A general thermal index for maize

Thermal indices predict and describe development rate more accurately than time in days and are commonly used to rate maize (Zea mays L.) for maturity . Separate temperature response functions for the vegetative and grain-fill ing periods predict more accurately time to maturity than a single function for the two periods combined, However, use of tao functions requires a pri ori knowledge of the silking date, which becomes the transition date from t he vegetative Function to the grain-filling function The objective of this study sas to evaluate the sensitivity of estimates of silking and maturity dates to the transition date between vegetative and grain-filling functions and to develop a protocol to combine the two temperature response function s in a general thermal index (GTI) for maize. Frequency distributions of me an daily air temperatures for five 20-d periods spanning mid-June to late S eptember at 19 locations in the northern USA. and southern Ontario from 199 2 to 1995 indicated few days (less than or equal to 12%) with mean daily ai r temperatures less than 15 degrees C before late August. This aas signific ant, as the two response functions diverged significantly at temperatures b elow 15 degrees C, Standard errors in estimating maturity date using differ ent transition dates remained small (<75 d) unless the transition date was delayed beyond the first week of September. Based on this analysis, a stand ard transition date of 1 August,vas proposed for the Gn. Testing on an inde pendent data set indicated that the GTI and a transition date of 1 August p rovided more accurate estimates of the planting to maturity period than gro wing degree days (GDD) or crop heat units (CHU)1 with a standard error of 8 .2 d (compared with 14.5 d using GDD and 12.5 d using CHU).