Weather Variability and Storage Capacity: Are You Designed for the Year You Actually Get?

 

We don’t farm averages. We farm variability. Livestock produce manure every day, mostly predictably. Weather adds water whenever it wants. Storage has to handle both. If we want fall application into cooling soils to remain an agronomic decision, not a reaction to storage pressure, we need to run the math now, not in September.

In our Spring Storage Planning article, we talked about the concept. Here, I want to punch some numbers and run the math.

Start with manure production.

How many head do you have? How many gallons per head per day are you generating? How many days until November 1?

Swine Finishing

For finishing swine in a deep pit, this is relatively predictable. Multiply head count by gallons per head per day and by days to your desired fall window, then divide by the surface area of your pit. Finishing pigs in a wean-finish barn, relatively tight in terms of water use, are often around 1 gallon per pig per day (your mileage may vary — I see barns from 0.85 up to 1.4 gallons per head per day). As I write this on February 23, there are 251 days until November 1. That’s roughly 300,000 gallons of manure.  doesn’t feel like much until you convert it to inches of pit depth. Have a 50x 190 barn holding 1200 head, that’s 51 inches of manure. Do you have that in your barn?

Liquid Beef

For liquid beef deep pit systems, it’s the same kind of math, just change the manure production number. Deep pit beef barn, put your manure production at around 6.5 gallons per head per day. That’s about 7.3 feet of storage space needed. But now we introduce uncertainty.

Are roof downspouts tied into the system? Over the next 251 days we average about 30 inches of rain. If about 80% of this turns into runoff and you are catching water in the pit from half your downspouts, that’s another foot of rainwater added. Do you have 8 feet of usable space so you can make it to your fall application window?

Liquid Dairy

For dairy, you add layers. Manure volume. Parlor wash water. Loafing lot runoff. Silage bunker runoff and leachate. What is the shape of the manure storage?

Let’s start around 22 gallons per head per day for manure and generated wash water. Looking at the ISU dairy — no loafing area outside, so I’m in luck. Silage bunker runoff is directed through vegetative filter strips. Yes, I chose this farm to make it easier.

With around 400 cows, I need roughly 18 feet of slurry storage space in our tank to get to November, plus whatever rainfall accumulates on the surface, maybe close to zero in a dry year, maybe several inches or more in a wet one.

But what if we were also handling runoff from the silage bunker area? That’s around 40,000 square feet. If roughly 80% of rainfall becomes runoff, I’d need close to another 5 feet of storage space in our manure storage.

If your projected level in September or October approaches your limit, the fall window is already compromised. Planning now can save some headaches latter.

Is Spring a Strategic Drawdown?

If the math is tight, what is the plan? Is our best approach to move some manure this spring to protect our fall window? If you do get full before November, what will you do? Do you have acres available? Are you confident harvest will start early? Will cover crops be established in time to receive early fall manure and protect water quality? This is not an argument that everyone should switch to spring application. It is an argument that some operations may need to use spring strategically to protect fall.

Fall application can work very well, if we actually reach cooling soil temperatures. That only happens when storage capacity gives us the ability to wait. Run the numbers. Then decide intentionally.