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

Abstract

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.

Acknowledgments

The assistance and cooperation of all the growers who allowed us to work on their fields is gratefully acknowledged. Funding for the project was provided by Agriculture and Agri-Food Canada, the Canada-British Columbia Green Plan for Agriculture, Coast Agri Ltd., and the Fraser River Action Plan of Environment Canada. The assistance of M. Betts, BCMAFF in conducting trials on Vancouver Island is gratefully acknowledged. Laboratory and field assistance was provided by B. Harding, L. Birston, D. Chapple, G. Telford, and C. Watson.