PHENOLOGICAL TEMPERATURE RESPONSE OF MAIZE

Variability of development rate estimates across locations and years u sing the current heat unit system of growing degree-days (GDD) with ma ximum and minimum temperature thresholds of 30 and 10 degrees C (GDD(3 0,10)) limits predictability of maturity in hybrid maize (Zea mays L.) . Data sets of daily maximum and minimum air temperatures and dates of maize development stages were collected for a range of hybrids at loc ations in Canada and the northern USA (39 degrees to 45 degrees N lat) . Data were analyzed to improve the temperature response functions for maize at different stages of development. Results indicate that durin g vegetative growth, phenological response to mean daily air temperatu re followed a sigmoidal curve beginning below 5 degrees C, with maximu m response to temperatures between 25 and 30 degrees C. During reprodu ctive growth, the temperature response function was nat from 0 to 12 d egrees C and rose significantly only with mean daily air temperatures greater than this range. A general thermal index (GTI) based on these two response functions improved estimation of maturity dates by 50% ov er estimates made using GDD(30,10) (SE of 6.7 d for GTI and 13.6 d for GDD(30,10) in estimating time from planting to maturity). The greates t improvement using GTI occurred for the reproductive period (SE of 5. 8 d using GTI; compared with 12.1 d using GDD(30,10)). These results s uggest that incorporating the temperature response function reported i n this paper would improve prediction of maize development.