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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. |
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