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Introduction Rain-fed Lentil (Lens culinaris Medik.) is vulnerable to weed competition because of its tiny stature, slow establishment, and limited vegetative growth. Weed control is necessary for maximum seed yield and seed quality. Crop yield losses are primarily a result of competition with weeds for nutrients, moisture and space. Therefore, weed control at this period plays an important role to gain high roduction. Although the vast majority of lentil production is under rain-fed conditions, there is a little published information on weed control with herbicides in rain-fed lentils. Herbicides due to efficiency and expense savings play a essential role in weed control. The purpose of this research was to determine the best time to application effective herbicides in weeds control for the maximum rain-fed Lentil production in Khorramabad. Materials and Methods The Lentil (Lens culinaris Medik.) field experiment was carried out as split plot based on complete randomized block design with three replications during 2017-2018 in Khorramabad (48.21°E, 33.29°N, 1170 m above sea level, 450 mm average yearly precipitation) Iran. The following herbicides were used: Oxyfluorfen (EC 24%) 1 Li ha-1, Isoxaflutole (SC 480) 200 Ml ha-1, Flumetsulam(WG 800) 20 g ha-1, Metribuzin (WP 70%) 500g ha-1, Pendimethalin Aria(EC33%) 4 li ha-1, Pendimethalin Prowl (CS45.5%) 4li ha-1, Imazethapyr (SL10%) 350 ml ha-1 and Pendimethalin prowl 2li ha-1plus Imazethapyr 350 ml ha-1. Herbicide applicatioin time at 3 levels; IAP application (Immediately after planting), PRE application and IBCE (eraly post herbicide application at fourth lentil node stage) was assigned to the main and sub plots respectively. The fields were in wheat cultivation in the year before the experiments. The soil at the test sites was a silty loam with a pH of 7.9 and organic matter of 1.11%. Lentil was sown at a density of 75 kg seed ha-1 by hand on December 21, 2017. Herbicides were sprayed with an electric knapsack sprayer MATABI (calibrated to deliver 300 L ha-1. In order to evaluate the effect of treatments on weeds density and biomass, at the beginning of lentil flowering, sampling was done from a surface of 0.3 m-2 area and weed density and weed biomass recorded. At lentil physiological maturity, the yield and yield components were measured by harvesting lentil plants from a 1-m2 area in each plot (Izadi & Maghsoudi, 1400). The data were subjected to the analysis of variance using SAS. Means were compared using Duncan's Multiple Range test at P=0.05 level of significance. Results and Discussion The results indicated that the lowest weed density and weed biomass were associated with Flumetsulam and Prowl plus Imazethapyr. The average weed densities for the postemergence and preemergence treatments of the eight evaluated herbicides were 14.9% and 31.0% lower, respectively, compared to the herbicide application immediately after planting. The least herbicide injury effects on the Lentil crop were assigned to the Pendimethalin Aria, Pendimethalin Prowl, Prowl plus Imazethapyr, Imazethapyr and Flumetsulam. In the case of Oxyfluorfen herbicide, weed density for early postemergence application treatment was 84.1% lower than in PRE application. In the case of Isoxaflutole herbicide, the mean weed density for application immediately after planting was 79.3% lower than for early post-application. In the case of Flumetsulam herbicide, the mean of weed biomass for application immediately after planting was 73.3% and 66.6% less than preemergence and early post-application, respectively. With the exception of Flumetsulam herbicide, there was no significant difference between the different herbicide application times in terms of weed biomass. The highest average Lentil grain yield per unit area was observed when Flumetsulam herbicide was applied immediately after planting. On the other hand, the lowest average Lentil grain yield per unit area was recorded when Isoxaflutole herbicide was applied immediately after planting. No significant differences in Lentil grain yield per unit area were found between the different application times for the herbicides Metribuzin and Imazethapyr. Based on cluster analysis grouping of relative efficacy of control treatments, immediately after planting application or preemergence application of Flumetsulam and early postemergence application of Imazethapyr were better than the other herbicide treatments for Lentil weed control, whereas the lowest relative efficacy was related to preemergence application of Oxyfluorfen, immediately after planting application of Isoxaflutole and Metribuzin, and preemergence application of Pendimethalin Arya. Conclusion Based on the results of this research, the application of flumezolam immediately after planting (20 g ha-1) and imazatapir(350 ml ha-1) as pre-emergence application have had more efficiency for weed control in lentil than other herbicide treatments, while the pre-emergent application of oxyfluorfen, the application immediately after planting of isoxaflotel and metribyozin have had the effects of plant burning on lentil crops and the pendimethalin Aria herbicide is not recommended in lentil farms because of to the lack of control of broadleaf weeds under normal conditions in lentil fields. In general, among the experimental treatments, in terms of the level of control efficiency and the level of safety for the lentil crop, the application of flumetsulam immediately after planting was the best herbicide treatment. [ABSTRACT FROM AUTHOR]