End-of-Season Corn Stalk Test for Excess Nitrogen in Silage Corn

The concentration of nitrate in the lower portion of corn stalks 1 to 3 wk after physiological maturity is a reliable tissue test for detecting optimal or above-optimal supplies of available N for corn (Zea mays L.) grown for grain. This test, the end-of-season corn stalk test, has not been evaluated for silage corn. The test would be especially valuable for silage corn because this crop is frequently grown on manured fields where assessing the N status is difficult due to unquantified rates of N application and variable mineralization of organic N. This study reports nitrate concentrations in lower corn stalks shortly before harvest of silage and evaluates their use as an end-of-season corn stalk test for silage corn. Also explored is the possibility of collecting corn stalks up to 24 h after a field is harvested. Lower corn stalk samples were collected from a total of 19 N-response trials on corn fields with a history of manure applications in Connecticut in 1994 and 1995. In 1996, corn stalk samples were collected at harvest and 24 h after harvest from 20 farmers’ fields. Three commonly used methods for defining a critical nitrate concentration, the quadratic-plateau model, the linear-plateau model, and the Cate-Nelson method were used to describe the relationship between relative yield and stalk nitrate concentrations. Three different critical concentrations were calculated by the three methods, which suggests that an optimal range of 500 to 1000 ppm nitrate N is appropriate for this test. There was no significant change in the nitrate concentration of stalk samples collected at harvest and 24 h after harvest. These results suggest that the end-of-season corn stalk test can be used as a convenient method to define excess N availability and to improve N fertilizer recommendations for silage corn fields. Research QuestionThis study evaluates the use of nitrate concentrations in the lower portion of corn stalks 3 to 4 wk before physiological maturity to estimate the N status of corn grown for silage. Also explored is the possibility of collecting corn stalks up to 24 h after a field is harvested. Literature SummaryThe concentration of nitrate in the lower portion of corn stalks 1 to 3 wk after physiological maturity is a reliable tissue test for detecting optimal or above-optimal supplies of available N for corn grown for grain. This test, the end-of-season corn stalk test, has not been evaluated for use with silage corn. The test would be especially valuable for silage corn because it is more frequently grown on manured fields where assessing the N status is difficult due to unquantified rates of N application and variable mineralization of organic N. Excess N availability in corn fields has been linked with nitrate loss from the soil profile. Most diagnostic tissue tests indicate N deficiency. While these tests can be economically beneficial for corn fields with N deficiency, environmental problems and unneeded economic outlays can exist where excess N availability is detected. Diagnostic tools for identifying unnecessary applications of N fertilizer and manure are needed to refine N fertilizer recommendations and management. Study DescriptionIn 1994 and 1995, N response trials were conducted on a total of 19 fields in Connecticut. Each trial was established on a field with previous dairy manure history and was managed by the farmer, except for fertilizer application and silage harvest. Six rates of N fertilizer (0, 30, 60, 90, 120, and 150 lb N/acre) were hand-applied as ammonium nitrate. Ten corn stalk samples were collected from each plot at the time of silage harvest. The corn stalks were collected by cutting plants at 6 and 14 in. above the ground; the resulting 8-in. segment was saved and analyzed for nitrate. The dried leaves were removed from each 8-in. segment. In 1996, two sets of corn stalk samples were collected, at harvest and 24 h after harvest, from 20 farmers’ fields to measure the effect of time of cutting on stalk nitrate concentrations. Applied QuestionsIs the end-of-season corn stalk test an effective indicator of optimal and above-optimal supplies of available N for silage corn? Yes. This test provides reliable information about optimal and above-optimal supplies of N for silage corn. Because of variable increases in stalk nitrate concentrations from additions of fertilizer N when excess N was available, the test cannot be used to predict the amount of excess N availability. What is the optimal range of stalk nitrate concentrations? Three commonly used methods for defining a critical nitrate concentration (the concentration at which luxury consumption begins) were used to describe the relationship between relative yields and stalk nitrate concentrations (Fig. 1). These methods suggest that an optimal range of 500 to 1000 ppm nitrate N is appropriate for this test. Corn fields with stalk nitrate N concentrations less than 500 ppm probably would be deficient in N; corn fields with concentrations between 500 and 1000 ppm probably would be optimal in N; and corn fields with concentrations greater than 1000 ppm probably would be excessive in N. Fig. 1Relationship between relative silage yields and nitrate N concentrations in lower corn stalks at 19 trials; (a) Cate-Nelson method, (b) Linear-Plateau model; (c) Quadratic-Plateau model. Nitrate values truncated. Is it possible to collect corn stalk samples within 24 h after a silage corn field is harvested? At 20 trials there was no significant difference between average nitrate concentrations in corn stalks at harvest and 24 h after harvest (P = 0.80). These findings suggest that nitrate transformations do not occur rapidly in corn stalks within 24 h after harvest. Therefore, collection of samples for the end-of-season corn stalk test is possible for 24 h after harvest. Collection of stalk samples from stubble makes this test much easier to use and should increase the adoption of the test by corn producers. The stubble sampled has to be at least 14 in. tall to enable cutting the stalk at the height used in this study. Cutting corn stalks lower or higher than the 6 and 14-in. height will result in incorrect assessments of the N status. Unpublished data from this lab indicates that stalk nitrate concentrations are greater in the 6-in. segment of the stalk located 4 to 10 in. above the ground than in the 8-in. segment cut at 6 and 14 in. above the ground.