تأثیر تراکم کاشت شاهدانه (Cannabis sativa L.) بر دوره بحرانی کنترل علف‌های هرز

 Introduction: Industrial hemp (Cannabis sativa L.) is an annual herbaceous crop of Asian origin considered to be one of the oldest crops known to man and it is traditionally grown in many regions of Europe for its fiber production. In agricultural ecosystems, weeds known as a main factor that reduces quantity and quality of products. The competition of weeds with crop is always one of the most important problems in achieving maximum yield. Therefore, weed management is essential to increase production. Increasing plant density is one of the strategies to reduce weed damage. The critical period of weed control refers to a part of crop growing season in which weeds should be removed in order to prevent crop loss due to weed competition. Crop density may be shorten weed critical period. Therefore, this study was conducted for evaluating the impact of crop density on hemp weed critical period.
Materials and Methods: A field experiment was conducted in 2016 at the Agricultural Research field of University of Birjand, Birjand. The experiment was conducted as a factorial design with the treatments arranged in a randomized complete block design with three replications. The experimental factors consisted of a quantitative series of both increasing duration of weed interference (WI) and length of weed-free (WF) periods, and hemp plant density at two levels, 8 and 16 plant m-2. Hemp seeds were planted on May 5, 2017. Each plot consisted of six rows spaced at 60 cm between rows and 10 cm (16 plant m-2) and 20 cm (8 plant m-2) inter row. Each plot consisted of 3 m wide and 6 m long (18 m2). Each plot divided two parts, one part used for measuring morphological traits, and the other part used for measuring final yield. 180 days after sowing the hemp plants was harvested. Equations describing crop yield response to weed interference were fitted to the hemp yield data using a nonlinear regression. The Gompertz equation was used to describe the effect of increasing duration of weed-free period and the logistic equation was used to describe the effect of increasing lengths of weed-infested period on the seeds yield of hemp.
Results and Discussion: Analysis of variance results showed that grain yield per hectare was significantly affected by plant density and competition at 1% level, but the interaction effect of density in competition was not significant (Table 1). The highest grain yield was obtained from treatments that most of the growth was free of weeds. The highest grain yield was WI2, WFC, WF10 and WF8 treatments were respectively the highest yield. WF2, WIC, WI10 and WI8 treatments had the lowest yield due to more weed competition. WFC treatment increased the grain yield by 258.28% and WIC compared to WIC (Fig. 2). Martin et al. (21) reported a decrease or increase in grain yield due to the reduction or prolongation of the agronomic competition with weed. Density also had a significant effect on grain yield. The grain yield was 16 plants m-2, 56.57% higher than 8 plants density (Fig. 3). Martin et al. (21) reported that decrease or increase of grain yield by prolongation or reduction of the crop competition with weed. The grain yield of 16 plants m-2 was 56.57% higher than grain yield at 8 plants m-2 (Fig. 3). This increase in grain yield at 16 plants m-2 can be attributed to more plant number and more flower per square meter, which increases grain yield. Likewise results of this study, Mirzai and Madhajj (23) reported that decreasing the intervals between and within rows increases the grain yield per unit area and decreases in each plant. According to the results of analysis of variance, biological yield per plant (Table 2) had a significant effect on density and competition at 1% level (P <0.01), but the interaction between density and competition was not significant. Biological yield per plant at 8 plants m-2 density of was 14.86% higher than 16 plants m-2 (Fig. 5). The results of the present study indicated that to prevent yield losses higher than 5%, an “efficient” weed control for the hemp could be achieved by keeping the crop weed free between 616 and 988 GDD for 8 plant m-2 and 667.5-889 GDD for 16 plant m-2 , respectively. To prevent yield losses higher than 10%, an “efficient” weed control for the Hemp could be achieved by keeping the crop weed free between 547 and 1093 GDD for 8 plant m-2 and 616-935.5 GDD for 16 plant m-2 , respectively.
Conclusions: According to the results of this study, grain yield per hectare was significantly affected by plant density and competition at 1% level, but the interaction effect of density in competition was not significant. The results of this study revealed that weed critical period was decreased in 5% yield losses from 38 days (8 plant m-2) to 31 (16 plant m-2) and in 10% yield losses from 29 days (8 plant m-2) to 19 (16 plant m-2). These finding support the early suppression of the weeds in order to avoid dramatic crop yield losses.