Impact of Starter Fertilizers on Optimum Rate and Timing of Nitrogen Fertilizer in Grain Sorghum.

Information on optimum rate and timing of nitrogen fertilizer for grain sorghum production in Texas is limited. General recommendations suggest that side-dress nitrogen applications should be made near panicle initiation for optimum yield response. However, starter fertilizer solutions containing nitrogen could meet early season crop nitrogen demand and affect optimum rate and timing of side-dress nitrogen applications. Separate field studies were established in Burleson County, Texas to determine the optimum rate and timing of nitrogen fertilizer in grain sorghum with and without starter fertilizer application. A strip-plot design was used to impose three starter treatments (control, in-furrrow, 2x2) as strip plots and nitrogen rate or timing treatments as main plots. Strips were 4 rows wide (76-cm rows) and main plots were 20.1 m in length. Nitrogen rates included 112, 168, 224 and 280 kg N ha^-1 (less starter N) plus an untreated control. Fertilizer (urea) was applied by hand at the 12-leaf stage (wet field condition delayed application). For the nitrogen timing study, 224 kg N ha^-1 (less starter N) of urea was applied at the 3-leaf, 6-leaf, 10-leaf, boot and bloom stage by hand. Starter fertilizer used was ammonium polyphosphate (11-37-0) applied at 56 L ha^-1 applied in furrow and 168 L ha^-1 applied 5-cm to the side of the seed trench and up to 5-cm depth. Plant color and biomass were monitored using various spectral sensors. Grain weight, moisture and test weight were determined at harvest. Linear and non-linear regression was used to identify optimum rates and timing of nitrogen fertilizer. Nitrogen response curves were compared for contrasting starter fertilizer treatments.