ABSTRACT

US cotton growers are adopting COTMAN, a COTton MANagement system developed at the University of Arkansas, to monitor crop development and aid in making end-of-season decisions. Currently, research-based decision guides have been developed to aid in identifying the last effective boll population and determining dates for safe termination of insect control and the application of defoliants based on physiological cutout, or NAWF=5. An area of cotton production that may benefit from COTMAN is the decision of when to stop irrigating the crop. The objective of this research was to investigate a crop-based recommendation for timing the final irrigation on cotton. Studies were conducted during the 2000 through 2003 growing seasons in four mid-South states (Missouri, Arkansas, Mississippi, and Louisiana) to investigate the response of upland cotton to late-season irrigation. Irrigation treatments consisted of different irrigation termination dates at each site, with the first termination treatment targeted for approximately NAWF=5 or physiological cutout. Data from 12 experiments were included in this analysis. Marginal yield was calculated for treatments and the resulting data was modeled with a cubic polynomial to indicate the relationship between marginal revenue and degree day heat units (DD60) after NAWF=5 for various cotton lint prices. The resulting marginal revenue equations were set equal to the marginal cost of an additional irrigation and solved for the optimal termination points. For prices ranging from $0.35 to $0.75 per pound of lint, the optimal termination point was 545 ± 16 DD60 past NAWF=5.