Comparison of late-season diagnostic tests for predicting nitrogen status of corn

We compared six late-season diagnostic tests for determining N adequacy in corn (Zea mays L.) in a 3-yr study in Pennsylvania. The six tests were: (i) the NO3--N concentration of stalk sections at black layer; (ii) the NO3--N concentration of stalk sections at the one-fourth milk line growth stage (MLGS), which allows corn grown for silage to be tested; (iii) the chlorophyll meter (CM) test at the one-fourth MLGS; (iv) the relative CM test (normalized values) at the one-fourth MLGS; (v) a visual test based on the number of green leaves below and including the ear leaf at the one-fourth MLGS; and (vi) a relative visual test (normalized values at the one-fourth MLGS), We found that with a critical level of 250 mg kg(-1) N-3(-)-N, the stalk NO3- test separated N-sufficient from N-deficient sites with approximately 93% accuracy when sampling was done at either the one-fourth MLGS or within several weeks after black-layer formation. It appears that the 250 mg kg(-1) NO3--N critical level can be used to accurately predict N adequacy for any sampling time between the one-fourth MLGS and a few weeks after black-layer formation. When drought-stressed fields were excluded or the CM readings normalized with a high-N reference plot in the field, the accuracy of the CM test at tire one-fourth MLGS was approximately 92%, The visual test at the one-fourth MLGS was an accurate predictor of corn N status only when visual readings were normalized with a high-N reference plot. These results demonstrate that there are several late-season N tests that are suitable for making relatively accurate assessments of N sufficiency for corn silage and grain yields.