Comparison of seasonal thermal indices for measurement of corn maturity in a prairie environment

Abstract

This study was initiated to determine (i) which thermal index — corn heat units (CHU), modified CHUs (mCHU) or growing degree–days (GDD) — was most consistent across years and locations and (ii) which maturity index — 50% silk, time to 30% grain moisture or time to 100% milk line — was the most consistent measure. Fifteen corn hybrids were evaluated at Morden and Brandon, Manitoba, over 3 yr (1988–1990). Correlation coefficients between total accumulation of thermal units from planting to first frost and thermal units required to reach a specific stage of maturity ranged from 0.283 to 0.969. The lowest correlations were between CHU and time to 50% silk. Coefficients of variation (CVs) were always lower for CHU than for either mCHU or GDD. CV was highest for time to 100% milk line but similar between time to 50% silk and time to 30% grain moisture. Correlations between ranking of hybrids across site–years was highest for mCHU. Overall ranking of thermal indices for consistency was CHU > GDD > mCHU. Time to 50% silk was the most consistent of the measures of maturity. Time to 30% grain moisture was a more consistent measure of maturity than time to 100% milk line.Key words: Maize, heat units, growing degree–days, milk line, grain moisture