Hay Tarp Size Guide: Cover the Top, Protect the Stack

Cover the top, leave the sides open

University of Minnesota’s Morris study found that covering bales on gravel cut dry-matter loss from 10.9% to 4.8% over about eight months, a reduction of about 6 percentage points. Oklahoma State’s round-bale fact sheet puts uncovered ground storage at 5–20% dry-matter loss in the first nine months, versus 2–4% for covered bales kept off the ground.

Those gains come from keeping rain and snow off the top, not from sealing the pile. OSU is direct: plastic does not breathe. Hay stored above about 20% moisture under a sealed sheet can heat and mold. UMN’s storage target is below 20% moisture after baling at 15–20%.

The outer layer is most of the weather risk. UMN notes that the outer 4 inches of a 6-foot-diameter round bale hold about 25% of the bale’s volume. That is the band a top cover is trying to keep dry, together with a drained base so the bottom does not wick.

Rectangular stack: measure the top

1. Build or mark the final stack. Measure the top length and the top width, not the ground footprint if the pile tapers.
2. Add overhang on all four sides so drip falls clear of the outer bales. 1 to 3 ft is a common handling range, not a standard.
3. Plan a crown: a ridge board, a row of extra bales, or a slight peak so water runs off instead of sitting in the middle.
4. Choose anchors that will not tear bales apart or create trip hazards. Weight the edges rather than cinching the sheet into a drum.

Worked example: a rectangular stack top measures 20 × 40 ft. With 2 ft of overhang on every side, the planning cover is 24 × 44 ft. If the only stock option is 25 × 45 or 30 × 50, pick the size you can still handle and tension. A sheet that is technically big enough but too heavy to crown will pond in the first storm.

UMN’s rectangular piles in the Morris study were stacked on their 4-foot sides with two extra bales laid over the cracks on top. That top layer is the watershed. A tarp has to cover those cracks and still drip past the outer faces.

Round bales: measure the row and the crown

1. Set the bales how they will sit: a single row end-to-end, or a wider pile. UMN’s outdoor advice for uncovered round bales is end-to-end in lines at least 3 ft apart, not a pyramid. A pyramid traps moisture when there is no cover.
2. Measure row length along the ends, bumper to bumper down the line.
3. Measure crown path with a tape over the top of the bales, from the intended drip line on one side to the drip line on the other. That path is longer than the floor width of the row. Do not use diameter as if the bale were a box.
4. Add end overhang and a little extra at the sides so water clears the equator of the outer bales without wrapping them into a sealed tube.
5. Keep the stack off wet ground (gravel, pallets, tires). Ground contact can account for a large share of the loss; OSU notes that as much as 12 inches of the bottom of a bale can be lost to wicking.

Worked example: six 5-foot-long round bales in one row sit about 30 ft end to end. The tape over a 5-foot-diameter bale from drip line to drip line, stopping near the widest point on each side, is on the order of 8 ft of fabric, not 5 ft. With 2 ft of overhang at each end and a little extra at the sides, you are in 34 × 10 ft planning territory, then round up to a stock sheet you can still pull over the crown. A second row beside the first changes the crown path; measure it.

OSU’s covering-area table is a cost tool (square feet of cover per bale at different stack geometries), not a substitute for a tape on your pile. Use it to think about how stacking changes cover area, then measure.

Which measurement to use

Feed the rectangular numbers into the size calculator’s Hay Stack mode. For a round row, enter the crown path as the width rather than the floor width, or the sheet will come up short over the curve.

Field tips

• Create a crown so water runs off.
• Weight edges; do not cinch the cover tight (OSU).
• Retension after the first rain and wind.
• Inspect chafe where rope crosses bale edges.
• Keep bales off the ground. Covering the top does not fix a wet base.
• Do not assume a tarp makes wet hay safe to store.

Storage and covering: University of Minnesota Extension, “Preserving the value of dry stored hay” (15–20% baling moisture; covering on gravel 4.8% vs 10.9% DM loss; outer 4 in of a 6 ft round bale ≈ 25% of volume; uncovered pyramids trap moisture). Round-bale covers and sealed plastic: Oklahoma State University Extension, “Round Bale Hay Storage” (covered vs exposed loss ranges; plastic does not breathe; do not tie a cover down tight).