Planting and Irrigation Termination Timing Effects on the Yield of Upland and Pima Cotton (1997)

There have been conflicting results reported about the effect on cotton ( Gossypium spp.) lint yield of altering planting and irrigation termination (IT) timing. The objectives of this study were to identify a planting window (PW), on a heat unit (HU) basis, and IT timing, as a function of crop growth stage, for optimum yield potential of Upland ( G. hirsutum L.) and American Pima ( G. babadense L.) cotton. Two PWs of Upland ‘Deltapine 90’ (DPL 90), Pima ‘S‐6’, and IT treatments were included in field experiments for 11 site‐years. Planting windows were defined as PW1 and PW2 for plantings prior to and following 600 HU accumulated after 1 January, respectively. Two IT treatments were imposed for each planting. Irrigation termination in the desert Southwest generally results in cessation of growth (crop termination). The first IT treatment (IT1), was imposed to ensure full development of bolls set up to cutout, and the second (IT2) was after two additional irrigations. From covariate analysis, there was no evidence of interaction between PW and IT, indicating that these treatments responded the same across the different environments for both cotton species. There were, however, differences in lint yields among treatments. For DPL 90, PW1 IT2 yielded 83 and 97 lb/acre more than PW1 IT1 and PW2 IT2; and for Pima S‐6, PW1 IT2 was 118 and 204 lb/acre more than PW1 IT1 and PW2 IT2, respectively. Early planting is necessary for optimum yield potential of full‐season cotton varieties; with the greatest yield coming from early planting and termination after the development of a second fruiting cycle (PW1 IT2). However, if a reduction in input costs and the avoidance of late‐season insect pests are important considerations then cotton should be planted early (300 to 600 HU after 1 Jan) and terminated at the end of the first fruiting cycle (approximately 600 HU past cutout) to maintain the lint yield potential of full‐season maturity types of Upland and Pima cotton. Research Question The traditional approach to cotton production in many areas of the desert Southwest (Arizona, California, and northern Mexico) has been to fully use the long growing season. There are several incentives for the cotton industry in this region to adopt a reduced season approach; these include increasing water costs, limited water availability (for some irrigation districts in some years), nutritional needs of the crop that must be extended the full length of the season, late‐season insect problems, adverse weather associated with the summer monsoon season, and increasing concerns over maintaining high quality lint. Entomologists have often called for alterations in cotton planting and irrigation termination timing from an insect management point of view. Economic pressures and other resource management constraints continue to force producers to find the most efficient means of production. Obviously, a consideration of a shorter season of production involves the alteration of planting and irrigation termination (IT) timing, however, there is considerable disagreement regarding the effect these may have on lint yield. For this reason it is necessary to identify optimum planting windows (PW) and irrigation timing from an agronomic perspective. Literature Summary Agronomists have reported conflicting results on the effect that changes in planting or IT timing have on lint yield for Upland and Pima cotton. Most of the work done in this regard has identified planting and IT in relation to calendar date. Scheduling on a calendar basis does not take into account the inherent year‐to‐year variation. Current technology gives access to weather data so that PW can be identified on a thermal or heat unit (HU) basis. The progression of cotton itself through the completion of the first fruiting cycle (cutout) can then be used to determine when irrigation should be terminated. Research has shown that approximately 600 HU (86/55°F thresholds) are required to develop a fresh flower into a full‐sized, hard green boll (the point at which fiber development is complete). By identifying the flowers that one intends to harvest, adequate soil moisture is required over the entire 600 HU period. In this way, crop growth stage, in relationship to cutout, and the 600 HU “time” interval can be used to schedule the final irrigation. Termination of irrigation results in cessation of crop growth in the irrigated West. Study Description From 1989 through 1992, field experiments were conducted at the University of Arizona Agriculture Centers at Yuma Valley (150 ft elevation, Gadsden clay loam), Maricopa (1100 ft elevation, Casa Grande sandy loam), and Marana (2000 ft elevation, Pima clay loam). Early and late planting, IT, and cotton species (Upland and American Pima) were included as treatments. Early planting followed as near the legal planting date for each location as conditions allowed, a second followed after about 2 wk. Two IT treatments were imposed for each planting and were based on the end of the first fruiting cycle. Therefore, the first IT (IT1) was imposed to maintain adequate soil‐moisture for full fiber development of final blooms at cutout and IT2 was imposed after an additional two irrigations (6 acre‐in. each) were applied in order to promote the development of a “top‐crop.” The Upland cotton was “Deltapine 90” (DPL 90) and “S‐6” was the Pima cultivar. Data from weather stations at each site were used to calculate HU accumulations (86/55°F thresholds). From this data, two PW were defined based on whether the planting occurred prior to, or after, 600 HU accumulated after 1 January (HU 1 Jan). Thus, the first planting window (PW1) ranged from 226 to 571 and PW2 ranged from 705 to 1141 HU 1 January. Analysis of covariance was then completed for each cotton species using these PW and IT timing combinations to define the main effects and the environmental mean lint yield (EM) as a covariate. This EM represents the mean lint yield of all treatments and replicates separately for each study and site‐year. Applied Questions Which planting window and irrigation termination timing combination resulted in the greatest lint yield of full‐season maturity type cultivars of Upland and Pima cotton? Figure shows how lint yield for each treatment varied as a function of EM. A low EM denotes those seasons (or site‐years in this case) that yielded less than in a good cropping season. This becomes a visual way to determine how each treatment affects lint yield over a range of conditions. Figure indicates that all of the treatments responded the same across yielding environments, the lines are parallel (having the same slope), but there were differences in lint yield, the elevation (different y‐intercepts) of each line. On average, for DPL 90, PW1 IT2 yielded 83, 97, and 193 lb lint/acre more than PW1 IT1, PW2 IT2, and PW2 IT1; for Pima S‐6, PW1 IT2 was 118, 204, and 273 lb lint/acre more than PWl ITl, PW2 IT2, and PW2 IT1, respectively. Which is the most efficient planting window and irrigation termination timing option for full‐season cotton varieties in the irrigated West? In this work we have shown that early planting (<600 HU accumulated after 1 Jan) resulted in the highest yield potential of full‐season maturity types of Upland and Pima cotton (Fig. ). Soil and weather conditions in the desert Southwest should be favorable by the time 300 HU have accumulated after 1 January, in order to consider initial plantings. However, soil temperature and weather forecast information should always be incorporated into the implementation of any planting strategy. Late planting and IT (PW2 IT2) did not yield significantly less than the PW1 IT1 treatment for either cotton species, but late termination (IT2) extended the length of the growing season by at least 26 d. Therefore, if termination prior to the rise of late‐season insect pests, or to reduce production inputs, is an important requirement in the production system to improve efficiency, then IT timed to mature flowers set at cutout (IT1) should be employed. From an entire production system standpoint, the PW1 IT1 combination will maintain lint yield potential of full‐season maturity types of Upland and Pima cotton and yet avoid costly late‐season inputs to maximize production efficiency. Upland (a) and Pima (b) cotton lint yield as a function of environmental mean for each planting window (PW) and irrigation termination (IT) combination. The first PW (PW1) was <600 heat units (HU), and PW2 was >600 HU accumulated after 1 January. The first IT (IT1) was slightly after cutout and the second (IT2) was after two additional 6 acre‐in. irrigations. image