Evaluation of leaf chlorophyll index for making fertilizer nitrogen recommendations for silage corn in a high fertility environment

Two experiments were conducted to evaluate the potential to use leaf chlorophyll index (LCI), measured using a Minolta SPAD-502 meter, for making fertilizer nitrogen (N) recommendations for silage corn in a high N fertility environment. In Experiment 1, treatments were 0, 40, 80, 120, 160, and 200 kg N ha(-1) as NH4NO3 broadcast at corn emergence. At the six-leaf stage, soil inorganic N to 90 cm depth, total Kjeldahl N concentration of the fifth leaf, sixth leaf, and the above-ground portion of the plant, and LCI of the fifth and sixth leaves, were determined. In Experiment 2, LCI and LCIR (ratio of LCI to LCI from a reference plot receiving 200 kg N ha(-1) at corn emergence) were measured at approximately the six-leaf stage in 87 trials 1994-1996. Treatments consisted of different fertilizer N rates applied at sidedress, and corn yield and pre-sidedress soil nitrate test (PSNT) values were determined. In Experiment 1, nitrogen concentration of the above-ground plant increased with increasing soil inorganic N content up to about 350 kg N ha(-1), and LCI generally increased linearly with leaf total N concentration. However, the relationship between LCI and leaf N concentration varied among years, and the relationship between leaf total N concentration and above-ground plant total N concentration was inconsistent. In Experiment 2, above-ground plant total N concentration was a poor predictor of soil inorganic N supply and corn relative yield when measured over a number of trials. Leaf chlorophyll index at the six-leaf stage therefore has significant limitations as an indicator of corn N status or soil inorganic N supply. Despite these limitations, LCI had a 68% success rate in identifying fields which were responsive and unresponsive to sidedress N using a critical LCI value of 43.7 SPAD units. This success rate is similar to that obtained with the PSNT. However, LCI cannot be used to recommend rates of fertilizer N for LCI values below the critical value. Use of LCIR was of little benefit compared to use of LCI. Leaf chlorophyll index may be most suitable as a preliminary screening tool, used on-site to rapidly identify fields which do not require additional sidedress N, whereas other fields would require a PSNT soil sample to be taken.