# Effect of Controlled Defoliant Application on Cotton Fiber Quality
## 1 Introduction  
Cotton (_Gossypium hirsutum_ L) is a very important industrial crop [1]. It is a perennial crop which has indeterminate growth habits. However, in most parts of the United States of America (USA), it is grown as an annual crop with the application of growth regulators [2]. It has a long history of cultivation and domestication [3]. Cotton has a huge role in the economy of the USA as the country is the third largest cotton producer and one of the leading cotton exporters in the world [4]. Cotton is grown primarily in 17 southern-States known as the \"Cotton Belt.\" Among them, Texas is the largest producer contributing approximately 40 percent of USA cotton, then followed by Georgia, Mississippi, and Arkansas. The USA produced approximately 20 million cotton bales contributing nearly 7 billion dollars to the USA economy during the marketing year (MY) 2019, i.e., August2019-July 2020 [5]. Even though South Carolina is not a top cotton-producing state, cotton is one of the main cash crops of this state. Defoliation is the falling of leaves mainly because of maturity, senescence, and injury. During defoliation, water-conducting tissues remain alive until the leaf drops-defoliation differs from desiccation. Defoliation is a physiological phenomenon in which an abscission layer develops between the petiole and stem. The enzymatic action of plant enzymes such as cellulase and pectinase then digest the cell wall and middle lamella of the abscission zone, which causes leaves to separate from the stem and drop from the plant. Plant hormones, such as abscisic acid and ethylene, play a major role in the development of abscission layers, whereas auxins discourage abscission layer development. [6]. Although defoliation is a natural phenomenon, untimely leaf fall requires the spraying of chemical defoliants to prepare the cotton plant for mechanical harvesting. Typically cotton defoliants are applied approximately two weeks prior to the expected harvest date. Appropriate and timely defoliation plays a major role in increasing harvesting efficiency, minimizing trash content, and improving the fiber quality [7]. Depending on the mode of action, defoliants are herbicidal or hormonal. Herbicidal defoliants injure the plant causing ethylene production which encourages leaf drop [8]. Some examples of herbicidal defoliants are Carfentrazzone-ethyl, Thidiazuron, Diuron, and Tribufos. For the herbicidal defoliants, which do not move within the plant, adequate chemical penetration and the appropriate spraying pattern are necessary for adequate defoliation. Therefore, each leaf should receive the chemical for a successful leaf drop. However, hormonal defoliants encourage ethylene production and inhibit auxin transport in the plant, which encourages leaf abscission [9]. In the USA, chemical defoliants such as Tribufos, Dimethipin, and Ethephone are widely used [10; 11]. The defoliants do not directly influence boll opening thus, they must be applied in combination with a boll opener, to provide satisfactory defoliation and boll opening [12; 13]. Poor defoliation can lower the fiber quality while defoliating too early lowers yield and micronaire [14]. As leaves are the major source of carbohydrates production through photosynthesis, early defoliation can interfere the production and movement of energy from leaves to growing cotton bolls [15]. Therefore, cotton defoliation is often practiced when 60% of bolls are opened to avoid loss in yield and fiber quality [16]. However, [PERSON] et al. reported that defoliation is possible even when 40% of the cotton bolls were opened as they found insignificant results on seed cotton yield, pinning percentage, and fiber quality across different percentages of boll opening [14]. Although several factors affect cotton yield and fiber quality; timing of defoliation application [17], type of chemicals used [18], and spraying technologies are major factors [19].