Evaluation of Weed Interference Effects on Yield and Yield Components of Hemp (Cannabis Sativa)

Introduction
Hemp (Cannabis sativa L.) is one of the oldest cultivated annual crops in the world belonging to the cannabaceae family and annual wind-pollinated broadleaf. It is originated in central Asia and considered a multi-purpose crop including stalk fiber (apparel, fabric, bags), stalk hard (animal bedding, mulch, biofuel, filter, and so on), seeds (nut, and snacks), oil (salad oil, cosmetic, shampoo, soap, oil paint, industrial oil, and solvent), leave and inflorescence (anti-microbe (hand soap), and agrochemical products (insecticide and herbicide). Weed competition for nutrients, water, and light led to decreased growth and yield of hemp. Mechanical and agronomical techniques are commonly used for weed management in hemp. Since mechanical and agronomical techniques are expensive and time-consuming, finding a new approach to save time and money is essential. Determination of weed critical period may facilitate arriving at this approach. The critical period of weed control (CPWC) refers to a part of the integrated weed management (IWM) program and is defined as a part of the crop growth cycle in which weeds must be removed for avoiding economic yield losses due to weed competition with crops for water, light, and minerals. On the other hand, CPWC is a period in which crops must be growth-free of weeds. Numerous agents' effects on CPWC include crop species and growth characteristics, crop density, crop variety, and crop planting pattern. The main goal of this study was to determine the effect of hemp density on CPWC and hemp yield and yield components' responses to the critical period of weed control.
 
Materials and Methods
A field trial was carried out in 2016 at the research field of Birjand University in South Khorasan Province, Birjand, Iran (latitude 32°, 86′, 49″ N, longitude 59°, 22′, 62″ E, and altitude 1491 m). Before sowing seeds, the field was fertilized with urea (250 kg ha^-1) split at two times (sowing time, 2 mount after emergence), P₂O₅ (100 kg ha^-1), and K₂O (100 kg ha^-1). The field was plowed with a mouldboard plough (30 cm depth), and then harrowing by a disk used for seedbed preparation twice, and finally, leveling the soil surface by leveler tools. Hemp has never been cultivated in this field. The hemp seeds were sown on May 4, 2016, in a cluster, 5 seeds in each hole with 60 cm between rows and 10 and 20 cm between plants in the row for 8 and 16 plants/m², respectively. Sown depth was 3 to 4 cm. Crop emergence began about 2 weeks after planting. Two weeks after emergence (four weeks after planting), the plants were thinned to reach a determined density. The experiment was arranged in a randomized complete block design (RCBD) in a factorial arrangement with three replications. Each experimental plot was 6 m long, 3 m in width, and included 5 rows. Data were collected in three central rows. Treatments included six weed-infested periods (2 (WI₂), 4 (WI₄), 6 (WI₆), 8 (WI₈), and 10 (WI₁₀) weeks after crop emergence (WAE)), in which weeds were allowed to grow to 2, 4, 6, 8, and 10 weeks after that control conducted by hand hoeing until harvest time. In weed-infested control (WIC), weeds were allowed to grow in all growing seasons duration. Six weed-free periods (2 (WF₂), 4 (WF₄), 6 (WF₆), 8 (WF₈), and 10 (WF₁₀) weeks after crop emergence (WAE)), which weeds were hand hoeing control to 2, 4, 6, 8, and 10 WAE and then weeds were allowed to grow until harvest time. In weed-free control (WFC), weeds control in all season growth duration. The experimental treatments during the growing season of hemp are described in Figure 1. Seed hemp yield and yield components were determined at the end of the growing season.
 
Results and Discussion
Weed-infested and weed-free periods treatments have significant effects on the seed yield and yield components including flower number and seed number per plant of hemp (Table 1). Moreover, hemp density has a significant effect on the seed yield and yield components including flower number and seed number per plant of hemp (Table 1). Higher crop density led to limited CPWC since under weed-infested fields using higher crop density led to improved crop competition ability (Zimdal, 1993). (Hayat et al.,2003) reported that high crop density led to decreased seed yield of crops by increasing inter-species competition. (Purcell et al.,2002) demonstrated the seed yield of soybean depends on crop density whereas by increasing crop density firstly seed yield was increased but under high crop density seed yield was decreased. Therefore, using proper crop density is known as an important challenge to crop production. Flowers per plant, seed number per plant, and seed yield were affected by crop density. Results of this study demonstrated that under weed-infested fields using 16 plants/m^-2 is more suitable than 8 plants/m^-2 compared to weed-free in Birjand conditions (Dry climate)(Table 2).
 
Conclusions
A greater understanding of weed behavior can help with IWM strategies. The critical period of weed control determination is a key factor for IWM strategies that led to less introduction of herbicides into the environment, saving money and time, and so on. Data obtained from the results of this study revealed that more crop density led to a decrease in the duration of the critical period of weed control in hemp.