Dustin Harrell answers common N management questions

Dustin Harrell, LSU AgCenter rice specialist

DR. DUSTIN HARRELL
LOUISIANA
Extension Rice Specialist
dharrell@agcenter.lsu.edu

Nitrogen is one of the largest expenses in a rice production budget. Efficient use of fertilizer N not only helps maximize grain yield, but it also helps lower fertilization rates, lower fertilizer expenses and minimize negative effects on the environment.

There are a handful of questions that I get asked annually regarding N fertilization, and I thought I would take the time to answer a few of the most common.

Q: Are starter fertilizers beneficial and should they be counted toward your seasonal N rate goal?

A: First, starter N applications most often do not result in a yield increase at the end of the year. However, an increase in early season vegetative growth is often observed; therefore, a flood can be established earlier. This is particularly beneficial in weed management. Secondly, starter fertilizer N applications are the least efficient of all N application timings and should not be counted toward your targeted N rate total.

Q: When should I use a urease inhibitor on my preflood N?

A: Urease Inhibitors should only be used when urea is the fertilizer source and when potential volatilization (gaseous) losses make the use of them economically viable. The use of a urease inhibitor typically is economical when the potential exists for approximately 10 percent of the N to be lost to the atmosphere. In most years, this generally occurs somewhere around three to five days on a dry soil.

If urea is applied on wet or moist soil without any standing water, a urease inhibitor should always be used. If urea is applied into standing water, a urease inhibitor should not be used because it will not be beneficial. Approved urease inhibitors for Louisiana include one or more of the following active ingredients: NBPT, NPPT and Durimide.

Q: Why is it OK to apply mid-season N into the water but not OK to apply pre-flood N into water?

A: By mid-season, rice has developed an extensive root system that can actually out-compete N losses at this stage of development. So flying N fertilizer into the water at midseason is not a problem. In fact, research has shown that N applied at midseason is taken up almost completely in as few as three days.

Conversely, when pre-flood N is thrown into standing water early in the season, rice does not have a very big root system, and it will take approximately three weeks for this N to be taken up by rice. A lot of N can be lost by volatilization and nitrification-denitrification over a three-week period.