Using the Most Profitable Nitrogen Rate in Your Cotton Production System
By Cliff Snyder and Mike Stewart
The agricultural community is viewing higher nitrogen (N) prices this spring with a bit of “sticker shock”. Because of the 2003 price increase in natural gas, the major cost in ammonia production, the production cost of N fertilizers has risen, just as it did in 2001. Anhydrous ammonia is the foundation for all other major N sources.
Farmers, their fertilizer dealers, and crop advisers are considering ways to maintain or improve yields while managing the N rate, source, and timing optimally. Some are trying to determine if cutting back on the N rate will affect economic returns. Research results from across the Cotton Belt consistently show that the optimum N rate is not as affected by N price and cotton price as one might think.
Farmers need to remember that dryland and irrigated cotton take up between 14 to 20 lb of N/A to produce each 100 lb of lint/A. Without optimum N, water, other nutrients, pest control, and all other production inputs will be used inefficiently. Adequate N uptake early in the season is critical to the development of a healthy root system and an expanding leaf surface area for maximum photosynthesis as flowering begins. A large portion of the N uptake occurs after flowering, when the peak uptake rate can approach 4 lb/A/day. About 32 lb of N/A/bale are removed in the harvested cotton seed and lint.
Soils store and release N differently during the growing season depending on the organic matter levels, soil texture, rooting depth, available moisture, and temperature. The optimum N rate and timing will depend on these soil and climatic conditions, and will also vary with tillage systems and the way the N application(s) and sources are managed. Research results from across the Cotton Belt can help farmers understand what the optimum N rates are for the best agronomic performance and economic returns. The following example is used to illustrate the cotton yield response to N rates, and a range in N and cotton prices. More illustrations for the Southeast, Midsouth and High Plains can be found at the PPI website: www.ppi-ppic.org
A 12-year (1988-1999) study in southeast Arkansas evaluated cotton response to N under a dryland and two irrigated regimes on an alluvial Rilla-Hebert silt loam. Urea was applied in two or three split applications: preplant, early squaring, and first bloom, depending on the total N rate. Table 1 shows the furrow-irrigated cotton lint yield estimated from the production function at N rates ranging form 0 to 150 lb/A. Note the sizeable yield increase even on this relatively fertile soil. Based on the N response in each of the irrigation regimes (Figure 1), economic optimum N rates were estimated. The profit maximizing N rate on this alluvial soil ranged from 108 to 120 lb/A for dryland cotton, 83 to 92 lb/A for high-frequency pivot irrigated cotton, and from 112 to 119 lb/A for the furrow irrigated cotton. The corresponding lint yields at these N rates were about: 1,200 lb/A for dryland, 1,310 lb/A for high frequency pivot irrigated, and 1,460 lb/A for furrow irrigated.
Tables 2A, 2B and 2C show that, regardless of whether cotton is dryland or irrigated, the economic optimum rate changes little even with a 125% range in N price ($0.20 to 0.45/lb of N). For example, when the N price was $0.25/lb, the most profitable N rate changed only 4 lb/A as cotton price changed from $0.52/lb to $0.82/lb for dryland cotton (Table 2A). Similarly, a decrease in cotton price from $0.62/lb to $0.52/lb changed the most profitable N rate for dryland cotton only 6 lb/A, even when the N price was $0.45/lb.
Table 1– Estimated cotton yield at different N rates for furrow irrigated cotton on a Rilla-Hebert Silt Loam in Southeast Arkansas (Research by J. Scott McConnell, 1988-1999).
Figure 1- Cotton response to N on an alluvial Rilla-Hebert silt loam in Arkansas, 1988-1999.
(Research by J. Scott McConnell, University of Arkansas)
Table 2 - Optimum cotton N rate on an alluvial Rilla-Hebert silt loam under dryland and
irrigated culture in Arkansas: 1988-19999. (Research by J. Scott McConnell, University
of Arkansas)
Things to remember about N prices, cotton prices, and N rate selection:
Changes in N price or cotton price have a relatively small impact on the most profitable yield or the optimum N rate. Significant reductions in the N rate used on cotton could lead to large economic losses.
Account for all sources of N in your cotton production system. Different soils have different N storage and release capacities. Residual soil N tests, which work best in drier regions of the Cotton Belt (i.e. western and High Plains), may help identify opportunities to alter N applications. Local research and extension specialists can provide information on sampling methods and the most appropriate time for sample collection. Credits from legumes and manure applications should always be considered when planning fertilizer N applications.
Keep other essential plant nutrients in balanced supply. Research by USDA-ARS scientists in Mississippi showed that optimum potassium (K) fertilization increased the efficiency of fertilizer N use by 19%. The lint yield production per pound of N applied increased 13% with adequate K fertilization. Other studies have shown that with optimum K management, the amount of lint produced per inch of water can increase from 20 to 50% over the production where no K was applied to a soil with a medium to high soil test K level. Improvements in water use efficiency can enhance the returns to all production inputs … not just fertilizer inputs. Balanced N and K nutrition can also help reduce the potential for a buildup in residual soil nitrate, which could pose a risk to ground and surface water quality.
There are many best management practices (BMPs) that can help improve farm profits and protect the environment. Appropriate timing of N, and other nutrient applications, and proper placement can have a significant impact on efficient crop use and profitability. Nitrogen input decisions should be planned wisely, and should be specific for each cropping system, to reach yield goals that result in maximum profit and preservation of soil and water quality.