NITROGEN FERTILIZATION OF CONSERVATION-TILLED WHEAT. II. TIMING OF APPLICATION OF TWO NITROGEN SOURCES

ABSTRACT

Nitrogen (N) applications to wheat (Triticum aestivum L.) need to be timed for accelerated N uptake by the plant if highest yields and highest N use efficiency are to be attained. Applying N before accelerated uptake may allow soil losses while applications after accelerated uptake may restrict plant N utilization both of which may restrict yields. Research was conducted between 1997 and 2000 on Collins silt loam (Aquic Udifluvents). Pioneer 2552 was planted in 1997, Pioneer 2580 was planted in 1998, and Pioneer 2684 was planted in 1999. The experimental design was a split plot with treatments replicated five times. Main plots were N application timing (Feekes' growth scale of 5, 6, 8, 9, and 10) with ammonium nitrate (AN) and urea-ammonium nitrate (UAN) applied at 101 kg N ha⁻¹ the sub-plots. Spring N application timing affected yields, test weights and disease severity. Yields over the three years were higher for broadcasting AN than UAN. Early N application at GS 5 lowered yields one of the three years while late N applications at GS 10 reduced yields two of three years. Take-all diseases may have reduced yields associated with delayed N timing one of the two years. Soil N losses from the early applications may have reduced yields. Reduced yields from late applications may be the result of applying N past the optimum N uptake period. Wheat test weights varied with N source but generally decreased with N application timing. Take-all severity increased with delayed N timing. However, the severity of glume blotch was not affected by timing of N applications.

ACKNOWLEDGMENTS

The authors are grateful for the financial support provided by Mississippi Chemical Corporation and appreciate their interest in the soil fertility research conducted in Tennessee.