If your grain sorghum emerged about 30 days ago, it is entering into the growing point differentiation or reproductive stage. Around this time, the total number of leaves is being defined and nitrogen needs to be in place as the plant will start using nitrogen rapidly. Grain sorghum seems to be more sensitive than corn to the timing of sidedress fertilization. If possible, sidedress grain sorghum a week earlier than you would corn.
Our current nitrogen recommendations for grain sorghum are based on yield goal and soil texture. For yield goals of 130 bu/acre or higher, apply a total of 160 lb/acre. For yields under 130 bu/acre, apply a total of 135 lb/acre. These recommendations are for irrigated grain sorghum grown on soils of light texture (silt loams, sandy loams). For soils of clayey texture, increase such rates by 45 lb N/acre. For non-irrigated grain sorghum, our recommendation is 110 lb N/acre, but this rate may need to be increased since good yields can be obtained under non-irrigated conditions with timely rainfall.
If you find a window to apply urea right now, there is a good chance the soil will be wet. Under such conditions a urease inhibitor is recommended to reduce the risk of volatilization losses. Urease is the enzyme involved in the conversion of urea to ammonium. Research has shown that the most efficient urease inhibitors are those with NBPT as the active ingredient. Currently, there are several NBPT-based products available in the market under different names.