Using Optical Sensors to Enhance N Use Efficiency in Sorghum.

Research in the Central Plains region has shown grain sorghum (Sorghum bicolor L. Moench) to be more tolerant of water-stress than corn (Zea mays L.). As a consequence sorghum is commonly grown in the region in rotation with winter wheat (Triticum aestivum L.) and soybeans (Glycine max) in areas and soils where corn does not perform well due to regular drought stress.  Sorghum yields vary widely from year to year, making traditional pre-plant, yield goal based, N recommendations a challenge. The objective of this study was to develop sensor based mid-season N recommendations utilizing two active, optical sensors (GreenSeeker, NTEK Industries and

Crop Circle, Holland

Scientific). Eight field experiments were conducted across Kansas in 2006 and 2007. Specific treatments used consisted of factorial combinations of pre-plant and side-dress N rates to supply a total of 0-168 kg N ha^-1. Pre-plant N treatments were applied immediately prior to planting at all locations.  Side-dress N treatments were applied 30-40 days after planting.  Grain yields ranged from 630 to 10,600 kg ha^-1. Grain sorghum yields were responsive to N at only a few sites. Non-responsiveness of grain sorghum yields at the rest of the sites were mainly due to high levels (> 73 kg N ha^-1) of soil profile residual NO₃^-, and water stress conditions during the growing season. The sensor based mid-season N recommendation system developed provides a much closer fit to observed N response than the traditional pre-plant N recommendation system.