Assessing Post-Maturity Stalk Nitrate in Different Corn Hybrids

by E. D. Nafziger

• Introduction
• Materials and Methods
• Results and Discussion
• Summary
• Table 1
• Conclusions
• References

Introduction:

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 grain fill - is considered a useful trait agronomically, probably due to the ability of such hybrids to produce carbohydrate late in the grain filling period, thus helping to keep stalks healthy as grain fill 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 1996 and 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.

Materials and Methods

Fifty corn hybrids entered by commercial companies in the corn hybrid performance trials at both Brownstown and Urbana were chosen for inclusion each year, from about 65 hybrids common to both locations. About half (23) of these hybrids were also included in both the 1996 and 1997 studies. 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. Urea was applied during land preparation at the rate of 160 lb N/acre. The Urbana trial was planted on a Flanagan silt loam soil. Pre-plant N was applied at the rate of 200 lb N/acre.

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. Within a week after harvest, stalk samples were taken for nitrate analysis. Stalk 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.

Results and Discussion

At Brownstown in 1997, 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 (Table 1). Though variability was high, there were significant differences among hybrids in stalk nitrate. 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 much lower than in 1996, and stalk nitrate levels were much higher (Table 1). There were significant differences in stalk nitrate among hybrids at Urbana in 1997, with a range of 960 to 4593 ppm. As was the case at Brownstown, most of the hybrids (45 of 50) had stalk nitrate levels above 2,000 ppm. Grain moisture and green leaf area were positively correlated at Urbana, and unlike the case at Brownstown, moisture and stalk nitrate were positively correlated (r=+0.34) in 1997. There were no other significant correlations between measured parameters.

Summary

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 these hybrids. Among the four environments, stalk nitrate levels did not correspond well with overall yield level (Table 1). 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 grain fill 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.

Table 1. Yield and lower stalk nitrate (LSN) levels averaged over 50 hybrids in each of the four environments.

Conclusions

• Lower stalk nitrate content varied among hybrids, but not consistently among environments.
  
  
• No hybrid "indicators" were found to help adjust or interpret the LSN test.
  
  
• Wide variations in LSN among environments were not predictable, raising questions about the usefulness of the LSN test as currently used.

References

• Binford, G.D., A.M. Blackmer, and N.M. El-Hout. 1990. Tissue test for excess nitrogen during corn production. Agron. J. 82:124-129.
  
  
• Binford, G.D., A.M. Blackmer, and B.G. Meese. 1992. Optimal concentrations of nitrate in cornstalks at maturity. Agron. J. 84:881-887.
  
  
• Eichelberger, K.D., R.J. Lambert, F.E. Below, and R.H. Hageman. 1989. Divergent phenotypic recurrent selection for nitrate reductase activity in maize. II. Efficient use of fertilizer nitrogen. Crop. Sci. 29:1398-1402.
  
  
• Tsai, C.Y., D.M. Huber, D.V. Glover, and H.L. Warren. 1984. Relationship of N deposition to grain yield and N response of three maize hybrids. Crop Sci. 24:277-281.