Diagnostic efficiency of the blacklayer stalk nitrate and grain nitrogen tests for corn

Protecting water quality and maintaining profitable corn (Zea mays L.) production requires diagnostics that can distinguish between N deficiency, sufficiency, and excess. This study evaluates initial recommendations on blacklayer basal-stalk NO3-N ranges and critical concentrations for diagnosing N status, and it compares the performance of this test with grain analysis. Observations (428) were collected from 13 N-response experiments. Linear response and plateau (LRP) and binary logistic regression (BLR) were used to characterize the relationships between yield and tissue-test values. With the LRP, stalk NO3-N and grain N concentrations separating deficient from sufficient observations were 0.42 and 13.1 g kg(-1), respectively, and the success rates of the two tests were comparable (77 and 75%, respectfully). The BLR also identified critical concentrations, but the values increased with decreasing yields, a desirable decision-rule attribute given that extreme deficiency can result in higher-than-expected tissue concentrations. The success rates of multiple BLR functions using yield and stalk or grain analysis as factors were again comparable (88 and 87%, respectively), but they were significantly greater than with the LRP analysis. Stalk analysis was superior to grain analysis for distinguishing sufficiency from excess. A constant stalk NO3-N concentration (1.67 g kg(-1)) separated sufficient from excessive cases, and fertilizer efficiency approached zero at 2.9 g kg(-1). Premature sampling resulted in stalk NO3-N levels that were 40 to 600% greater than levels observed after blacklayer formation, with the greatest error occurring when N fertility was low. When not testing for N excess, the advantages of grain analysis are the ease of sampling during harvest and the reduced risk of error associated with premature sampling.