Bronzing is a symptom of zinc deficiency in rice.
This year, there are a number of fertilizer recommendation changes coming out of Dr. Dustin Harrell’s research. They can be found in the 2011 version of Rice Varieties and Management Tips, which can be downloaded at www.lsuagcenter.com/en/crops_live-stock/crops/rice/Publications.
One complaint heard for the past couple of years has been about the lodging problem in CL151. It was suspected that nitrogen fertilizer was being applied at higher than necessary rates. After summarizing three years of variety by nitrogen rate trials, Dr. Harrell changed the nitrogen recommendation for CL151 from 120 – 160 pounds per acre to 90 – 130 pounds per acre. In his studies, economic return was maximized at this level without sacrificing gross yield. In verification fields planted to CL151 in 2009, we used 140 pounds and experienced minimal lodging, but this was on silt loam soils that provide little residual nitrogen.
Another change this year is the phosphorus soil test based fertilizer recommendations. The change was made by the LSU Soil Testing and Plant Analysis Laboratory to create a stand-alone calibration for the Mehlich III soil test extraction. Now the numbers reported by the Soil Testing lab at LSU can be compared to the results from other laboratories, using the same soil test extraction. This information is in a table in the Rice Varieties and Management Tips publication.
In the same table that provides an interpretation of the soil testing results for phosphorus are similar ratings for sulfur and zinc. However, there is an error in the zinc row. Soil test levels greater than 2.25 are considered high for zinc, not greater than 16 as it shows in the table.
Dr. Harrell has been studying the interaction between soil pH and soil test extractable zinc and its effect on zinc fertilizer recommendations. It has long been known that pH affects zinc availability. After summarizing his current data, Dr. Harrell has found that at soil test levels of one ppm or less and if the pH is greater than 7, it will require 15 pounds of granular zinc per acre. At the other end of the spectrum, if the pH is less than 7 and the soil test value is greater than 2.25, then no zinc is recommended. He only has five site years of data and has plans to continue the study for at least two more years.
Dr. Harrell was questioned last year at a grower meeting about varietal sensitivity to zinc deficiency. At the time, he was unable to answer if variety selection played a part. This past growing season, he set up a study to find out the answer. The results were interesting. Jupiter, CL111, Cocodrie and CL151 appeared to be moderately resistant to zinc deficiency. Neptune, Cheniere, Catahoula and Wells appeared moderately sensitive. Jazzman and CL261 appeared to be very sensitive.
As was mentioned in a similar article last year, the easy questions regarding fertility and fertilizer recommendations have already been answered. Now the tough ones are being addressed.
Over the last two years, we have had a good look at CL151 under two extremely different harvesting conditions. In 2009, the harvesting conditions were very wet, and, in 2010, the conditions were extremely dry. In both of these years, lodging was a severe problem with CL151. This variety has exceptional yield potential, but lodging has severely hindered this potential in CL151.
Over the last three years, numerous studies have been conducted in trying to mitigate the lodging problem with this variety. To help solve the issue, nitrogen rate research with CL151 has been of great focus.
When looking at nitrogen, rice yields and nitrogen rate do not have a linear relationship, which means rice yields will not exponentially increase for each pound of nitrogen applied. The principle of the law of diminishing returns is a more proper way to explain nitrogen response in rice. The basis of this principle is that the first added increment of nitrogen will result in the largest yield response, and with each added increment of nitrogen, the yield response will be less.
In rice, the highest yield response comes from the first 50 lb N/Acre, and then with each additional pound of nitrogen applied, the response becomes lower and lower until it is not economically feasible to apply any more nitrogen. Also, as the nitrogen rate is increased, other factors, such as lodging and disease, become problems that reduce yield.
Historically, most varieties have reached their highest economic return at approximately 150 lbs N/Acre on silt loam soils and 180 lbs N/Acre on clay soils. CL151 is a variety that has shown to respond less to nitrogen, meaning that suitable rice yields can be achieved with lower rates of nitrogen. To explain this further, 90 percent of maximum yield with CL151 can be achieved with only 88 lbs N/Acre on a clay soil and 95 percent of the maximum yield can be achieved with only 118 lbs N/Acre.
To compare this to another variety such as Catahoula, it takes 124 lbs N/Acre on a clay soil to reach 90 percent of the maximum rice yield and 159 lbs N/Acre to reach 95 percent of the maximum yield. With CL151, the research shows that it takes less nitrogen to reach maximum rice yields.
In response to this research, Mississippi State University has reduced its nitrogen rate recommendation for CL151. The new recommendation for CL151 is a total of 120 lbs N/Acre on clay soils and 90 lbs N/Acre on silt loam soils.
Reducing the nitrogen rate for CL151 will help reduce issues with lodging, but it is not the total solution for preventing lodging issues with this variety. Proper seeding rates (30 seed/ft2), fungicide application and timing also need to be considered.
Proper rice nutrient management is essential to growing a successful crop – main and ratoon. Complications arise because the majority of rice in Texas is grown under two irrigation methods. Early in the season, rice is flush-irrigated, while later in the season. rice is floodirrigated. Flushing usually results in soils drying out between flushes (if no rainfall occurs), which fosters aerobic soil conditions. Flooding results in saturated soils, which foster anaerobic soil conditions. Plant nutrients applied by rice farmers behave differently when exposed to aerobic and anaerobic conditions.
The first step in efficient use of fertilizers is to sample your soil for phosphorus (P), potassium (K) and micronutrient needs. P is an essential element and is found in rice genes, cell membranes and enzymes. K also is an essential element and is necessary for starch formation, activates many enzymes and regulates the opening and closing of rice leaf pores (helps with water and gas exchange). Virtually all non-organically grown rice requires large amounts of synthetic nitrogen (N); thus, pre-season soil testing for N is not very useful. If your soil test comes back indicating no need for additional P and/or K, don’t apply these nutrients. However, apply P preplant when concentration is less than 15 parts per million (ppm) in sandy soils and less than 10 ppm in clay soils. Apply K preplant when concentration is less than 50 ppm in sandy soils. Generally, additional K is not needed in clay soils. Flooding soils makes both P and K more available to the rice plant and changes soil pH towards neutral.
N is another essential element found in rice genes, chlorophyll and proteins and is required in the highest synthetic fertilizer amounts compared to other nutrients. Cost projections for the 2011 growing season hover around $500 per ton for urea (Toni Spencer, M & J Fertilizer, Winnie, Texas), which is the most common form of N applied in Texas rice fields. The two major N-loss processes in Texas rice production are denitrification and ammonia volatilization, so if you can reduce these losses, you can maximize the efficiency of your N fertilizer program. If left on a dry soil surface for an extended period of time, urea can be converted to ammonia gas and N lost to the atmosphere (ammonia volatilization). These losses increase during warm, dry, windy conditions. Thus, soil incorporation of urea at planting and flushing immediately following urea application maximizes the efficient use of urea at planting.
For preflood urea, apply on dry soil and flood immediately for the greatest N efficiency. Application of urea to wet soil also can increase ammonia volatilization. The flood (or flush) serves to incorporate the urea, which minimizes ammonia volatilization. If the flood is lost or paddies are intentionally drained after flood, urea can be converted to the nitrate form of N and leached out of the root zone. After reflooding, the nitrate can be converted by bacteria to a gas and lost to the atmosphere (denitrification). You may consider treating your urea with products that reduce ammonia volatilization and denitrification. Please consult with a plant nutrition expert on this matter.
N application at panicle differentiation (PD) can be tricky. Be sure to check your rice plants often after flood for stage of growth. Remember, a green ring stage occurs before each internode elongates, and our Texas rice plants may develop five internodes before PD. But, what if your plants seem to be running out of N before PD? Please consult a plant nutrition scientist for your specific situation! However, I do know that any stress – biotic or abiotic – can affect plant health and ultimately yield and quality. The problem is weighing inputs and outputs in terms of economics. If you have questions, contact your local university and USDA scientists involved in rice nutrition, find out what your neighbor is doing and constantly stay abreast of new rice research and technology.