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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[sup -1] as NH[sub 4]NO[sub 3] broadcast at corn emergence. At the six-leaf stage, soil inorganic N to 90cm 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[sup -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 350kg Nha[sup -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... [ABSTRACT FROM AUTHOR]

The purpose of this study was to measure corn response to nitrogen (N), to evaluate the feasibility of using the pre-sidedress soil nitrate test (PSNT) for making fertilizer N recommendations for silage corn, and to evaluate the environmental and economic implications of adopting fertilizer N recommendations based on the PSNT in south coastal British Columbia, Canada, a region of high soil N fertility. Eighty-seven trials were conducted from 1994-1996 with corn or forage grass as a preceding crop on fields having a history of manure application. Non-manured trials received no spring manure application whereas manured trials received a spring application typical of that farm field. In 1994, treatments included sidedress N rates of 0, 30, 60, 90, 120, and 150kg N ha-[SUP1] replicated twice. In 1995 and 1996, trials with less than 30mg NO[SUB3]-N kg[SUB-1] soil to 30cm depth prior to sidedress received the same treatments as in 1994, whereas trials with concentrations above this value received two treatments, 0 and 150kgNha[SUP-1] replicated four times. Trials received less than 50kgN ha[SUP-1] at planting. Yield response to sidedress N was limited, with 71, 87, and 55% of trials with 90% or higher relative yield (yield at 0kgNha[SUP-1] divided by yield at 150kgNha[SUP-1]) in 1994, 1995 and 1996, respectively. The limited yield response was attributed to high soil N mineralization, which averaged 166 and 146kgNha[SUP-1] for non-manured trials in 1995 and 1996, respectively. The critical PSNT value (above which no yield response to sidedress N is expected) was predicted to be 19 and 23 mg NO[SUB3]-Nkg[SUP-1] soil using the Cate-Nelson procedure and a linear-plateau regression procedure, respectively. A critical PSNT test value of 32 mg NO[SUB3]-N kg[SUP-1] soil was predicted using a linear-plateau regression of the estimated N rate which would maximize economic return (N[SUBMER]) against PSNT test value. In addition, a model designed to minimize the risk of yield... [ABSTRACT FROM AUTHOR]