Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2004
Publication Date: 3/1/2005
Citation: Viator, R.P., Nuti, R.C., Edmisten, K.L., Wells, R. 2005. Predicting Cotton Boll Maturation Period Using Degree Days and Other Climatic Factors. Agronomy Journal. 97:494-499. Interpretive Summary: Cotton is tropical crop that requires high temperatures for development. The amount of heat the cotton plant receives daily is called a heat unit; a cotton plant requires a certain number of heat units to reach different growth stages. Heat units can be used for variety selection; scheduling insect, disease, and weed control; and planning irrigation, defoliation, and harvest. A model is a mathematical method of explaining a natural process such as fruit formation. The current heat unit model uses only a lower temperature threshold of 60°F. This research was initiated because it was believed that the 60°F lower threshold was not an appropriate lower limit for cotton growth and that an upper threshold should also be used. This study determined that cotton growth is more accurately modeled if an upper threshold of 86°F is incorporated into the exiting model with an adjusted lower threshold of 55°F. This alternative monitoring method that uses both daily high and low temperature will improve the accuracy of cotton growth monitoring. This will aid cotton researchers and crop producers in monitoring growth and in making management decisions based on the crop status.
Technical Abstract: Heat units are often used for cotton (Gossypium hirsutum L.) growth monitoring and management. The objectives of this research are to determine if the degree-day 15.5°C model is an accurate method to monitor reproductive growth in cotton in the northern, rain-fed region of the Cotton Belt, to evaluate other degree-day models, and to investigate other weather variables that may improve the accuracy of the current system. Cotton was planted at three different timings in 2001 and 2002 to provide different climatic regimes during the boll-filling period. On ten typical plants per plot, all first position flowers were individually tagged with date of flower opening and were then harvested at full maturity. Weather data consisted of maximum, minimum, and average air temperature; maximum and average soil temperature; average soil moisture; maximum and average solar radiation; and maximum and average photosynthetically active radiation. All climatic factors except soil moisture were significant in terms of effects on boll maturation period (p<0.0001). The 30/13°C degree-day model, which used 30°C and 13°C as the upper and lower thresholds, provided the best adjusted r2 of 0.9399 of all the single variable models; the degree-day 15.5°C model had an adjusted r2 of 0.9268. Modifying the DD15.5 model by adding either minimum air temperature or maximum and average air temperature increased the adjusted r2 to 0.9632 and 0.9934, respectively, while also reducing mean square error and coefficient of variation. Producers should consider use of one of these modified degree-day models to improve their monitoring and management decisions.