Crop managers need to determine the most profitable time to defoliate cotton (Gossypium hirsutum L.) in a high rainfall environment such as the coastal region of Texas. In cotton production, delaying defoliation exposes open bolls to a higher probability of rainfall, and thus, reduces lint yield and fiber quality. Premature defoliation, however, has detrimental affects on lint yield and fiber quality. A more recent method to determine defoliation is based on heat-unit (HU or DD15) accumulation after physiological cutout or five nodes above white flower (NAWF=5). Results have been inconsistent across a wide range of field environments when utilizing HU accumulation past cutout; therefore, adoption of this method has been limited. Many regions of the Cotton Belt have maximum day time temperatures during the growing season that are above optimum for maximum growth. Field studies were conducted for three consecutive growing seasons in the Brazos River Valley and Upper Gulf Coast regions of Texas. The purpose of this research was to identify an upper temperature threshold (UTT) for calculating degree days for defoliation timing. The experimental design consisted of a split-plot design with four replications. The main plots consisted of three upper temperature thresholds (32°C, 35°C, and no upper limit) and the subplots were five HU timings (361, 417, 472, 528, and 583) accumulated from date of cutout. Utilizing an UTT to calculate daily HU failed to explain differences in the optimum time to defoliate based on accumulated HU from cutout for the upper thresholds investigated. Accumulated HU had a significant impact, however, on defoliation timing. Comparison of the two locations showed that maximum lint yield was obtained at 472 HU and 52% open boll at Wharton County versus a maximum of 528 HU and 62% open boll for the Burleson County location. Employing the NACB=4 method to time defoliation at both locations would have resulted in premature application of harvest aids and reduced lint yields. No differences were observed in adjusted gross income values at Wharton County among the 417, 472, 528, and 583 HU treatments. For Burleson County, adjusted gross income peaked in value at 528 HU.

Among the available methods of creating selectable variation, induced mutagenesis has been historically under-utilized in cotton improvement. Dick Auld showed that chemical mutagenesis could be used to enhance fiber length of a medium staple cotton cultivar without sacrificing yield. The goal of this project was to determine if mutagenesis could be used to improve the fiber quality of a germplasm line already considered to be at the upper-limits of fiber length. Seed of TAM 94 L-25 were treated with EMS in 2001 and the M2 generation was produced at Lubbock, Texas in 2002. More than 1200 M3 plants were grown at College Station, Texas in 2004, harvested individually, and HVI fiber properties determined. The top and bottom 1 percent for UHML, strength, and elongation were selected and the seeds of these individuals planted as an M4 progeny row nursery in 2005. Approximately ten individual plants per progeny row were harvested for re-evaluation of fiber parameters. From the approximately 1600 individual TAM 94L-25 M4 plants harvested in 2005, 61 were selected and subsequently treated as pure lines. Agronomic performance trials were conducted on 61 of those TAM 94L-25 M lines along with the M0 check and two commercial cultivar checks, Fiber Max 832 and Phytogen 355, in 2008 and 2009 in College Station and Weslaco, Texas. Within-boll yield components were examined for 13 representative mutant lines and the three checks. TAM 94L-25 averaged 751 kg lint ha^-1, 31.1 mm UHML, 303 kN m kg^-1 fiber bundle strength, and 6.0 percent elongation. The 61 mutant lines yielded from 366 to 932 kg lint ha^-1, exhibited UHML from 24.3 to 34.9 mm, fiber bundle strengths of 261 to 333 kN m kg^-1, and elongations from 5.4 to 8.1 percent. Seed surface area of the TAM 94L-25 M-lines was between 99 and 124 mm^2, and fibers per unit seed surface area from 123 to 168 fibers mm^-2. The M0 parent, TAM 94L-25 averaged 125 mm^2 seed-1, and 128 fibers mm^-2. The data presented herein demonstrate that EMS-induced mutagenesis was successful in creating TAM 94L-25 M-lines with superior fiber and yield traits to that of the non-mutated, high fiber quality parent, TAM 94L-25.

This research was focused on increasing the efficiency of upland cotton production in the northern cotton belt through the use of new fertilizer formulations, placement, and timings. The objectives of the experiments reported in this thesis were to: 1) evaluate the effects of side-dress potassium (K), sulfur (S), and boron (B) formulation and application timing on tissue nutrient levels during the bloom period; 2) evaluate lint yield response of cotton to different formulations of nitrogen (N), K, S and B applied at side-dress; and 3) compare 5x5 banding (5 cm beside and 5cm below the seed) and deep placement of complete N-P-K-S blends to current nutrient management strategies on early season plant growth, nodes above white flower, total nodes, petiole nutrient concentrations during bloom, and lint yield. Tissue S and B concentrations were increased more often than K concentrations when the nutrients were applied with side-dress N. When evaluating P and K placement, petiole P levels were found to be significantly higher in unfertilized plots when no side-dress N was applied. Phosphorus and K placement and/or rate had no effect on lint yield when N was applied at side-dress during the study. Environmental conditions potentially influenced the response to P and K placement as 5x5 placement produced yields significantly higher during 2013 growing season at location 1, while deep placement produced significantly higher yields in 2014 at location 3. As a result, Virginia nutrient management recommendations for cotton have been updated to incorporate management strategies to maximize lint yields. / Master of Science

Identifying the proper tillage system which provides the best agronomic benefits for cotton production in the coastal plain soils of Virginia was the basis for this research. Strip-tillage was evaluated from 2015-2016 on-farm to determine the effects of annual and biennial treatments on plant growth and lint yield, as well as measuring the impacts on soil compaction. Also, small plot tillage experiments were conducted from 2013-2016 assessing no-till, conventional tillage, minimum tillage, and strip-tillage as well as the subsequent effects of these systems on four cotton varieties. Biennial strip-tillage produced similar lint yields to annual strip-tillage at 3 of 4 locations, with only one location showing a significant difference in lint yield of 135 kg ha-1. Persistence of subsoil tillage within the row from the previous year was observed at some locations and plant heights were not different at all locations, although annual strip-tillage provided deeper potential rooting depths both early season and at harvest. In short term tillage systems, minimal penalties in plant growth and lint yield were observed in no-till verses the other systems, primarily associated with greater soil compaction, shorter plant heights, and lower yields. An overall 8% reduction in yield was found with no-till systems, with no significant differences in yield among tillage systems observed in any year. Varietal effects on plant growth and yield were observed annually, with FM 1944 GLB2 being the shortest plants, and DP 1321 B2RF having the tallest plants. No tillage by variety interaction was observed, supporting the idea that varieties respond similarly across tillage systems. / Master of Science

Precision management of cotton production can increase profitability by decreasing inputs. The overall objective of this project is to improve cotton production by minimizing seeding rates while still maximizing yields and lint quality in water-limited soils. The research for this study was conducted at the Texas AgriLife Research IMPACT Center located in the Brazos River floodplain. In 2008 and 2009, 27 measurement locations were selected in production-sized center-pivot irrigated fields and planted in cotton variety Deltapine 164 roundup ready flex / bollgard II. Sites were selected based on soil apparent electrical conductivity (ECa) values, in a low, medium, and high ECa zones. Three seeding rates (74,100; 98,800; and 123,500 seeds ha-1) were established in each of the three ECa zones with three replications. In 2009, an additional seeding rate was added at 49,400 seeds ha-1. At each measurement location, soil texture, soil moisture (weekly), lint quantity and quality (High Volume Instrument) were measured. An additional replication for each ECa zone and seeding rate was selected for lint quantity and quality (HVI) measurements. Results indicated that cotton lint yield increased as ECa values, clay content, and water holding capacity of the soil increased. The seeding rates did not consistently affect cotton lint yield or quality. Seeding rates of 74,100 and 49,400 seeds ha-1 in a low and medium ECa zone for IMPACT-08 and -09 yielded more lint (300 kg ha-1), respectively. HVI lint quality parameters, such as, micronaire, fiber length, strength, uniformity, and elongation were significantly better in ECa zone 3. While the seeding rates did not affect the amount of soil water used throughout the season, lint yield variations between ECa zones can be explained by the rate at which soil water was used. Lower rates at which soil water was used within ECa zone 3 resulted in higher lint yields when compared to ECa zones 1 and 2, which used soil water faster and at greater depths. The findings suggest that irrigation applied to the low ECa zone was not sufficient to meet the plants demand, while in a high ECa zone, irrigation could have been reduced, resulting in cost savings through reduced inputs.