Managing Dairy Slurry to Maximize Nutrient Capture


Manure slurry is a valuable but difficult resource to manage on dairy farms.  Slurry pits must be emptied to make room for the never-ending stream of manure.  Manure is often not a top priority for most dairies and handling may have to wait until seasonal fieldwork is completed.  That leaves farmers with a fall and early winter window to empty the pits and get manure applied. This is crucial step as there must be adequate pit capacity for the coming winter.  Some farms have limited acres to apply manure and feel forced to apply at maximum regulatory rates to get the pits emptied.  With the current high cost of synthetic fertilizers, and even supply chain disruptions, managing the manure resource properly becomes a critical farm task.

Inherent Problems
Even though we can see tremendous crop yields with high rates of slurry, this comes with some soil health issues, which may include soil surface crusting, collapsed soil aggregates, impaired  water infiltration rates, mortality of soil invertebrates, and excessive soil phosphorus levels.  A shovel test will likely reveal compacted soil with plated layers because the application of slurry manure is a distinctive disruption to the soil microbiome.  The anaerobic state of slurry applied at high rates overwhelms native soil biology and the system (including soil aggregation) can collapse.

Soil microbiology that supports productive soil function thrives best in an aerobic environment.  Gaseous exchange with the atmosphere, which is essential for nutrient cycling, only functions well with good soil aggregation.  While some disruption is natural and can be beneficial in stimulating soil microbiology, a large surge of a foreign substance (i.e., anaerobic pit slurry) overwhelms and damages the soil microbial community and ultimately the soil aggregate structure.

PFLA analysis (a biological analysis of the soil) of fields that have had frequent and heavy applications of slurry often reveal poor fungal-to-bacterial ratios. The soil becomes bacteria dominated with bacterial counts far exceeding those of conventional row crop fields.

Manure is the primary fertility source in organic farming systems.  In the past, manure was always viewed as a “good thing” unless the farm surpassed regulatory limits.  Conventional wisdom asks, “What can go wrong with adding organic matter and natural fertilizer?”  And, “If a little is good, more should be better, right?”

The truth is, frequent application of heavy rates of any fertilizer facilitates a lazy soil microbe-soil-plant system.  This is called negative feedback, just like in the physiology of the cow.  The plants are getting all the nutrients they need through the manure applications and make little-to-no demands on the soil microbiology.  This causes the soil microbiology to go dormant.

Additionally, slugging soil with high rates of slurry manure stimulates weed growth.  The two biggest factors influencing weed growth are tillage and heavy doses of nitrogen.  Most plants we call weeds love nitrogen.

Mitigating Problems
So how do we avoid these issues?  First, rememeber that “moderation is best.”   Manure that is surfaced applied in moderate amounts to a living plant is immediately available for plant utilization.  Under this system, incorporation is not necessary and minimizes additional disturbance.  In many states this is allowed, but most farms have not taken advantage of this option.

Application during the late fall, winter or early spring to a field void of living plants requires tillage or injection to incorporate the slurry.  Regulations require minimizing volatilization and runoff loss.  Incorporation also helps to minimize odor that might be offensive to neighbors.

In line with the regenerative principle of minimal disturbance, the consultants at Understanding Ag work with farms to identify opportunities to apply manure that minimize soil disturbance, meet manure and water quality regulations, provide agronomic benefits and protect feed quality for livestock.

Practices we have seen work well include:

  • Keeping slurry application rates to 3000-4000 gal/acre/application.
  • Applying to living plants at least 4” tall during the growing season.

Application at these rates minimizes the risk of “burning” the crop.

Opportunities to apply include:

  • Applying after a hay cutting (with a 4” residual height).
  • Applying to cereal rye or triticale in the fall or spring.
  • Applying to a summer annual once it is up and growing.
  • Applying after a summer annual harvest (single cut scenario) that is then planted with annual ryegrass intended for a harvest later in the fall or next spring.
  • Applying to Italian ryegrass forage after each harvest.
  • Applying to perennial pasture after a high-stock-density graze.

For those who hire out their manure hauling, many of these options fall outside peak service demand.  This can be advantageous in that haulers may be looking for work at those times.

Side-dressing with lower rates to young corn is also possible with tools such as the Yetter Avenger.  Research in Minnesota and Wisconsin has demonstrated that a drag hose can be pulled over corn before the V4 stage without any significant loss in yield; as the corn growth point is still below the soil surface.

Grazing Options
Another option is incorporating livestock into your cropping systems (a key principle of regenerative agriculture).  This does not mean every animal needs to be out on pasture every day of the year.  Far from it.  Positive impacts can be made with just one or two adaptive grazing events per year on a given acre.

It may be logistically difficult to graze your milking herd, but what about heifers and dry cows?  Every day they are out on pasture or grazing cover crops is one less day of manure to handle.  Note that we are talking about pasture that has plenty of forage biomass to graze and not a dry lot pasture where you are feeding.

The manure that drops straight out of the heifer or cow is very different from slurry pit manure, or bed pack manure.  It is immediately symbiotic with the native soil microbes and is far more readily incorporated.  We capture a significantly higher percentage of the nutrients from their manure.  Slurry from a pit, or bed pack manure, can lose 50-90% of the original nitrogen (N) and up to 100% of the potassium (K) once the manure (including urine) leaves the heifer before we get it spread on the field.  With active grazing we lose very little of the K and only around 5-15% of the N, even when the ground is frozen.

In addition to adaptively grazing perennial pastures, hay fields in their last year of production prior to rotating to an annual crop, present an opportunity.  Grazing in lieu of taking the last cutting on a hay field provides significant fertility benefits.

We can graze cereal rye or triticale in the spring in lieu of mechanical harvest.  We can graze cover crops in a variety of forms, glean crop residue, or implement one of the many means of out-wintering.

Each of these methods can reduce the amount of manure we need to handle while capturing the greatest amount of nutrient value in our fields without the cost and labor of hauling.  If we manage livestock integration well, manure distribution can rival mechanical application.  With the rising cost of labor, machinery, parts, and fuel, anything we as managers can do to get the animals out on the landscape working for us helps the bottom line.

Making substantial adjustments on the farm requires changing how we SEE things.  When we see manure as an important and valuable resource, and work to maximize the benefits of this resource, we will adjust how we manage it.

Timing is a critical component of profitable agriculture.  Timing moderate slurry manure applications to living plants as much as possible during the growing season can maximize this resource while protecting soil biology.  Identifying new opportunities for slurry application helps minimize slugging the system simply to get the pit emptied.  Combining practices allows us to take advantage of the best opportunities to maximize crop production while keeping fertility, machinery and weed control cost to a minimum.

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