Variability in leaf chlorophyll concentration among fully-fertilized corn hybrids

Several studies have reported that corn (Zea mays L.) hybrids differ in leaf chlorophyll concentration (LCC) to the extent that Minolta SPAD 502 meter data have to be normalized (values obtained from a general field divided by those from a fully-fertilized area in the same field). However, situations exist where the normalization method can add needlessly to production expenses and even lead to nitrogen deficiencies and reduced yield. Little information was found to indicate how much potential variability exists among modern hybrids, so we evaluated 45 entries in a fully-fertilized and irrigated standard performance test over a 2-year period in northwestern Kansas to determine: (i) how much variability was present in a large sampling of commercially available, fully-fertilized corn hybrids and (ii) if results suggested alternatives to the normalization method. We used the SPAD meter to measure LCC twice during the vegetative phase and twice during the reproductive phase. Results indicated that genetic potential for maximum LCC was achieved. Quadratic regressions of LCC on days from planting yielded highly significant R2 values of 0.88 when adjusted to common growth stages and 0.91 when data were not adjusted. A cubic model did not add significant improvements, but we prefer this model because it provided a better fit for early-season data. For quick checks, we propose that the mean values (+/-standard deviation) obtained at growth stages R1 (57.9+/-4.5) and R6 (60.0+/-4.6), and to a lesser degree V6 (40.0+/-3.2) and V10 (48.6 +/-3.8), be used as first approximations for evaluation and refinement. As first approximations, they are subject to error and should not be used as the sole basis for deciding if or how much supplemental fertilizer nitrogen (N) needs to be applied. Although these standards are useful, their ranges were judged to be too broad for precise estimates, so we see little alternative to the normalization method.