Effects of various types of defoliants on some vegetative aspects and yield of cotton CV Varamin in Garmsar conditions

Introduction
The phenomenon of leaf drop, often observed in cotton plants, plays a significant role in the overall efficiency and productivity of cotton harvesting (Wang et al, 2023). As cotton plants mature, the shedding of leaves can influence the exposure of the cotton bolls to sunlight, thereby affecting their development and quality (Naderi Arefi & Abedini Esfahlani, 2013). Moreover, reduced leaf coverage can facilitate easier access for mechanical harvesters, ultimately increasing harvest speed and lowering labor costs (Nowrouzieh et al, 2021). Understanding the dynamics of leaf drop is crucial for farmers aiming to optimize their yield and enhance the quality of their cotton crop. In other words, seedcotton quality can be lost by trash and leaf contaminants through harvest process. Cotton indeterminate growth habit leads to coocurance of vegetative and reproductive growth. In the event of high vegetative growth (Naderi Arefi & Abedini Esfahlani, 2013), plant residue especially un-opened bolls acts as pest overwintering places. Defoliants are used for crop terminantion and reducing of these adverse effects of rank growth (Nowrouzieh et al, 2021). In order to examine the effects of defoliants on yield and earliness of cotton, this experiment was conducted in 2019 and 2020 growing seasons in Garmsar agricultural and natural research station in Davar Abad. Experimental design was RCBD with three replication arranged in split plot.
Materials and Methods
In order to investigate Effects of various types of defoliants on yield and earliness of cotton CV Varamin in Garmsar conditions, In the 2019-2020, a two-year experiment was conducted in split plots in a randomized complete block design with three replications at the Agriculture and Natural Resources Research Center of Tehran Province (ANRCT), located in central Varamin.
Main plots allocated to defoliant treatments (glyphosyte, paraquat, def, ethphone, glyposyte + papraquat, glyposyte + ethphone, ethphone + papraquat and glyposyte + papraquat + ethphone) and Sub plots were treatment at 50 and 85 percent open boll stages
To determine the vegetative characteristics and yield components, 5 plants were selected from each plot and the average of the measurements was recorded. To determine the yield, after removing the two side rows and 1 meter above and below the remaining rows, the entire plant was harvested. The results were recorded in Excel software and the data were analyzed with SAS 1.9 software. Due to the non-significance of the Bartlett test, composite analysis was not performed and the results of the two years were analyzed separately.

Results & Discussion
Results showed that treatment of glyposyte + papraquat + ethphone at 50 and 85 percent open boll in both years led to abscession of some vegetative and reproductive organs which result in increased plant height because of regrowth. In first year of experiment, open boll number increased with sprying of def and glyposyte + ehphone. In second year, the effect of def and ethphone on boll opening at 85% open boll stage was higher than 50% open boll stage. def treatment at 85% open boll resulted in highest yield (7662 kg h-1) among the all treatments. Def and ethphone were superior with respect to earliness. The earliest harvest in first year obtained from def and ethphone at 50% open boll, and def at 85% open boll (91%). In second year, the earliest treatment was ethphone at 50% open boll. At 85% open boll the earliness from def reached to 91.84% which was superior to the others at this stage. Defoliation at 85% open boll growth stage was superior to 50% open boll growth stage. Totally, defoliation with def or ethphone will result in higher yields and earliness.

Conclusion
Considering of suitable results from some treatments of this study, point of view of defoliation and improved boll opening, determining of dosage and sprying time of them, can provide new and low cost defoliants.