As has been common this time of year the past few years, there are recommendations being made to apply nitrogen (N) using a foliar spray application piggy-backed with a fungicide instead of applying urea at late boot to hybrid rice.
The reasoning is straightforward – it’s cheaper to add in a foliar N product if you’re already going across the field with a fungicide. The problem is, there is no evidence to support that a foliar N product can supply the needed amounts of N that a urea application can.
For hybrid rice, we recommend applying 30 lb N/acre (65 lb urea) at late boot (flag leaf collar visible) up to beginning heading. From midseason and beyond, rice is capable of taking up urea N with high efficiency (70% uptake). Foliar applications of N products cannot supply these N rate levels.
A foliar N product containing 32% N can only deliver 3.2 lb N per gallon (for a product with a weight of 10 lb/gal). Even if a foliar is taken up with the same high efficiency as urea, 70% of a 3.2 lb/gal foliar is just 2.2 lb N versus 70% uptake of 30 lb N as urea is 21 lb N.
Again, there are charts being sent around that suggest 1 lb N applied foliarly is equal to 5-10 lb N applied to soil. This is false – a pound is a pound.
If you attempt to increase the rate of a foliar N product to compete with urea, then you have the issue of it becoming very cost-prohibitive, and the chances of severe foliar burn to rice increase. Stick with urea for the late boot application on hybrids – years of data show increases in grain yield, head rice, and total rice. Each of these increases alone is small but put together generate a nice return on investment for the urea application.