the treatments including WEEDfree and WEED-infested were applied in the regular time distances of 15, 30, 45, 60, 75, 90, 105 and 120 days right after the beginning of thyme growth at spring. Two check treatments including full and no CONTROL of WEED during the whole season were also included. The nonlinear models kind of logistic and Weibull were applied to determine the start and end of CRITICAL PERIOD of WEED CONTROL respectively. Major WEED species were field bindWEED (Convolvulus arvensis), flixWEED (Descurainia sophia), mon groundsel (Senecio vulgaris) and prickly lettuce (Lactuca scariola) with greater biomass and size. The results showed that PERIODs of WEED- free and WEED- infested significantly affected the dry weight and number of WEEDs in all treatments. Considering 5% and 10% of the acceptable yield loss in the models, CRITICAL PERIODs of WEED CONTROL were between 41-90 days and 54-76 days for fresh weight, 12-94 days and 29-78 days for dry weights, and 20-105 days and 30-86 days for essence yield after the beginning of growth at spring.

The study was conducted in Mashhad (NE of Iran) during 1996 to determine the CRITICAL WEED CONTROL PERIOD in Sorghum. Treatments included WEED free and WEED infested PERIOD to 3,5,7,9 leaf stages and total growth season (WEED free and WEED infested check). Treatments were replicated four times in complete randomized block design. Results showed that a CRITICAL WEED CONTROL PERIOD in Sorghum between 20-31 days after sorghum emergence (about 3 to 5 leaf stage) with 5% allowance reduction for yield. By increasing allowance reduction for yield to 8%, the CRITICAL PERIOD was changed to a CRITICAL WEED CONTROL time (24 days after emergence or about 5th leaf stage). WEED free PERIOD up to 5th leaf stage decreased WEED dry matter and population to 90% and 70% respectively. Nightshade (Solanum nigrum), Redroot pigWEED (Amaranthus retroflexus), Common lambsquarter (Chenopodium album) and Field bindWEED (Convolvulus arvensis) were the dominant WEED of this experiment.  

WEED competition is one of the yield-limiting factors of sugar beet production in Iran. WEED competition may reduce the root and sugar yield, depending on the WEED density and the intensity of WEED competition with the crop. In order to determine the CRITICAL PERIOD of WEED CONTROL (CPWC) in sugar beet in Shahrekord region, an experiment was arranged in a randomized complete block design with 14 treatments and four replications in 1999 and 2000. Treatments  included WEED-free plots until 2, 4, 6, 8, 10 and 12 weeks after planting (WAP) in 2000 (in 1999 based on the stages of crop growth in the form of WEED CONTROL, treatments were until 4, 6, 8, 10, 12 and 14–leaf stages of sugar beet) and WEED-infested plots from 2, 4, 6, 8, 10 and 12 weeks after planting sugar beet in 2000 (in 1999 based on the stages of crop growth in the form of WEED interference until 4, 6, 8, 10, 12 and 14-leaf stages of sugar beet). Two CONTROL treatments (full-season CONTROL of WEEDs and full-season interference of WEEDs) were also included. The results showed that WEED infestation significantly (P<0.01) reduced the root and sugar yield of sugar beet in all treatments. Total root yield loss of the crop in full competition was 92.9% in 1999 and 61.16% in 2000 compared to WEED free CONTROL treatment. Based on 10% loss of yield, the beginning of the CRITICAL PERIOD of WEED CONTROL (CPWC) was 25 and 5 days after planting for the first year and the second year, respectively.  On this basis, the end of the CRITICAL PERIOD of WEED CONTROL was 78 days for the first year and 88 after planting for the second year. Dominant WEEDs in this experiment were Amaranthus retroflexus, Chenopodium album, and Echinochloa crus-galli. Considering the high density of WEEDs in the experimental site, a long CRITICAL PERIOD of 82 days existed five days after planting. Also, with regard to late planting date of sugar beet (from May to June), competition between WEED and crop may start sooner, causing considerable yield loss in sugar beet.