Effectiveness of different weed control practices on weed biomass and yield of onion (Allium cepa L.)

Introduction
Onion (Allium cepa L.) is a monocotyledonous and biennial plant of the Alliaceae family, which is used as a vegetable and seasoning. This plant needs a lot of water and fertilizer, which increases the risk of weeds. The decrease in onion yield due to weed infestation is estimated to be around 40-80% (Channappagoudar and Biradar, 2007, Angiras et al., 2008). The use of herbicides and agronomic measures are among the effective ways to control weeds in onion fields. Metallachlor is an inhibitor of cell division and this herbicide controls annual narrow-leaved weeds and broad-leaved weeds. Periact is a herbicide from the group of dinitroanilines and it is absorbed through the roots and leaves and by preventing cell division, it causes the death of the plant in a short period of time after germination. One of the non-chemical solutions to suppress weeds is the use of mulch. Soil covers include organic and inorganic compounds, according to existing reports, plastic soil covers are more important for controlling soil conditions and increasing yield (Darvishi et al., 2020). Since the research in the field of weed control in onions is limited and due to the increase in the cultivated area of this commercial plant, manual control of its weeds is difficult, and on the other hand, some herbicides may not be effective. This study was conducted with the aim of comparing the effectiveness of different methods of weed control.
Materials & Methods
With the aim of comparing the effectiveness of different methods of weed control and its effect on the onion crop in Ramhormoz region, an experiment was conducted in the form of a randomized complete block design with four treatments. The treatments include: T1 = black plastic mulch, T2 = transparent plastic mulch, T3 = S-Metalachlor pre-emergent herbicide and T4 = S-Metalchlor pre-emergent herbicide + pendimethalin and control treatment (T5) in four replication. The height of the plant and the total number of leaves per plant were measured at intervals of 15 days during three times after planting the seedlings. Bulb diameter and bulb weight at the time of harvest were estimated from a surface equivalent to one square meter. To check the effectiveness of experimental treatments on weed control, quadrats of 0.5 x 0.5 square meters were used and weeds were removed at the time of harvest. The measurement of dry biomass was calculated after placing in an oven at 70°C for 48 hours.
Results & Discussion
Onion yield changes were between 658 and 39150 in the field, which was more than the highest amount related to the black mulch treatment and the lowest was observed in the transparent mulch treatment. Control, Dolgold, Priact and transparent mulch treatments showed 23.8, 22, 8 and 40.5 percent reduction in yield compared to black mulch treatment, respectively. It seems that the use of black plastic mulch affects the absorption of nutrients by the roots by modulating the soil temperature, and also affects plant growth by controlling weeds and maintaining soil moisture. The lowest weed biomass (wet and dry weight) with 140.4 and 72.2 g.m-2 and with a reduction of 78.4 and 77.9 percent compared to the control in the polyethylene mulch treatment black was obtained. Applying the transparent plastic mulch treatment increased the fresh and dry biomass of weeds by 23% and 20%, respectively, compared to the control treatment. It seems that the transparent polyethylene mulch has warmed the soil with the passage of light and has made the conditions more favorable for the germination and growth of weeds. In such conditions, most types of weeds have been able to sprout and grow green. Unlike the transparent soil cover, the black polyethylene soil cover did not provide germination conditions due to the lack of sunlight transmission and caused the growth of germinated weeds.
Conclusions
The results of this research showed that the improvement of onion quantitative traits was affected by the experimental treatments. So that the use of black plastic mulch, due to preventing the light from reaching the soil surface, reduces the germination and density of weeds by 57.7% compared to the control treatment and up to 62% compared to transparent polyethylene soil cover.