Therefore, it is a necessary component of feed for your cattle and crops. Modeling of dairy systems has indicated environmental losses of nitrogen were equivalent to 50 to 77 percent of nitrogen imported on farm annually.
Nitrogen is imported onto the farm in three main ways. There is a large amount of nitrogen “imported” via biological fixation when leguminous crops, such as alfalfa, are grown. A typical dairy will also import a significant amount of nitrogen in the form of feed and fertilizer.
Nitrogen leaves the farm in the form of milk, cattle, manure (if exported) and feed (if excess feed is produced by the farm).
The average lactating cow has a nitrogen use efficiency of approximately 30 percent, with most of this nitrogen going into milk. Managing dietary nitrogen can be important, as each pound of nitrogen fed over what is needed for lactation and maintenance is excreted by the cow.
Typical nitrogen-use efficiencies of crops is less than 40 percent, with the remainder of nitrogen either being stored in the soil or lost through leaching, runoff and volatilization. Therefore, managing both feed and manure/fertilizer nitrogen is important.
The goal of on-farm nitrogen management is to maximize your return on investment while protecting air and water quality. Maximizing return on investment is not the same as maximizing yield. At some point, there are diminishing returns for each additional unit of nitrogen used to produce any given product. The ideal is to maximize your profit, therefore inputting enough nitrogen to get the best production for the least cost. At the same time, managing nitrogen to protect both water and air quality is an important part of the decision-making process.
The 4R principles of nutrient stewardship can help guide the on-farm decision-making process. This consists of using the right source, right rate, right time and right place. While these are good guidelines to follow and sound simple, the reality of on-farm management can be quite a bit more complex, and therefore solutions will need to be flexible to fit each farm’s needs.
Right source
The right source involves matching fertilizer type to crop needs. Most dairy producers will use a mix of both manure and synthetic fertilizers for crop production. Some of the more common fertilizers consist of urea, urea ammonium nitrate (UAN), monoammonium phosphate and diammonium phosphate. There are also specialty fertilizers designed to enhance nitrogen-use efficiencies in cropping systems such as slow-release fertilizers; fertilizers with nitrification, denitrification and urease inhibitors; and branded enhanced-efficiency fertilizers.
Organic nitrogen can come from a wide range of sources, including plant and animal, with each source having its own unique characteristics and applications. Two things to keep in mind with organic sources are: The nitrogen may not be available right away, as it will have to go through a mineralization/nitrification process in the soil, and some of these amendments may have other benefits, such as improving soil quality, which will provide benefit beyond just the nitrogen value.
The plant-available nitrogen in traditional fertilizers is readily available and therefore easy to account for in nitrogen budgets. Organic nitrogen sources may vary greatly and may not be readily plant-available; therefore testing prior to application and assuming some sort of availability factor will be necessary.
Right rate
The right rate matches the amount of nitrogen fertilizer to crop needs. A crop’s nitrogen requirements will be driven by factors such as yield, nitrogen uptake by the plants, and nitrogen availability from the soil, crop residues and the fertilizer source applied.
It is important to use a realistic yield goal based on past performance of the field. Once you have a realistic yield goal, then you need to determine the amount of nitrogen that will be taken up by the crop over the growing season, which can be based on previous tissue testing or book values. Remember, the same crop grown in the same field can take up different amounts of nitrogen from year to year due to changes in crop health, weather, irrigation, the nitrogen source used, the rate used, etc.
You also will need to know how much nitrogen you are starting with; therefore soil testing to at least 2 feet in depth should be done prior to applying fertilizer. For certain crops, such as corn, waiting until later in the season to soil sample may be advantageous to better estimate the amount of nitrogen that will be available from the soil, as testing later allows you to capture some of the nitrogen mineralized by the soil.
You will also need to calculate a nitrogen credit for any crop residue or nitrogen-fixing crop (such as alfalfa) that may have been in the field the previous year. When using organic amendments, you have to assume a mineralization rate for the current application as well as from any past applications. The amount of nitrogen mineralized will depend on the type of organic amendment, the soil type, climate, irrigation, crop rotation, etc. If possible, try to use estimates that were determined using similar materials on similar soils in similar climates.
Right time
The right time makes nutrients available when the crops need them. In an ideal world, you would slowly feed nitrogen to crops as they need it. For most crops, the majority of nitrogen uptake happens during the mid-stages of plant growth. Making nitrogen available during the times when the plant uptake is the greatest can enhance the nitrogen-use efficiency provided all other management factors are optimal. Doing split applications of nitrogen when possible can help better match the crops’ uptake patterns.
Other options include the use of slow-release fertilizers. Many organic nitrogen sources behave like a slow-release fertilizer, as nitrogen is slowly mineralized over time. Using in-season soil and plant tissue testing can also help better match nitrogen needs with nitrogen supply. Nitrogen mineralization can occur late into the year and may still be occurring after crops have been harvested; therefore post-harvest field management can be important to help reduce losses of this nitrogen.
Right place
Applying nitrogen in the right place keeps nutrients where crops can use them. There can be large variability in nitrogen fertility across a given field. Precision application of nitrogen can help target nitrogen to areas where it is needed, saving money and reducing potential losses.
However, the technology for variable application rates may be cost-prohibitive for many producers. A less expensive option is to divide your field into management units based on topography, soil type or other unique field features. By doing so, you can better evaluate the nitrogen status of those areas and estimate the nitrogen needed in those areas.
How you apply nitrogen can make a difference as well. Surface applications of urea and manures can be prone to ammonia (NH3) volatilization, therefore losing valuable nitrogen. Using urease inhibitors can reduce the losses of ammonia from urea by slowing the conversion to ammonium (NH4).
Injecting or incorporating manures immediately following application can reduce ammonia losses. For example, in on-farm studies where liquid manure was injected, there was a 67 percent reduction in ammonia losses compared to surface-applied manure. When solid manure was disked into soil immediately following application, ammonia losses were reduced 35 percent compared to surface application. However, there can be trade-offs between practices. Any nitrogen not lost through ammonia volatilization can be lost due to denitrification or nitrate leaching.
On-farm nitrogen budgeting is complicated. You have to balance all the on-farm inputs (soil, biological fixation, plant residue, organic amendments, fertilizer) with outputs (nitrogen removed by crops). As nitrogen-use efficiencies tend to be low, you will always have losses from the system. Overapplying nitrogen in the form of fertilizer or manure can create a situation where large amounts of nitrogen losses can occur, particularly through ammonia losses and leaching with either improper irrigation or large rainfall events. Therefore, keep good records of how much nitrogen is going into and coming out of the system.
Soil sampling should also provide information on how much nitrogen is being stored in the soil. However, keep in mind: Using soil sampling alone to determine whether you are doing a good job managing nitrogen is not a reliable practice. If you have low post-harvest soil nitrogen, that may mean you did a good job of matching inputs to outputs, but it could also mean you did a poor job with irrigation management or you had enough rainfall to leach the nitrate (NO3) out of the soil profile.
Keeping detailed records can help evaluate your overall nitrogen-use efficiency over time and help identify areas where you could make improvements. When more nitrogen ends up in the products you are producing, that is less money spent to produce those products and less nitrogen available for loss to the environment. 
PHOTO: It is very important to keep good records of how much nitrogen is going into and coming out of the system in order to evaluate overall nitrogen-use efficiency over time. Photo by Karen Lee.
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April Leytem
- Research Soil Scientist
- USDA-ARS Northwest Irrigation and Soils Research Laboratory
- Kimberly, Idaho
- Email April Leytem
