Vacuuming manure slurry cleans lanes and has useful benefits. In flush lane systems, manure on concrete is collected via flushing and creates a dilute waste stream. Use of a vacuum or scraper before the lane is washed maintains manure in a slurry form (Figure 1).
This action reduces the amount of manure entering anaerobic lagoons and reduces subsequent organic carbon converted to methane, a powerful greenhouse gas. The California Dairy Quality Assurance Program estimated methane reductions of 24% and 18% in freestall and open lot housing, respectively, are possible when converting from a flush to a scrape or vacuum manure removal system.
Removing manure from animal housing before it reaches the lagoon can also improve the functional capacity of the lagoon by slowing down accumulation of solids that need to be excavated.
Collecting manure before it enters the lagoon presents an opportunity to use those solids and their nutrients where it is more sustainable – even if that means away from the dairy farm. This last prospect provides several benefits to the dairy farmer but also raises formidable challenges.
Dairy farmers are collecting a valuable product when manure is diverted from the lagoon by vacuuming. The manure collected has nutrients and organic matter that might be sold to other farmers. The slurry form is more easily transported to fields at a distance than liquid manure.
Exporting nutrients off-farm may improve dairy nutrient balances, farm soil and plant health as well as make compliance with regulatory water quality objectives easier. However, there may be logistical challenges to address to make this system work beneficially.
The cost of manure vacuum equipment is approximately $250,000. Additionally, trained staff are needed to perform regular vacuuming and maintain the safe and effective operation of the machine. The vacuumed area must also have concrete strong enough to withstand the weight of the filled vacuum. Once the manure is vacuumed, handling the wet slurry requires enough warm space to dry it and alternative storage for when it is cold and wet outdoors. At times, land will be available to receive slurry application.
Alternatively, manure will probably need to be handled multiple times after vacuuming to convert it to a form easily trucked off the farm. Finally, when manure is manifested off-site, it requires land available for application and preferably an arrangement which monetizes its value. We have seen some creative ways of adding value to the vacuumed manure on-farm, such as composting it with separated solids, which can make it more attractive to buyers (Figure 2).
Monetizing manure
What is the fertilizer value of vacuumed manure? That depends on who you ask. Consider the plant’s needs for healthy growth and development. We assume the farmer wants a hearty yield and will provide the plant with all it needs. We can examine the vacuumed manure fertilizer value based on a realistic forage corn yield. Table 1 demonstrates some physical and chemical properties of manure vacuumed from four dairies in the San Joaquin Valley of California. Notice the total nitrogen (N), phosphate (P2O5) and potash (K2O) concentrations of 0.42%, 0.24% and 0.47%, respectively.
If we express these values as a ratio such that the phosphate is set to equal 1.0, we can see the proportions of these three nutrients is 1.7-1.0-1.9. That is, for every pound of phosphate, this material contains 1.7 and 1.9 pounds of nitrogen and potash, respectively. This ratio of macronutrients also happens to be similar to that of forage corn seasonal macronutrient uptake, which is 2.3-1.0-2.3. For every pound of phosphate corn removes from the soil in a season, it also removes 2.3 pounds of nitrogen and potash. Notice that the proportional nitrogen and potash requirements of a forage corn crop are higher than what is found in the vacuumed manure when compared on a phosphate basis.
Note, however, that during drying the nitrogen content is subject to some loss due to volatilization, or gassing off, as ammonia. This suggests that the vacuumed manure can be dried in order to be applied as a fertilizing material for corn silage based on meeting the crop’s phosphate need without over-fertilizing with nitrogen or potash. That is good for corn, but what about other crops besides forages, perhaps off the dairy?
Moving manure
Getting high-water content manure like vacuum solids to something as manageable as compost or another dried form is necessary to have viable transportation costs. For our purpose, we can imagine we have a finished compost product with a similar nitrogen (N)-phosphate (P)-potash (K) ratio as our vacuumed manure. That is, if we had a finished compost that was 40% moisture, our N-P-K would be 1.4-0.8-1.6. A ton of this compost would deliver 28 pounds, 16 pounds and 32 pounds of nitrogen, phosphate and potash, respectively. With current fertilizer prices costing at least 90 cents per pound of those nutrients, depending on the formulation, a fair price on the same nutrient balance as our ton of compost would be $68. Currently, a ton of compost is going for approximately $40.
We will not price freight here because it depends on the mode, such as rail versus flatbed, weight or premiums paid for consideration of what is being hauled. It is possible a long enough distance or a high enough price for freight would make the compost formulation more expensive than the fertilizers delivered to the farm. That said, we point you back to Table 1 to consider the added benefits of secondary macronutrients – sulfur, calcium and magnesium – and carbon, which are not accounted for in our cost equation. These additional nutrients would increase the actual fertilizer value of our hypothetical compost. Yet, we caution non-dairy forage farmers to consider salts and micronutrient levels in dairy manure products, as crops other than forages might be relatively more sensitive to salinity or trace metal toxicity.
It is also important to consider that, in general, the grand majority of nitrogen in dairy manure compost is in the organic form, which means it is not immediately plant-available. The organic nitrogen must first be decomposed by microbes before it is plant-available. Even though plants cannot distinguish the source of essential nutrients, be it compost or synthetic fertilizer, the two sources will behave differently in the soil before the plant benefits from either. Thus, dairy manure, like vacuumed slurry or a dried product made from it, should not be considered as a one-for-one replacement with synthetic formulations of fertilizer.
Vacuuming manure from freestall dairy housing is a viable solution for diverting solids from entering lagoons, reducing methane emissions and maintaining the functional capacity of the lagoon. However, there are significant costs and operational – and possibly infrastructural – changes for a dairy operator to understand and consider before switching from flush to vacuum. One major challenge is year-round slurry management. We proposed drying or composting as relevant solutions since drier materials might be transported further where the nutrients in the manure could be used by non-dairy forage farmers. This solution could satisfy local water quality protection objectives by reducing the land application rates of nutrients around dairies. Other farmers might also be happy to purchase a source of nutrients that saves them money when synthetic fertilizer prices or supply are not cost-effective.
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