Manure dairy nutrients to maintain yields while protecting water quality requires having the infrastructure in place that will enable the nutrients to be land-applied at appropriate rates.

Will your system allow you to successfully use dairy lagoon water as the primary nutrient source for your crop? Use the following checklist to see if you have all the necessary parts of the system in place:

1. Sufficient crop acreage to utilize the nutrients in the lagoon water.

If application is through an irrigation system, the system must be designed in such a way that lagoon water can be applied to the entire acreage at appropriate rates.

2. Sufficient storage capacity so the pond water can be applied during or just prior to periods of crop uptake and no applications made when crop nutrients are not needed.

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Your soil type, climate and irrigation practices will influence how far in advance of expected crop uptake you can safely apply lagoon water nitrogen. Extra capacity for storm water, higher-than-average rainfall years, storage of nutrients to cover peak use periods, additions of extra water to prevent the pond from becoming too concentrated, losses from evaporation, tailwater return flow and minimum operating levels needed for flushing and floating pumps all need to be considered when sizing ponds.

3. A source of fresh water for dilution.

In most cases, applying undiluted pond water to a field will result in over-application of nitrogen, phosphorus and salts. The speed of the irrigation, concentration of nutrients in the pond and desired application rate determine the dilution required. Required dilution rates vary greatly, but a dilution of 10 percent lagoon water in the mixed water is typical for an application rate of 50 pounds per acre available nitrogen. Winter irrigations typically have lower dilution rates – on the order of 25 to 35 percent lagoon water – because the potential for leaching is less from spring rainfall than with summer irrigation so more nitrogen can be applied in a single application.

4. Adequate mixing of fresh and pond water.

Pond water usually must be diluted prior to application. If flows of fresh and pond water come from opposite directions in a pipeline, they will probably not mix, even if both streams are put through one valve, and one side of the irrigated area will receive more nutrients than the other side. Lagoon water in a pipeline has a high electrical conductivity (EC) and an EC meter can be used to check mixing under your conditions.

5. Correctly sized pumps and pipelines to allow application of 30 to 120 pounds of crop-available lagoon nitrogen in a single application without plugging.

The low rate will be needed to prevent salt injury on pre- and first irrigations, and the high rate will be used in mid-winter applications. Most mid-season summer applications will be around 50 pounds per acre of available nitrogen. Very concentrated pond water or slow irrigation run times may make it difficult to apply appropriate rates of nutrients.

6. A solids removal system will be necessary in most cases.

This will keep solids from building up at the head of fields, help keep pipelines clear, provide a more uniform product for nutrient application, help prevent solids from building up in the pond, make it easier to manage the organic nitrogen fraction of lagoon nutrients and allow more nitrogen and phosphorus to be moved offsite if necessary.

7. A means of preventing build-up of sludge in the pond is essential for long-term nutrient management.

One method is to ensure that nutrients generated in a year are applied in that same year to prevent having to apply the leftovers from many seasons at one time when the sludge is cleaned out. Options to help minimize sludge build-up include agitation, movable floating pumps or flushing of ponds with freshwater. However, these methods can result in a lagoon water product which has too much nitrogen in the organic (slowly available) form. Maintaining yields without applying excess nitrogen when more than 30 to 50 percent of the total nitrogen is in the organic form is difficult because in order to provide enough available-form nitrogen to meet crop needs, an excessive total amount of nitrogen must be applied.

Another way to minimize build-up of solids in ponds is to keep them from entering in the first place. Traditional mechanical separators take out coarse particles but generally won’t prevent sludge build-up. Functional settling basins or very fine screen separation equipment in conjunction with a process pit are better options. A process pit collects and stores water from the milk barn to use for flush water, which then drains back to the process pit. The process pit water is sent over a low-throughput, high efficiency separator prior to transfer to the long-term storage pond.

An alternative method to land applying all nutrients generated in a year is to deliberately allow sludge to accumulate in the first pond of a two- or three-pond system, keeping the last longer-term retention pond relatively clear. Periodically the sludge from the first pond(s) would be pumped and land-applied at agronomic rates or dewatered and exported. Technology to economically accomplish the dewatering process remains to be demonstrated.

8. Uniform distribution of irrigation water down the furrow or check.

Because nutrients are being applied with the water, more nutrients will be applied in parts of the field where more water infiltrates, for example the head or tail end. This results in over-fertilization in some areas and under-fertilization in others. In some cases, injecting the lagoon water into the fresh water during only the last portion of the irrigation may improve uniformity of surface water nutrient applications.

9. A method of measuring how much lagoon water is being applied.

There are several methods for doing this. Measuring pond drop may seem to be the most straightforward method, but the results can often be inaccurate due to pond inflows and outflows, challenges with recording the correct drop, sides that are not straight or uniformly sloped, etc.

Another method is to calculate the application based on pump output and the hours the pump ran. This sounds simple, but it is often difficult to accurately determine the gallons per minute (gpm) of a lagoon pump because they cannot be pump tested using standard equipment, and also the pump output will vary according to the water level in the pond, amount of sludge, debris in the impeller, etc.

The easiest way to measure application is to install a flow meter on the pond outlet. This method, when coupled with a control valve, allows specific amounts of lagoon water to be applied and measures the total gallons applied to each field. In many cases, the cost of the flow meter can quickly be recovered in savings on commercial fertilizer.

10. A method of measuring the concentration of crop nutrients in the lagoon water.

The amount and forms of nitrogen and the amount of phosphorus in the pond can vary throughout the season and sometimes even over the course of an irrigation. Laboratory analysis offers the most accurate and complete information on a sample, while in-field rapid testing procedures for nitrogen can allow the application to be rapidly adjusted in response to changing concentrations or unanticipated run times.

11. A means of controlling the amount of lagoon water that is applied to the field by varying the proportion of lagoon water to fresh water.

A valve or variable frequency controller on the pump will allow the lagoon water flows to be regulated. Some valve designs are better at throttling flow than others, especially if it is sometimes necessary to apply very small amounts. A V-notch gate valve is preferred by most users for this purpose because it is less prone to clogging when throttled down to lesser flow rates and can be more accurately adjusted when nearly completely closed. Be sure the valve is made from a material that is resistant to corrosion.

12. A method of record keeping.

The best infrastructure is of little value for nutrient management without a functional record keeping system. You will need to record volume and concentration of each source of nutrients applied to each field. Sources include lagoon water, fresh water, manure, commercial fertilizer and legume-crop plowdown. Yield and nutrient concentration of the harvested crop also needs to be recorded.

A record keeping system that allows you to track the amount of crop nutrients as soon as they are applied to each field during each irrigation will enable you to make informed decisions about subsequent applications. Data entry associated with dairy nutrient management is considerable and time-consuming, and it is essential that someone with good computer skills be assigned responsibility for this task.

13. An understanding of when crops need nutrients.

Most plants will have periods during the season when their need for nutrients is greater than at other times. Timing applications to coincide with crop uptake will ensure the highest yields. Applying crop nutrients at times when the crop doesn’t need them can result in losses, especially of nutrients such as nitrogen that have the potential of leaching into groundwater.

14. A method of accounting for organic form nitrogen applied in both current and previous seasons.

Crops can only utilize available form nitrogen (ammonium or nitrate form). However, much of the nitrogen in liquid and solid manure is in the organic form. Failure to account for and manage this large portion of the total nitrogen can result in groundwater contamination. It will be necessary to estimate the amount of nitrogen that will be mineralizing during the cropping season and reduce the amount of applied nitrogen accordingly.

If considerable mineralization is expected to occur at certain times of the year or in between crops, plan to have a crop planted to take it up prior to major leaching events such as irrigation or winter rainfall. If too much of the applied nitrogen is in the organic form, maintaining yields without overapplying nitrogen can be difficult, especially if the field is subject to losses from leaching and denitrification.

15. A soil or plant tissue testing program to estimate how much plant-available nitrogen or other crop nutrients are already present in the soil.

Soil testing for nitrogen is most helpful in situations where nitrate leaching or denitrification losses are minimal. Plant tissue tests can help identify nutrient deficiencies and must be used to document the need for additional applications under current state regulations.

16. A tailwater return system to prevent discharge of nutrients off the property is a necessity in many locations.

17. Backflow prevention devices are required to prevent wastewater from contaminating freshwater sources.

An air gap between the discharge of the liquid manure and the standpipe is the most common method of backflow prevention.

18. Eliminate any unnecessary salt (especially sodium and chloride) from the cow’s diet and feed to minimize excessive phosphorus and nitrogen excretion.

Also reduce or eliminate other sources of salt such as cleaners or amendments.

19. Apply dry manure at agronomic rates.

Since it is difficult to predict when the organic form nitrogen in dry manure will become available to plants, apply between one-third to one-half of the needed nitrogen as dry manure or lagoon organic nitrogen, and use water-run commercial fertilizer or very low solids lagoon water to supply available nitrogen during periods of peak crop uptake. PD

References omitted due to space but are available upon request by emailing editor@progressivedairy.com .

— Excerpts from Superior Dairy California Review, Vol. 15, No. 4

Marsha Campbell-Mathews

Eric Swenson is in the nutrient management program with the University of California.