Corn's response to manure application can be a challenge to predict. Manure nutrient uptake is highly variable and impacted by both the year's crop growth conditions and the manure characteristics. Nitrogen (N) losses and corn N needs to be supplied by fertilizers vary significantly by year. N losses are maximized during warm, wet springs, as nitrate-N readily moves through the soil with water. Additionally, these conditions promote denitrification losses to the atmosphere if nitrogen is present in the nitrate form. Various tools exist to assist with N application rate decisions, such as the Maximum Return to Nitrogen (MRTN) Calculator. The MRTN Calculator is available in many Midwestern states but does not account for the season of N application and provides a nitrogen recommendation based on well-timed spring and side-dress applications.
Generally, it is best to apply manure as close to the growing season as
possible to limit nitrogen loss opportunities before crop uptake. A literature
review and meta-analysis were conducted using various studies investigating
different N rates during different application seasons to evaluate the impact
of nitrogen application timing on crop response to nitrogen. For a study to be
considered and used in this analysis, the study needed to use at least three
nitrogen rates and have performed nitrogen applications in at least two of the
three application seasons. Yield results were normalized to the percent of the
maximum yield obtained within a study year to facilitate analysis across
studies and time. The results of this review are shown in Figure 1.
The high variability in yield illustrates the
difficulty in accurately predicting N needs in a given year. Fall applications,
on average, achieved the lowest yield at a particular N application rate.
Sidedress applications, on average, had the highest yield at a nitrogen
application rate. This appears to be related to the risk of nitrogen loss
between the time of application and crop uptake. However, this wasn't the case
in every study or every year, as within at least one of the included studies,
spring manure application had the lowest yields.
An important point to consider is how this
impacts your farm, your bottom line, and how your crop will yield. For example,
200 lb N/acre applied in the fall yields 89% of the maximum on average, but
switching to a side-dress application results in 98% of the max yield. This 9%
increase correlates to an 18-bushel improvement if the field's yield potential
is 200 bushels per acre. While application timing significantly impacts yields
in the "average" year, varying weather conditions can make this
timing effect more or less significant in different growing conditions and different
years. Farmers need to weigh the pros and cons of each scenario to determine
which application strategy best suits their fields and the probable growing
conditions for the upcoming cropping year.
Figure 1. Scaled nitrogen response curves where application timing is coded by color and each study used is coded with a different symbol. Yield response was more significant at lower nitrogen application rates for nitrogen fertilizer applications closer to the corn growing season, but with higher nitrogen application rates similar yields could be obtained with different application timing.
Farmers benefit from longer, typically drier,
application windows in the fall. Soil conditions are generally favorable,
providing an ideal environment for application. As long as soil temperatures
have cooled to 50°F and are trending colder, microbial activity and
mineralization to nitrate are limited. Additionally, for organic matter-rich
manures (often solid manures), a fall application allows some time for the
mineralization of organic N, allowing it to break down into forms usable by
plants.
However, fall applications come with their
challenges. Snowmelt and other early spring weather introduce the risk of N
leaching or denitrification, leading to a greater risk of potential nitrogen
losses. As seen in Figure 1, fall applications, on average, tend to need higher
nitrogen application rates to obtain similar yields. Moreover, fall application
has the highest uncertainty about the exact amount of N that will be available to
the crop, complicating the decision-making process for farmers.
Spring applications offer their own set of
advantages. Less time between when the nitrogen is applied and when crops
uptake the nitrogen means fewer opportunities for N losses. By avoiding the
nitrogen being exposed in the field during early spring rains, leaching losses
are minimized. Improved N retention can lead to higher crop yields, and farmers
can better predict weather conditions for the season ahead, providing a higher
certainty level of potential nutrient need than fall.
Spring manure applications come with their own
set of difficulties. Wet spring conditions can hinder fieldwork schedules,
leading to delays or missed opportunities. Planting demands may leave little
time for additional field operations, creating conflicts in timing. Compaction
is another risk associated with spring applications. Heavy agricultural
machinery on moist soils can reduce soil porosity and negatively impact future
crop growth. While most of the N available in swine manures is available in the
first year, organic N takes time to mineralize, meaning only ammoniacal N is
immediately available to plants in spring application scenarios, potentially
delaying nutrient uptake.
In-season sidedress applications have become
another attractive option for manure application, avoiding some of the
challenges associated with fall and spring timings. Farmers have a clearer
picture of seasonal conditions by summer, facilitating adjustment to planned
nitrogen application rates to meet the expected demands of a particular growing
season better. Precision fertilizing allows tailored N application to meet crop
needs, while immediate crop uptake minimizes N loss opportunities.
However, side-dress applications come with
their challenges. Depending on the crop growth stage, specialized equipment may
be required to avoid crop damage during application. Dragline and tanker
applicators may be used up to the V4 stage in corn but should be avoided beyond
V4 to prevent crop damage as the corn matures unless specific equipment is used
to avoid running over corn. Specialized high-clearance irrigation equipment may
be used for late-season nutrient application. Without pre-plant N, fertilizer
response may be reduced, affecting overall crop health and yield. Additionally,
seasonal conditions can pose application challenges and compaction risks,
limiting effectiveness in wet summers.
Every application scenario comes with its
benefits as well as its risks. As fertilizer prices continue to change, weather
conditions become more unpredictable, and environmental stewardship becomes a
higher priority, it is crucial to recognize the pros and cons of each nutrient
management option. One strategy to mitigate some risk involved with fertilizer
applications could be multiple split applications, e.g., providing a partial N
rate in the fall and the rest in the spring. Predicting corn's response to
manure application can be a significant challenge. While the optimal manure
application strategy may change from farm to farm and year to year, it is
necessary to weigh the pros and cons of each strategy to determine the best fit
for your operation.
