Manure management: Getting the most out of your nitrogen
By Deanne Meyer
Almost a year ago, a large California study on nitrate contamination of groundwater was released. More recently, Washington State made the news related to nitrate and groundwater. Why are dairy farmers interested in these results? More importantly, what can producers do to maximize use of manure nitrogen (N) and minimize potential nitrate contamination?
What’s the problem? The Environmental Protection Agency (EPA) established maximum concentrations of nitrate in drinking water (part of the Safe Drinking Water Act). The value of 45 parts per million (ppm) or milligrams per liter nitrate is the upper limit for safe drinking water. This equals 10 ppm nitrate-nitrogen.
When consumed by at-risk populations, elevated concentrations of nitrate can result in blue baby syndrome and problems with respiration. (The nitrate interferes with the oxygen-carrying capacity of the blood.) When drinking water exceeds the maximum concentration, it should be prevented from being used by at-risk populations. A city with elevated concentration would use expensive treatment technologies to reduce nitrate concentrations or find an alternative drinking water source. There’s no good reasonably priced farm-scale technology to remove nitrate from drinking water.
Unlike most chemicals or compounds, the nitrogen (N) cycle is complex. This translates into no magic bullet (yet) for nitrogen management. N is excreted from cows in feces and urine. Around one-third of the N is excreted either in a plant-available form or in a form that will be hydrolyzed to make it available to the plant relatively quickly. The remainder of the N is included in organic materials (bound to carbon, etc.). Organic N must be mineralized to yield ammonium-N, and this is then nitrified to nitrate-N form. N mineralization is highly variable depending on the source, soil type and soil conditions.
Further decomposition can yield N2, the primary gas in our atmosphere. Incomplete activities may result in volatilization of N2O (a greenhouse gas). Ammonium-N can move from liquid to gaseous phase and ultimately volatilize (lost to the atmosphere). Nitrate-N can be leached when water is applied to land (inefficient and/or non-uniform irrigations or excessive rainfall).
One of the great challenges when managing manure-based organic-N is to closely match when organic-N becomes plant available and when plants need N. Once organic-N is converted to nitrate, the nitrate has no choice but to leach through the soil when excess water is present. Once it’s below the plant root zone, there is little opportunity to prevent nitrate from ultimately contributing to groundwater.
Maximize N uptake by plants
Hopefully you’ve included N management in your New Year’s resolution. Here are pointers to achieve better N management.
1. Estimate how much nitrogen is needed for a specific crop and determine targeted application rates. (This amount is typically based on previous crop yields and known environmental losses to volatilization and potential leaching).
2. Establish anticipated application times and amounts to achieve nitrogen application rates. It may be helpful to work with a crop adviser. You will want to account for both the organic-N and the ammonium in these calculations. The reliability and predictability of organic-N availability for plant use is not precise at this time. In some areas, operators may choose to limit organic-N application to less than 30% to minimize the impact of its use on crop fertilization.
3. Sample manure regularly (prior to land application) for organic-N and ammonium-N. For solid manure, be sure to ask for moisture content. The moisture in solid manure can vary considerably (10% to 85%). If groundwater sources have elevated nitrate, then also evaluate liquid manures for nitrate-N. Otherwise, this value should be relatively low.
4. Count how much manure (solids or liquids) are applied to each individual field for each crop.
If the facility is N rich with insufficient land for application, try to either generate less N within the facility or identify methods to remove N from the facility.
5. Determine total N applied (amount of material applied multiplied by N content).
6. Account for N in ground and surface waters used for irrigation events.
7. Manage irrigation water as efficiently as possible.
8. Sample crops removed from fields (for moisture and nutrient content) and quantify amount of forage removed.
9. Determine total N removed (forage amount removed multiplied by forage N content).
10. Source reduction: Evaluate all diets formulated for animals at the dairy. Depending on market conditions, it’s possible to pay a lower price for a commodity feed while increasing N intake (and subsequent excretion). Feeding appropriate amounts of protein will minimize the quantity of N to manage.
11. Collect some manure sources as solids and export (manifest) them off-site. Care must be used in collection corral manure to be sure the corral (soil) remains and only the surface manure is harvested. Box scrapers are common to minimize destruction of corral surfaces.
12. If additional N needs to be removed, use of gravity or mechanical separators may be advantageous. Be mindful that this will remove predominantly the organic-N. Do not count on 5% to 15% through this method.
Manure management is part of a biological system. From an N point of view, this means the amount of manure N in one form or another (organic, ammonium) will vary depending on diets fed, atmospheric losses, microbial decomposition, etc. Frequent collection of manure from wet areas (concrete lanes) may conserve ammonium-N (plant available) – flushing twice daily instead of once should improve N conservation. This may result in more plant available N applied to crops, although this does not guarantee the N will not be lost in another part of the system.
Ultimately, what we could really use in dairy nutrient management is a technology that will mineralize the organic-N (go from used plant food and dead bacteria) to ammonium-N. Then, the N would behave similar to anhydrous ammonia and it would be much more useful and predictable as a fertilizer.
• To contact Deanne Meyer, PhD, Livestock Waste Management Specialist, University of California, Davis, email her at firstname.lastname@example.org.