There is unprecedented interest and investment in applying nitrogen on corn later in the season.  By later, we mean applications that are beyond the traditional planting or sidedress timings and generally between the V10 to VT (tassel emergence) stages. Some key questions need to be considered as we aim to shift more of the corn crop’s N supply to later in the season.

1.   If the total amount of N applied is consistent across all strategies, do later N applications consistently out yield earlier N applications?  The answer to this one is no.  By that I mean that if you look across a range of data from Ontario and the Corn Belt states, you certainly can find instances where the later N application is significantly better, but on average, if you factor in some additional cost for late season N applications, then the timing strategies are often quite close in terms of net returns.

Split applications, where some of the N is applied in the planting window and the remainder is applied either as a sidedress N or a late N application, are often better in terms of yields and economics, but again, the advantage is not large.  That is, if your late N strategy is to simply take the 150 lbs that you are currently putting down at planting time and split it 75 lbs/acres on April 25 and 75 lbs/acre on June 20, you will generally be better off, but the improvement across years may be on average relatively small (i.e. $10-15 /acre).

2.   So how do I win bigger with late N applications?  The key to improving potential net returns is to ensure that the nitrogen rate that gets applied is adjusted for seasonal conditions.  In some years this will mean applying more than expected based on traditional rate approaches, and in some years, considerably less.  When these “late” N rates track N supply and demand more closely, then improvements in net returns may well be more in the $25-50/acre range.  For example, suppose a grower applies 125 lbs/acre as the planting time base rate, then this grower monitors rainfall, soil nitrate, crop NDVI, yield potential and comes to the conclusion that no additional late N is required, as opposed to the traditional 50 additional pounds that he normally applies.  In this situation, if yields slip 4 bushels per acre because no additional N was applied and product and application costs add up to $50, then net returns improve by about $30 per acre.  In contrast, if the signals (i.e. low soil nitrate, high rainfall, etc.) indicate a need for more N than normal, then the grower applies 75 lbs additional N.  Experience indicates that in the years when residual N supply is low, or when losses have been higher, that yield increases can easily increase net revenue even with above average investment in late nitrogen.  Perhaps the single biggest advantage to applying N later in the season is that growers can become increasingly more informed and can make better N rate decisions.

3.   How does a producer select the base N rate?  The OMAFRA N calculator is an excellent place to start as it gives you a comprehensive look at most factors that impact potential N requirements. One key to moving forward with greater success with late N is to adjust the base N rate to what I refer to as an “Opportunistic Base N Rate”; this implies a rate that will clearly carry the crop to a later season N application but that does not over supply N so that there is little “opportunity” to capitalize on seasonal adjustments.  For example, if a grower applied 175 lbs per acre on all his corn ground in 2012, he would have missed the opportunities to shave or completely eliminate late N in that growing season with zero yield penalty.  So a good Opportunistic Base N Rate generally is no less than 60 lbs/acre, and no more than 70-90% of the OMAFRA N Calculator pre-plant rate recommendation.

4.   What Factors Drive the Late Season N Rate?  The key factors in selecting the optimum late season N rate are: accumulated rainfall, soil texture, soil nitrate levels, yield potential and plant based indicators.

Accumulated Rainfall.  This needs to be relatively accurate rainfall data for the specific field and needs to be gathered from late April to the late N application timing.

Soil Texture.  This information is used for earlier base N estimates, but it also has a significant interaction with the rainfall amounts when determining late N rates.

Soil Nitrate Levels.  Soil nitrate levels that are taken from fields that have received earlier broadcast fertilizer N need to be interpreted with some caution, but if the levels are taken before plant uptake is significant (i.e. V8), they can be useful in estimating N requirements.

Yield Potential.  Planting date, plant population, uniformity of stand, field yield history and stage of development can all play a role in estimating potential yield and N requirements.

Plant Based Indicators. Tissue N concentration, leaf colour, or NDVI readings can help assess the status of N sufficiency or yield potential in the crop.

5.   Can N be applied too late for crop uptake?  This doesn’t get talked about much, but certainly there is evidence that if late N is applied to the surface of the soil, and the soil is dry and little rainfall is received after application, then the N may not make it into the plant in time for maximum yield improvement.  To be on the safe side, late N should be applied no later than 10 days prior to tassel emergence.  I realize that under good conditions, timely rainfall, etc., this can be pushed later, but I fail to see many advantages for this strategy.  That is, if we have made a well-informed rate decision on July 5th, it likely will not improve that much by July 15th and risk (especially in a dry July) increases.  It also should be noted that although this article has focused on “late” N, most of the ideas discussed can improve N strategies for any N that is applied from V5 on.

Moving forward.  If you would like to explore N strategies, please give your local Maizex dealer a call and we can set something up, or email me at

Greg Stewart
Maizex Seeds Agronomy Lead

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