Yield potential and fertilizer N management in rice

Yield potential and fertilizer N management in rice

If we have a “normal” planting year in terms of acreage, about half of the fields being planted to rice will have been fallowed the previous year. We have been studying yield potential and nitrogen (N) management in fallowed vs. continuous rice for the past couple of years. Here is some of what we have found.

First, yields are typically higher for rice following a fallow year. In the two years that we have been looking at this at the experiment station, rice after a fallow was two to five cwt per acre higher than rice after rice. In one of those years, we found that adding more N fertilizer to continuous rice increased yields but still not to the level of the rice after fallow. Plant-available N typically comes from fertilizer N applied and from the soil. Typically, about half of the plant N comes from each of these sources. 

Using labeled N fertilizer, we found that the fertilizer N was used similarly between the two systems. However, in rice after fallow, there was more N available from the soil from PI to harvest than for continuous rice. Thus, comparing these two systems, continuous rice may need more N to reach its yield potential relative to rice after fallow. Also, fallow fields are less likely to require top-dress N because more N is available during the last half of the season.

Second, as was previously mentioned, the yield potential was less in both years for continuous rice. While this may be in part due to N as mentioned above, disease was also a factor. Despite applying a fungicide, in fallow fields there was significantly less stem rot (severity score averaged 2.1) than in the continuous rice fields (severity score averaged 2.7). Such a difference in severity rating could lower the yield potential by one to two cwt per acre.

Fertility is a balance

Fertilizer prices appear to be improved — for now — compared to the 2022 season. We are still experiencing much higher prices compared to 2021, so folks will still be looking for adjustments to their fertilizer programs. Fertilizer market volatility is a given; we just don’t know which way it may go.

A return to moderate nitrogen (N) prices should allow for optimal N fertilization practices in the 2023 season, with less need to attempt “trimming” of rates. The goal for N application should be to divide the large preflood N rate into two application passes to avoid streaking. Also avoid dressing ends, which leads to overlaps where disease and lodging issues occur.

While N is the driver for rice yield, it doesn’t carry the load alone. Potassium (K) fertility needs to keep up with N fertility to ensure balance in the plant. An exact ratio between N and K is not the focus — just that one doesn’t dramatically outpace the other. Pushing N rates higher while reducing K rates could lead to problems with increased disease and lodging.

Phosphorus (P) is a key component for rice but perhaps not to the degree of N and K. Arguably the greatest concern with P fertility is that if we reduce fertilizer rates to the point of causing a deficiency, it can be difficult to correct in-season to achieve full yield potential.

Zinc (Zn) deficiencies have been less prominent in recent years, which is probably aided in the fact that it is needed in small amounts and most use a seed treatment to help out. However, Zn can cause some hidden hunger effects where plants aren’t deficient enough to display dramatic symptoms but are limiting yield. Trust your soil test and apply accordingly.

It is preferred to utilize long-term soil test results as a guide for fertilizer rates. Looking at five to 10 years’ worth of soil test results for a field gives a much better picture of where we are and how we’re actually doing. A single soil test can be influenced by previous crop, sample time and soil conditions at the time of sampling, which may lead us astray to a degree.

Side note for furrow-irrigated rice: the soil test recommendations for P, K and Zn should be the minimum amounts applied. Why? These nutrients are not available to the same degree in furrow-irrigated (upland) situations as they are in flooded rice.

Use N management tools such as N-STaR, GreenSeeker, etc. for N management, and rely on multiple years of soil sampling data for other nutrients. Find a nutrient balance for maximum success and profitability in 2023.  Let us know if we can help.

Managing fertility in our furrow-irrigated rice

In my mind, fertility becomes increasingly important in furrow-irrigated rice (FIR). While we largely have it “figured out” in traditional rice production, introducing oxygen into the equation changes our philosophy with several key nutrients. Nitrogen (N), phosphorus (P) and potassium (K) availability are not going to be the same in the aerobic portions of a furrow-irrigated rice field due to basic chemistry, which I will promise not to dive too far into. 

It’s been said that we’re beating a dead horse when speaking on N management in FIR, and to a certain degree, I’d concur. However, it’s too important of a topic to neglect. I spent 2018-2020 focusing probably 80% of my time on the topic, and I feel like we have some solid answers. Regardless of soil type, a three-way split is very appropriate; but, the way we apply that three-way split matters much more on our heavier clay soils. 

On clay, 160 pounds per acre of urea at the general, “pre-flood” timing, followed by 160 pounds of urea two weeks later, followed by 100 pounds of urea one week after the second application has — time after time — produced the greatest rice grain yield for us. That is 100 pounds per acre more urea than generally recommended for flooded rice, but the extra application added an average of around 20 bushels per acre grain yield. 

On our lighter soils (i.e., silt loam), the extra N was not warranted. Essentially, any three-way split is acceptable, with our general recommendation being three applications of 100 pounds per acre of urea spaced seven to 10 days apart. Also worth noting is that all applications should absolutely be treated with a urease inhibitor (NBPT) to limit volatilization losses. 

Phosphorus and potassium issues can be more common in FIR, and we are currently working on answering questions surrounding those issues. A flood increases availability of both nutrients, so it stands to reason that the typical rice soil test will overestimate the availability of P and K. For now, our recommendation is to stick with current soil tests and to pull plant samples if you have reason to believe a deficiency could occur, especially for K. A solid K fertilizer response can be seen as far along as the late boot stage.

If you have fertility questions now or in-season, you can always contact us via e-mail at jchlapecka@missouri.edu or catch us at some of the upcoming meetings and conferences through the end of February. As always, eat MO rice!

Potassium deficiencies

Potassium deficiencies can occur in Louisiana rice soils. The high cost of nutrients has led to leaving out applications. Rice plants deficient in K are lighter green, and the leaf edges contain rust-colored spots that give the plant a brown appearance. Potassium is a highly mobile element in the plant and is translocated from the older to younger tissue. Consequently, potassium deficiency symptoms usually occur first on the lower leaves of the plant and progress toward the top as the severity of the deficiency increases. A preplant or early season K application is recommended.  A single application (30 to 60 pounds per acre) is usually sufficient to maintain adequate K in the rice plants. 

LSU AgCenter soil test-based potassium recommendations do not consider the ratoon rice crop. Recent research has shown that rice grown on soils that test very low, low or medium in soil test potassium will need an additional 30 pounds of K (as K2O) to maximize ratoon yields. The additional phosphorus and potassium fertilizer can be applied with potassium in the first crop or can be applied after first-crop harvest.

Potassium behavior in the soil is influenced little by water management. Potassium is a very soluble nutrient and is accumulated by the rice plant throughout the growing season. Potassium increases crop yields because it increases root growth and improves drought tolerance. It activates at least 60 enzymes involved in growth. Potassium is essential in nearly all processes needed to sustain plant growth and reproduction. Potassium builds cellulose, which also helps to reduce lodging. It aids in photosynthesis and helps translocate sugars and starches in the plant. Plants deficient in potassium are less resistant to drought and high and low temperatures. 

The role of K in plant nutrition is also very important as it relates to disease resistance. Because potassium improves the overall health of growing plants, it helps them fight against diseases and insect pests. Soil testing on a regular basis can reduce overfertilization while producing the best yield possible.