Rick Mascagni, Brenda Tubaña, Michael Salassi and Michael Deliberto

Nitrogen fertilization is a critical management practice required for producing maximum corn yield. Many factors, including soil type and crop management systems, determine optimum rates. Nitrogen is typically applied soon after crop has emerged and an adequate stand has been established. After fertilization, uncontrollable factors, such as too much or too little rainfall, may produce soil conditions conducive to nitrogen fertilizer loss through leaching, denitrification or inefficient plant uptake. Sometimes nitrogen applications are delayed or omitted due to inclement weather. At other times, growers apply the recommended nitrogen rate for an expected yield potential; however, as the crop develops, yield potential may be higher than expected and additional nitrogen may be required. In each of these situations, the question arises: How late can nitrogen fertilizer be applied and still be effective? The objective of this study was to determine the effectiveness of supplemental nitrogen applied at tassel emergence on a Mississippi River alluvial soil.

Eight field experiments were conducted from 2006 to 2012 on Sharkey clay at the Northeast Research Station near St. Joseph to determine the efficacy of late nitrogen applications (Table 1). Early-season rates ranging from 150 to 270 pounds of nitrogen per acre were injected using urea ammonium nitrate solution at the two- to three-leaf seedling stage, which is when most growers apply their nitrogen fertilizer. Additionally, one treatment consisted of a split application nitrogen of 180 pounds per acre applied early-season plus a supplemental rate of 60 pounds per acre applied at tassel emergence, giving a total of 240 pounds of nitrogen per acre. The second fertilizer application was activated soon after application with furrow irrigation to enhance plant uptake efficiency and minimize any nitrogen loss through volatization. Grain yield is reported at 15.5 percent moisture. Grower net returns for nitrogen applications above variable costs were determined using corn prices of $4.40, $5 and $6 per bushel and nitrogen fertilizer prices of 59 cents per pound of nitrogen. A 20 percent crop share was also taken out of added returns for rent. The trials were furrow irrigated, and the previous crop was cotton. All LSU AgCenter management recommendations were followed.

Rainfall was at or below normal each year except in 2007, when 16 inches of rain occurred in July. Average yields ranged from 143 bushels per acre in 2010 to 203 bushels per acre in 2012. Yield responses to nitrogen treatments averaged across years are presented in Figure 1. Although 180 pounds of nitrogen per acre was not adequate for maximum yield on this Sharkey clay soil, a supplemental application of 60 pounds of nitrogen per acre at tassel emergence increased yield in seven of eight trials. Yields were not sacrificed in most trials when supplemental nitrogen was applied at tassel because yield showed little difference between equivalent nitrogen rates when applied early-season (240 pounds of nitrogen per acre) versus early-season plus supplemental nitrogen at tassel emergence (180 pounds plus 60 pounds of nitrogen per acre) or the highest early-season rate of 270 pounds of nitrogen per acre. In a few trials, yield for the split application was less than the equivalent rate applied early season. In these cases, an earlier supplemental timing or a higher rate may have been more effective.

As expected, the highest net return for nitrogen fertilizer applications occurred when corn market price was $6 per bushel, with the highest net return occurring for the single, earlyseason application of 240 pounds of nitrogen per acre (Figure 2). Even though yields were similar when 240 pounds of nitrogen per acre were applied regardless of timing, net return to the grower for the single application compared to the dual application was increased by $14.95 per acre.

The present AgCenter nitrogen recommendation is 180-240 pounds of nitrogen per acre on irrigated alluvial soils, with the higher rates suggested for the heavier clay soils. Findings from this study confirm the present nitrogen recommendation of about 240 pounds of nitrogen per acre for irrigated alluvial clay soils. Although 180 pounds of nitrogen per acre applied early season was not adequate in these trials, supplemental nitrogen applied as late as tassel emergence was effective in most years, not sacrificing yield. Oftentimes visual nitrogen-deficiency symptoms late in the season can alert the grower that additional nitrogen is needed. Also, recent advances in  diagnostic tools such as remote sensing are being refined to aid in identifying late-season nitrogen problems. Determining optimal nitrogen needs is challenging because of the complex interaction between climatic factors affecting the availability of soil nitrogen and plant demand. An in-season nitrogen fertilizer management system permits the grower to make decisions based on the plant’s needs for the current season, maximizing return and minimizing environmental concerns.

Rick Mascagni is a professor at the Northeast Research Station, St. Joseph, La.; Brenda Tubaña is an associate professor in the School of Plant, Environmental and Soil Sciences; Michael Salassi is the Fairbanks Endowed Professor and Michael Deliberto is a research associate in the Department of Agricultural Economics and Agribusiness.

(This article was published in the 2014 winter issue of Louisiana Agriculture magazine.)