Determination of the Critical Period of Weed Control in Linseed (Linum usitatissimum L.) in Yasuj

Due to the high sensitivity of linseed to weed competition, high costs are imposed for weed management of linseed production. Determining the critical period of weed control is a practical strategy that is considered as one of the main pillars of the integrated weed management system due to the limited chemical control of weeds. The critical period is the period of the plant growth season during which weeds must be controlled so that yield damage does not exceed a threshold. This threshold is determined based on the cost of weed control. Critical period estimation can play an important role in achieving sustainable and integrated weed management in crop production and reducing the costs. Therefore, the aim of this study was to determine the critical period of weed control in order to stabilize the system as much as possible, minimize the use of chemical herbicides and reduce farmer costs on linseed in Yasuj.

This experiment was conducted in a randomized complete block design with 14 treatments and three replications in 2017 at the research farm of the Faculty of Agriculture, Yasouj University.Treatments included periods of weed control (WF) and weed interference (WI) up to 10, 20, 30, 40, 50 and 60 days after linseed emergence and also two treatments of weed control and weed interference for the whole season (110 days). Three ridges (4 m long and 50 cm apart) were planted in each plot and two rows of linseed (row spacing and plant spacing of 25 and 4 cm) were planted on each ridge. Sampling was done to calculate the density and dry weight of weeds in the control treatment group (WF) at the end of the growing season and for interference treatments (WI) at the end of each stage before weeding. At the time of linseed maturing, yield components, biological yield, grain yield, oil yield, element content and protein of flaxseed were measured.The critical period of weed control was determined using Gampertz nonlinear regression equations (to determine yield response to weed-free periods) and Logistic nonlinear regression equation (to determine yield response to weed interference periods).

Weed control and weed interference periods had a significant effect on weed dry weight. With increasing control and interference periods weed dry weight increased and decreased, respectively. Throughout the whole season weed interference treatment produced the highest amount of weed dry biomass (149 g m-2), and among the control treatments, the highest dry weight of weeds was observed in the 20-day control treatment.
With increasing control periods, weed plants have the opportunity to grow in a situation where the relative competitiveness of linseed plants has improved due to the expansion of lateral branches, increase in height and more intense shading. For this reason, weed growth is limited. However, increasing the length of interference periods, weeds can grow easily due to the weakness of the initial linseed seedling at the beginning of the season and the lack of sufficient growth resources. The effect of weed interference and weed control treatments on grain content (P, K, Zn and Fe), grain protein, yield components, biological yield and grain yield, oil percentage and oil yield were significant. With increasing control periods and decreasing interference periods of all elements in linseed seed, the number of capsules per plant, the number of seeds per capsule and 1000-seed weight increased. Biological yield of linseed in 10 and 20 day interference treatments was not significantly different from 10 and 20 day control treatments, because weed competition in the first season has a very negative effect on growth and biomass production of linseed and seedlings linseed have poor competitiveness against weeds. Therefore with weed control measures at the beginning of the growing season can improve seed and biological yield of this plant.The critical period of weed control for linseed in Yasuj, based on a 10% reduction in grain yield, was determined from 5 days to 44 days after the emergence linseed, equivalent to 142.8 to 589.85 GDD from the time of linseed planting.

In order to prevent weeds growth, improve the quality (elements, oil and protein) and quantity (yield) of linseed grain and reduce the costs, it is recommended that weed control start from five days after linseed emergence to 39 days after aforementioned time.