IMPACT OF STARTER FERTILIZERS ON OPTIMUM RATE, TIMING AND EFFICIENCY OF NITROGEN FERTILIZER FOR GRAIN SORGHUM IN TEXAS

Nitrogen (N) management is critical for producing high yielding grain sorghum in Texas. Many producers utilize planter options for starter fertilizer application, including in furrow or 2x2 placements of N and phosphorus (P). N and P applied as starter fertilizers could affect optimum rate, timing and nitrogen use efficiency (NUE) of N fertilizer in grain sorghum. Advances in technology for remote sensing enable rapid acquisition of crop canopy spectral measurements. Analysis of spatial variability of crop canopy reflectance may enable site-specific nitrogen (N) management in grain sorghum. Two field studies were established in Burleson County, Texas during 2016 and 2017 to impose contrasting N status in grain sorghum. The first study employed a single side-dress application of increasing N fertilizer rates (0, 112,168,224, 280 kg ha^-1) for grain sorghum using three starter fertilizer applications. For the second study, timing (20, 42, 56, 65, 81 (2016) 32, 49, 66, 75, control (2017) days after planting) of side-dress N fertilizer application (168 kg N ha^-1) was evaluated for grain sorghum under three starter fertilizer applications. Starter fertilizer treatments (sub-plots) included ammonium polyphosphate (11-37-0) applied at 0, 56 (in-furrow) or 168 (2x2) L ha^-1. Spectral measurements (visible and NIR) of the sorghum canopy were made using ground-based at multiple dates during the growing season. Handheld sensors were also used to monitor N status throughout the season. Spectral indices and the handheld sensor values were used to evaluate and relate to crop biomass, grain yield and N content. The optimal N rate in 2016 was 168 kg ha^ -1, there was no significance across increasing rates in 2017. Starter fertilizers did not affect the increasing rates. The optimal timing of N in 2016 was