Henry J. “Rick” Mascagni Jr., Gary Breitenbeck and Bubba Bell
Nitrogen fertilization is a critical component of cultural practices required for producing maximum corn yield. Many factors including climatic conditions, soil type and tillage systems determine optimum nitrogen rates. Nitrogen is typically knifed-in soon after the crop has emerged and an adequate stand is established. After fertilizer application, uncontrollable factors such as excessive or lack of rainfall may produce soil conditions conducive to nitrogen fertilizer loss through denitrification and inefficient plant nitrogen uptake.
Sometimes nitrogen applications are delayed or omitted because of inclement weather. At other times, growers apply the recommended nitrogen rate for an expected yield potential. As the crop develops, however, yield potential may be higher than expected and additional nitrogen may be required. In each of these situations, the question arises: Can nitrogen applications as late as reproductive growth stages be effective? Trials to evaluate late nitrogen applications on a Mississippi River alluvial soil attempted to answer this question.
Field experiments on Sharkey silty clay at the Northeast Research Station near St. Joseph were conducted in 2006 and 2007 to evaluate the influence on corn yield of early-season and late-season nitrogen applied at different rates. Early-season nitrogen was injected at about the two-leaf growth stage as 32 percent urea ammonium nitrate solution at nitrogen rates of 150, 180, 210, 240 and 270 pounds per acre. Late-season nitrogen was broadcast at the early-tassel stage as ammonium nitrate at rates of zero or 60 pounds of nitrogen per acre. Treatments were evaluated with and without irrigation.
Furrow irrigations were scheduled based on a 1.5-inch soil moisture deficit. Late nitrogen was watered-in soon after application in the nonirrigated trial in 2007 and was in the irrigated trials both years.
Late nitrogen received rain soon after application in the non-irrigated trial in 2007 and was watered-in in irrigated trials both years. Rainfall, which was needed to activate the late-season treatments in the non-irrigated trial in 2006, did not occur for three to four weeks after the applications. Yields are presented as two-year averages, since yield responses to both the early- and lateseason nitrogen treatments were similar each year.
When irrigated, average yields ranged from 152.2 to 204.5 bushels per acre for the no-late-nitrogen treatment and 183.6 to 209.7 bushels per acre for the late nitrogen (Figure 1). The optimum early-season nitrogen rate was about 210 pounds of nitrogen per acre. The late applications of 60 pounds of nitrogen per acre consistently increased yields for the lower early-season nitrogen rates, with the largest response occurring for the 150-pounds-per-acre early-season nitrogen rate. Yields were increased by 20.7 percent – ranging from 152.2 to 183.6 bushels per acre. Yields for total nitrogen applied (early season plus tassel emergence) were similar regardless of whether applied in single application at early season or split between early season and tassel emergence. For example, yields for the 210-pounds-per-acre rate applied either once at early season or split between early season and tassel were 192.4 bushels per acre for early season and 183.6 bushels per acre for the split application.
Under dryland conditions, yields were much lower than the irrigated yields, ranging from 120.7 to 141.9 bushels per acre for the no-late-nitrogen treatment and 127.4 to 130.2 bushels per acre for the late nitrogen treatments (Figure 1). The optimum early-season nitrogen rate was about 180 pounds per acre. Late nitrogen applications at tassel emergence did not increase corn yield. The low yields in the nonirrigated trials were primarily because of little May and June rainfall. If applying nitrogen as late as tassel emergence, it is important that the fertilizer be activated either by rainfall or irrigation as soon as possible for maximizing plant availability of the nitrogen fertilizer and minimizing any yield loss associated with late-season nitrogen deficiency.
These findings indicate that nitrogen fertilizer applications as late as tassel emergence may increase corn yield if the plant is nitrogen deficient. The efficacy of split nitrogen applications depends on many factors; however, climatic conditions are probably the most influential because they affect both soil nitrogen availability and plant nitrogen demand. Plant monitoring using remote sensing techniques is being evaluated for monitoring the plant nitrogen status late in the season. More extensive research is needed to determine if late-season plant monitoring can be used to determine if late nitrogen applications are warranted.
(This article was published in the winter 2008 issue of Louisiana Agriculture.)