strawEditor’s note: Welcome Dr. Ronnie Levy. He is taking over LSU AgCenter Extension rice specialist duties from Dr. Dustin Harrell, who was promoted to resident coordinator of the H. Rouse Caffey Rice Research Station in Crowley, Louisiana. Ronnie is no newcomer to rice, having spent more than 20 years as an LSU AgCenter county agent in Acadia Parish working in rice.
Rice planting is not far away. Optimum planting dates in Louisiana are March 10 to April 15 in Southwest Louisiana and April 1 to May 5 in North Louisiana. While planting early can have many advantages, such as higher main and ratoon crop yields, problems from early planting can occur.
Zinc (Zn) deficiency can be a serious problem in rice, resulting in reduced yield if not corrected. Plant growth is limited when the soil supply of Zn is low or adverse soil conditions (such as cold weather or continuous flooding) prevent plant uptake.
Zinc is an essential micronutrient for both plants and humans. It is involved in more than 300 plant enzyme activities including chlorophyll production, carbohydrate metabolism, maintenance of cell turgidity, protein synthesis, auxin regulation and pollen formation.
Rice affected by Zn deficiency initially becomes chlorotic (yellow) followed by a bronzing (brown) appearance in water-seeded fields. The plants are weakened, and leaves tend to float on the water rather than grow erect.
Severely zinc-deficient plants may die, leaving a sparse stand with poor vigor. In dry-seeded rice, zinc-deficient plants show an initial lightening of color, followed by bronzing of the leaves. These symptoms usually become visible in dry-seeded rice soon after the permanent flood is applied.
Plants can exhibit Zn deficiency symptoms at any stage of growth. If plants become deficient at heading, the leaves and glumes will show the typical chlorosis, brown flecking and spotting. When Zn deficiency becomes severe at heading, the florets are affected. A condition resembling straighthead may occur where the panicles of affected plants remain upright as fertilization fails to occur or kernel development is aborted.
Soil testing is an important tool in assessing nutrient needs. Zinc fertilizer recommendations are based on using a granular Zn sulfate. Other zinc sources can be used; however, inorganic Zn sources should be greater than 50% water soluble.
Liquid inorganic or chelated Zn fertilizers can be soil applied at lower rates compared with granular sources, generally between 2.5 to 5 pounds, because they can be applied more uniformly.
When Zn deficiency symptoms begin to occur (bronzing), it is recommended to immediately drain the field. When the rice begins to show signs of recovery (new growth), a foliar zinc application can be made to rice at rates between 1 and 2 pounds Zn per acre.
Granular Zn applications at this time have also shown to be equally effective. Application of nitrogen fertilizer should also be made prior to reflooding to account for losses associated with draining and reflooding.
Fertility management considerations for 2021
I would like to address a few fertility management considerations for this upcoming year.
First, I saw a lot of straw being bailed and removed from rice fields this past fall. Rice straw contains roughly 34 pounds K2O per ton, compared to 7 pounds per ton of grain.
A field that yields 100 hundredweight per acre has about 5 tons of rice straw. Replenishing the potassium (K) that is removed is important for sustainably managing rice fields. Low K results in lower yields, increased lodging and higher disease pressure.
This past year, we developed two fact sheets related to this topic that provide much more detail than I have here. The first is “Nutrients in Rice Grain and Straw at Harvest” and the second is “Managing Potassium in Rice Fields.” They can be found at http://rice.ucanr.edu/FactSheets/.
Second, we have been having a dry year. For those farmers who rely on good winter rains to decompose rice straw (as opposed to purposeful winter flooding), there may be still a lot of straw in the field when it comes time for land preparation.
Having a lot of rice straw in the soil at the start of the growing season can bind up applied fertilizer nitrogen early in the season. Therefore, if you have a lot of straw still in the field, you may need to apply a bit more nitrogen.
Third, topdress N applications are needed if the plant is N deficient or you suspect it will become so. At panicle initiation (45 to 55 days after planting), the crop should be assessed to determine if topdress N fertilizer is necessary.
A good assessment is important because not applying N when needed can lead to a reduction in yield; however, applying N fertilizer when it is not needed can lead to lodging, delayed maturity, increased incidence of disease and reduced yields.
The Green Seeker, which measures the NDVI (Normalized Difference Vegetation Index) of the canopy, is a tool we have been testing for this purpose. In the past (based on preliminary data), we suggested a response index of 1.1 or greater indicates the need for top-dress N application.
Based on a more complete analysis, we are adjusting this figure and now suggest a response index of 1.07 or greater warrants a top-dress N application. The response index is the NDVI reading of an enriched N strip (representing a crop with unlimited N) divided by the NDVI reading from the field test area.
The N enriched strip is an area where extra N was added to the field. This could be done by overlapping an area with an aqua rig or a small area where you intentionally added extra N.
For example, if the N enriched strip gave an NDVI value of 75 and the field test area gave an NDVI value of 71, the response index would be 1.06 (75/71 = 1.06) and this would indicate no need for a top-dress N application.
Additional information on nutrient management as well as other aspects of crop management can be found at http://rice.ucanr.edu/.
Soil sample, use urease inhibitor to steward nutrients
I am no expert on rice fertility, but I do know it is absolutely essential to test your soil for N, P and K. If you have not done so, now is the time to take your samples. Make sure you take them according to Extension or laboratory instructions.
If you recently have moved dirt from high to low areas, be sure to take samples from both areas. In addition, if you plan to ratoon crop, you may want to adjust your N, P and K according to your state’s guidelines, which can be obtained from your local Extension Service or rice research/Extension faculty and/or publications.
And of course, there are variety, soil type/pH and planting date considerations, all of which will impact your given fertility program…again, check with your local university folks and fertilizer dealerships for more detailed information.
I know some Texas rice crop consultants include fertility recommendations in their programs. You may want to hire one of these highly trained and valuable experts to help you make important fertility decisions.
Most rice farmers in Texas apply a urease inhibitor to preflood urea to minimize volatilization of ammonia, which results in less uptake of N by the crop and an increase in greenhouse gas emissions. It’s best to apply urea to dry soil and flush or flood immediately after application.
If urea is applied to wet soil, crop uptake is reduced. Also, it may take a few days before you can get water across your fields. In this situation, you should employ a urease inhibitor. The longer you leave urea on the soil surface before a flush/flood, the more N you lose to volatilization.
Here is the science behind volatilization of ammonia gas from urea:
1. Microorganisms naturally occurring in the soil produce an enzyme called urease.
2. Urease in the presence of water breaks down urea to ammonia.
3. Some of the ammonia reacts with water to produce ammonium, which is taken up by the rice plant, but a lot of the ammonia escapes into the air.
4. High pH soils and soils with a lot of organic matter increase ammonia volatilization.
5. Urease inhibitors decrease the amount of urease in the soil, which results in less N volatilization.
One last comment — kudos to Corrie Bowen (Wharton County Cooperative Extension agent) and Veronica Galvan (with U.S. Rice Producers Association) for putting on an informative virtual Rice Production Conference Jan. 20!
Make an ‘A’ on your soil test
The 2020 season was particularly notable for the amount of potassium (K) deficiency observed in rice. Both the 2018 and 2019 season displayed their share of it, but it seemed to be in overdrive this past year.
Interestingly enough, K deficiency even showed up in fields and areas where adequate fertilizer K was applied. This would seem to emphasize that the extremely wet conditions played havoc on the availability of K and the ability of plants to take it up.
The nature of the soil and nutrient availability can be difficult to pin an easy target on.
But the wet season definitely left us with a shallow-rooted rice crop that would have difficulty maximizing K uptake even with seemingly plenty available given soil test values and fertilizer K rates applied.
These occurrences seem to promote the idea of increasing our grid sampling of fields to better indicate potential problem areas. Potassium is one nutrient in rice where variable-rate application may have some merit to better address problem areas rather than a blanket application over the whole field.
In contrast, phosphorus (P) is not a good candidate for variable-rate application in rice due to the fluctuations in availability as the soil becomes saturated. Plus, the current soil test procedures are less sensitive at predicting the need for this particular nutrient.
Luckily, a number of commercial fertilizer application rigs do have the ability to apply a variable rate of one product, such as K, while applying a blanket rate of another product, such as P.
This could be a great way to manage input costs while putting your investment where it is most needed.
Typical visual potassium deficiencies are expressed as chlorosis along the margins of the older lower leaves and will often appear as a yellow-red firing of the leaves. Low K levels in rice can lead to increased disease susceptibility and severity.
One key indicator of potential K deficiency is brown spot, but in 2020 Cercospora was more common. Rice with low P levels, in contrast, typically has short, dark (green) colored plants with few tillers.
Stay the course with soil test recommendations for P and K. Where possible, variable-rate K may help us invest in improving weak areas of the field.
In-season applications of P and K can still provide a benefit if deficiencies are detected and verified, but stick with granular products to achieve the best results even then.