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Blocks were removed at 0.15-m depth increments until no further roots were observed. All roots were identified by eye, hand picked from the soil, stained with Congo red (Ward et al. 1978) and their length estimated using the grid technique tested by Tennant (1975

Several systems are extensively used to rate maize (Zea mays L.) maturity in North America, including growing degree days, crop heat units, and Minnesota relative maturity rating days. Correspondence between the different systems varies with the temperature range of the environment, because of different bases of calculation and unit size. However, general guidelines for conversion from one system to another would aid communication among researchers, producers, distributors, and extension personnel, particularly where regional preference for one system exists, but information developed in another region is shared. Simple regression equations were developed from a data set with 4 years, 28 hybrids, and 19 locations (between 39° and 48° N lat). Coefficients of determination were ≥0.91, indicating that these equations could be used as a general guideline to compare maturity ratings developed using the different systems.

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Production estimates based on photosynthate accumulation are affected by changes in photosynthesis rates related to aging in different cultivars. The influence of leaf and plant age on photosynthetic rates was investigated in six maize (Zea mays L.) cultivars with maturity ratings ranging from <2300 to >3500 CHU (corn heat units). Photosynthesis and irradiance measurements were fitted to rectangular hyperbolas using a nonlinear fitting technique. These curves were analyzed as a function of cultivars, and leaf and plant age. For comparison among cultivars and age groups, photosynthesis was represented by P2000 (net photosynthetic rate at irradiance (I) = 2000 µmol m−2 s−1). In all cultivars, early leaves had lower P2000 values at a given leaf age than late maturing leaves. The P2000 values tended to increase to a maximum at the time of full individual leaf expansion and then decrease. The duration of maximum P2000 values for leaves nearest the ear were longer than for earlier or later developing leaves. Leaves nearest the ear for early maturing cultivars tended to have lower P2000 values than later maturing cultivars at comparable leaf ages. Plant photosynthesis patterns were the same for all cultivars in relation to plant age and phenological development. Photosynthesis (P2000) was low for the six‐leaf stage, reached a maximum just before tasselling, and then declined during brown silk, blister, and dent stages. A quadratic equation described the relationship between P2000 and plant age. The trend was for latermaturing cultivars to have significantly (P < 0.05) larger P2000 values than earlier‐maturing cultivars at comparable phenological stages. Significant cultivar differences were found in leaf development rate and length of vegetative period (P < 0.10).