Response of Cotton Varieties to Varying Plant Growth Regulator Programs in South Carolina

The application of plant growth regulator (PGRs) chemicals have become common practice in cotton production across the cotton belt. Plant growth regulators appear to be important in stimulating early-season growth and in manipulating resources into harvestable yield. Proper use of PGRs often reduces plant height (Heilman, 1981), which may result in reduced yield losses caused by boll rots, hardlocks, and shading commonly associated with dense canopies (Snow et al., 1981). Although the physiological responses of cotton plants to PGRs have been fairly consistent throughout the Cotton Belt, reports on the yield response to PGR applications have been inconsistent. Some researchers have found yield increases (Briggs, 1980, Walter et al., 1980) with the application of PGRs, while others have found yield decreases or no yield effects (Crawford, 1981; Feaster et al., 1980, Thomas, 1975). Improved yields with the applications of PGRs have often been attributed to improved photosynthetic capacities (Gausman et al., 1979), more vigorous early-season growth (Oosterhuis and Zhao, 1994), higher boll retention (Kerby, 1985; Kerby et al., 1986), better light penetration (Reddy et al., 1990), improved earliness (Kerby, 1985), and increased root mass (Oosterhiuis and Zhao, 1994). However, it appears the proper rate and timing of PGRs depends upon many environmental factors, such as soil moisture, ambient air temperatures, and fertility (Cathey and Thomas, 1986). Yield responses appear to be related to environmental factors encountered by the plant throughout the growing season, with increased yields occurring under conditions that favor excessive vegetative growth (high N rates, excessive rainfall, thick stands, etc.) and yield decreases occurring under conditions that limit proper growth (excessive drought, high temperatures, N deficiency, etc.). The objective of this study was to compare and contrast yield performance and crop development of experimental and commercial cotton varieties in South Carolina in response to various plant growth regulator programs. A replicated field study was conducted in 2008 at the Pee Dee Research & Education Center located in Florence, SC on a Norfolk loamy sand soil type. The plots were planted on May 20, 2008 with a JD 1700 vacuum planter at a rate of 4 seed per row foot. Plots consisted of 4 rows, spaced 38 inches apart and were 40 feet long. Three plant growth regulator programs and ten cotton varieties were arranged as split-plots in a randomized complete block design with four replications. The three plant growth regulator programs were in main plots, and the ten varieties were in sub-plots. Varieties consisted of five commercially released varieties (DPL 555BR, DPL 143BR, ST 4554B2RF, DPL 0924B2RF, and DPL 0935B2RF) and five experimental varieties (DPLX 07X440DF, DPL 0912 B2RF, DPL 0949 B2RF, MCS 0711B2RF, and MCS 0702B2RF). Plant growth regulator programs consisted of: 1) untreated, 2) a standard program, and 3) an aggressive program. The standard program consisted of Pentia sprayed at a rate of 8 oz/a on July 1, 2008 at the matchhead square stage followed by Pentia sprayed at a rate of 16 oz/a on July 10, 2008 at the early bloom growth stage. The aggressive program consisted of Pentia sprayed at a rate of 8 oz/a on June 20, 2008 at the 6 to 8 leaf stage, followed by Pentia sprayed at a rate of 16 oz/a on July1, 2008 at the matchhead square stage, followed by Pentia sprayed at a rate of 16 oz/a on July 10, 2008 at the early bloom growth stage. Although weather conditions in SC in 2008 were optimal for cotton production (3 bale or greater lint yield), the aggressive PGR program (3 sprays of 40 oz/a total) reduced lint yield 6 to 7% (Table 1) compared to a more standard PGR program (2 sprays of 24 oz/a total) or the untreated (no PGRs applied). PGR applications appeared to change the internal partitioning of dry matter within cotton bolls. The use of Pentia in this study reduced gin turnout and micronaire, and increased fiber length. Although not statistically significant, the lint yields of many of the new B2RF varieties evaluated in this study were greater than DPL 555BR. The yield potential of these new B2RF varieties is extremely promising to many SC cotton growers who have relied so heavily on DPL 555BR as their primary variety over the last five or six years. DPL 555BR will not be commercially available after 2010. No PGR x variety interactions were found for any of the parameters measured in this study, with the exception of seedcotton (Table 2). Seedcotton production was reduced 21% with DPL 0912 B2RF by the aggressive use of PGRs during the growing season compared to the untreated plots. Likewise, seedcotton production was reduced 15% with DPL 143B2RF by the use of an aggressive PGR program compared to a more standard PGR program. Seedcotton production was increased 14% with DPL 555BR and DPL 0935B2RF by the use of a standard PGR program during the growing season compared to no PGRs applied at all (untreated).