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 NO − 3 –N concentration of stalk sections at black layer; (ii) the NO − 3 –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 NO − 3 –N, the stalk NO − 3 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 NO − 3 –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 the 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.