Illinois
Fertilizer Conference Proceedings
January 26-28, 1998
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E.D. Nafziger1
The use of a lower-stalk nitrate test, run on samples taken after corn reaches physiological maturity, to detect when excess N has been used on a field has been described by Binford et al. (1990). Though they reported this test to be quite useful for this purpose, they did not acknowledge the possibility that hybrids may differ inherently in the amount of N remaining in the stalk at maturity. They suggested that the optimal range of stalk nitrate was 0.25 to 1.80 g NO3-N per kg (parts per thousand), but later modified that range to 0.7 to 2.0 parts per thousand (Binford et al., 1992). The latter study included a number of different hybrids, but they were in different fields and so could not be directly compared.
While corn hybrids are known to differ in their ability to use N efficiently (Tsai et al, 1984; Eichelberger et al., 1989), little is known about possible genetic effects on lower stalk NO3 content. Stay-green—the ability of hybrids to retain green leaf color late in grainfill—is considered a useful trait agronomically, probably due to the ability of such hybrids to produce carbohydrate late in the grainfilling period, thus helping to keep stalks healthy as grainfill ends. At the same time, it could be that green leaves, which retain substantial amounts of N, might also be associated with higher stalk nitrate content at maturity.
This study was run for the second year in 1997 in order to provide an estimate
of the variation in post-maturity lower stalk nitrate among commercial corn
hybrids. Green leaf area was estimated late in grainfill to see if there might
be a correlation between green leaf area and lower stalk NO3 that
could improve the usefulness of this diagnostic test.
Fifty corn hybrids entered by commercial companies in the corn hybrid performance trials at both Brownstown and Urbana were chosen (from about 65 hybrids common to both locations) for inclusion. About half (23) of these hybrids were also included in the 1996 study. These trials were planted, managed, and harvested by the crop variety testing personnel of the Crop Sciences Department, University of Illinois.
The Brownstown trial was located on Cisne silt loam soil, and was planted on April 29 and harvested on October 3, 1997. Urea was applied during land preparation at the rate of 160 lb N/acre. Summer rainfall was near normal, and was distributed quite well, resulting in very good yields for this location. The Urbana trial was planted on April 18 in a Flanagan silt loam soil. Pre-plant N was applied at the rate of 200 lb N/acre. Growing conditions were quite good, but yields suffered moderately from a period of dry weather in July. The trial was harvested on October 6-7, 1997.
Plots at both locations consisted of four rows about 22 ft. long, with rows 30 inches apart. Plant populations of 24,000 and 28,000 per acre were established by thinning at Brownstown and Urbana, respectively. The percentage of the leaf area that remained green was estimated visually at Brownstown on September 3, and at Urbana on September 18. Within a week after harvest, stalk samples were taken for nitrate analysis. Stalks segments were cut from 6 to 12 inches above soil surface, with six stalks from the two outside (border) rows sampled per plot. Stalks were ground and analyzed for nitrate by A & L Labs, Ft. Wayne, IN.
Compared to 1996, green leaf area remaining was assessed in a more timely manner in 1997, with an average of 58 percent of the leaf area green on that date at Brownstown, and 46 percent green at Urbana (Table 1). Stalk nitrate levels of the 50 hybrids ranged from 1750 to 8137 ppm, and averaged 3868 ppm, with 47 of the hybrids having more than the 2,000 ppm considered to be the upper limit of the ideal range (Binford et al., 1992). Grain yields in 1997 were almost double those in 1996 at this location, and stalk nitrate was some 50% higher than in 1996. Though variability was high, there were significant differences among hybrids in stalk nitrate (Table 1). Correlations between yield and grain moisture, yield and green leaf area, and moisture and green leaf area were significant, but stalk nitrate level was not correlated with any other parameter measured.
At Urbana, yields in 1997 were only 148 bu/acre in 1997 compared to 225 bu/acre in 1996, and stalk nitrate levels were much higher in 1997, averaging 3267 ppm compared to only 1736 ppm in 1996. There were significant differences in stalk nitrate among hybrids at Urbana, with a range of 960 to 4593 ppm. As was the case at Brownstown, most of the hybrids (45 of 50) had nitrate levels above 2,000 ppm (Table 1). Grain moisture and green leaf area were correlated at Urbana, and unlike the case at Brownstown, moisture and stalk nitrate were positively correlated (r=+0.34). None of the correlations between measured parameters were significant.
While there were significant differences in stalk nitrate concentration among hybrids in three of the four environments over two years, these differences correlated poorly or not at all with yield or green leaf area of the same hybrids. Among the four environments, stalk nitrate levels did not correspond well with overall yield level. Though the lowest nitrate level was found in the environment with the highest yield (Urbana, 1996), both nitrate levels and yields were much higher at Brownstown in 1997 than in 1996. If, as these results seem to indicate, general weather and crop conditions late in grainfill affect yield and nitrate levels by different mechanisms, then the usefulness of using lower stalk nitrate concentration as an indicator of N sufficiency or excess may be called into question.
1 E.D. Nafziger is a Professor of Crop Production, Dept. of Crop Sciences, University of Illinois, Urbana, IL.
