A General Thermal Index for Maize

Abstract

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‐filling periods predict more accurately time to maturity than a single function for the two periods combined. However, use of two functions requires a priori knowledge of the silking date, which becomes the transition date from the vegetative function to the grain‐filling function. The objective of this study was 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 functions in a general thermal index (GTI) for maize. Frequency distributions of mean daily air temperatures for five 20‐d periods spanning mid‐June to late September at 19 locations in the northern USA and southern Ontario from 1992 to 1995 indicated few days (≤12%) with mean daily air temperatures less than 15°C before late August. This was significant, as the two response functions diverged significantly at temperatures below 15°C. Standard errors in estimating maturity date using different transition dates remained small (<7.5 d) unless the transition date was delayed beyond the first week of September. Based on this analysis, a standard transition date of 1 August was proposed for the GTI. Testing on an independent data set indicated that the GTI and a transition date of 1 August provided more accurate estimates of the planting to maturity period than growing degree days (GDD) or crop heat units (CHU), with a standard error of 8.2 d (compared with 14.5 d using GDD and 12.5 d using CHU).