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Fluid Journal : Fluid Journal 1999-2001
Winter 2000 Summary: Nitrogen (N) starter applications increased grain yield an average of 2.2 bu/A, suggesting that starter N is a viable input for late- planted, double-cropped soybeans. Our results also suggest that an indeterminate growth habit may be superior to determinate types for ultra-late planting dates. Although N starter applications in these systems appear warranted, we have not pinpointed a nitrogen rate expected to promote maximum economic and/or agronomic yield. Future efforts should focus on determining appropriate starter N rates in late-planted, double- cropped soybean systems across a variety of environments. _______________________________ Recent estimates show that up to half the U.S. soybean crop is double-cropped. In the Deep South, it is becoming more common to find soybeans grown behind full- season corn. In this cropping system, planting date is delayed to mid-or- late-July from the optimal mid-May- though-late-June period. Because of this delay in planting, yield reduction is commonly associated with double- cropping. Previous studies report that yield reduction in late-planted, double-cropped soybeans is associated with a lack of sufficient vegetative growth. Results of comparing the performance of determinate and indeterminate soybean cultivars in late-planted environments have been varied. Determinate types have produced high yields in some studies while other studies have shown indeterminate types to be superior. Although soybeans obtain N through symbiotic fixation, application of N as a starter has produced increased vegetative growth and grain yield in numerous studies. Additional studies have shown N starter to be viable where late planting occurs. Sufficient research, however, has not been conducted in a single study in the southeastern U.S. to evaluate the combined effects of growth habit and N rate on late- planted, double-cropped soybeans. Our objective in this study was to determine the interactive effects of N starter (0 and 45lbs/A of N) on soybean growth and yield when following corn in a late-planted, double-cropped system. Yield N starter increased grain yield 9 percent when averaged across all environments. As shown in Figure 1, yield was affected by both cultivar and rate of application. Au86-2397I (indeterminate) was superior to Au86- 2397D (determinate) across all environments, averaging 29.1 bu/A compared with 26.7 bu/A for Au86- 2397D. Greatest increase came in the highest-yielding environment (WGS96), where application of N starter boosted grain yield from 32.4 to 36.7 bu/A (data not shown). Our highest yielding environments were those receiving supplemental water sources, and those receiving rainfall within 24 hours of N application. Early-season growth Plant height at R1 was increased by application of N starter. Averaged across cultivars, starter application resulted in R1 plant heights of 15 inches, while plants not receiving starter averaged 13 inches. Cultivar selected affected plant height at R1. The Cook cultivar had greater height Michael E. Starling, Dr. C. Wesley Wood, Dr. David B. Weaver Late-planted Soybeans Respond to Nitrogen Starter Cultivars are compared with varying responses to applied starters at low-N sites. Future studies will focus more on optimum N rates.
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