Thermal time, or growing degree days, have been used for years in agriculture to predict crop growth stages and guide management decisions. This presentation briefly covers the history of degree days in cotton, describes some of the issues in relying on heat units to predict growth stage and guide management decisions, and explains a Beltwide effort to re-evaluate the use of heat units in U.S. cotton production. The base of 60F (which has been used in cotton production since the 1980s) still appears to be one of the simplest approaches for calculating heat units, but the number of heat units required to reach each key growth stage has changed. While it generally takes more heat units to reach emergence, squaring, flowering, cutout, and cracked boll now than previously published, the crop generally moves more quickly from cracked boll to 60% open than previously published. (Additional information is available in this
open access Agronomy Journal article).
