Flood wash systems for wintering barns. Specs, separation and lessons from a 900-cow Southland open day

Southland winters are hard on cows, paddocks and people. At Jersey Girl in Northope, near Winton, Leanne, Jason and Toby Erb winter 900 Jersey cows in two rubber matted barns. Each barn flood washes clean in under five minutes.

On 8 October 2026, Numat and DairyNZ ran an open day on the farm. More than 100 farmers came to walk the barns, watch a flood wash go, and ask the Erbs what they would tell anyone planning one.

This page sets out how the system works, what it is built from, what it took to get there, and what the farmers and DairyNZ said on the day. Where DairyNZ research or good practice guidance applies, it sits alongside the Jersey Girl numbers, so you can see how a working build compares.

Written by Milan Renton, Off-Paddock Facilities Specialist, Numat. DairyNZ sections reviewed by Nathan Nelson, DairyNZ. Based on the open day recording, figures supplied by the Erbs, Numat project records and published DairyNZ research, all referenced at the end of this page.

Key Takeaways

Water volume and fall work together. At Jersey Girl, 60,000 litres per flush on a 1.6% fall cleans a 90 m barn in under five minutes.

Separation decides how clean the barn stays. In the first season the flood wash did not work as hoped, because solids were not being separated from the greenwash. Separation is the key to success: if the flood wash is not working, cows will not lie down and can become lame.

Surface softness drives lying time. In DairyNZ research, surfaces with a Gmax reading above 140 dropped cows to about 8 hours of lying a day, the welfare minimum.

The switch from composting roughly doubled capacity, from about 460 cows to the whole herd, and ended about $130,000 a year spent bringing in bedding and carting it out.

Plan 12 to 18 months ahead. DairyNZ's case study farmers nearly all hit delays from consents, weather or construction.

Inside a 900-cow flood wash barn: open day highlights

Jersey Girl at a glance

Why cows come off the paddock in winter

Lying time is the main measure of cow comfort. DairyNZ guidance is that housed cows need 10 to 12 hours a day lying down, and a minimum of 8.

On a wet winter crop paddock, cows often cannot get there. A study of cows wintered on kale and fodder beet, run by AgResearch and DairyNZ and co-authored by Dawn Dalley, tracked lying time over 32 days:

Source: Neave et al. (2022), Journal of Dairy Science, an observational study of four groups of 99 dry, pregnant cows on one farm. Lying time fell as paddock conditions got worse, especially as surface water pooled. The Erbs saw the same problem on their own wet farm.

How the Jersey Girl flood wash works

A flood wash cleans a barn floor with a wave of water instead of a scraper or a tractor. Stored greenwash is released at the top of the barn all at once and runs the full length of the floor, carrying manure off the mats and into a effluent channel at the bottom end.

At Jersey Girl that wave is 60,000 litres per barn. The floor falls at 1.6 per cent, the water reaches the bottom in under five minutes, and the barn is clean. The Erbs flood wash one to three times a day, and more when it is needed.

From the catch system, the greenwash passes a weeping wall that holds back solids, and goes back into the effluent system to be used again. The same water cleans the barn over and over, which is why separation, covered below, matters so much.

Numat designed the fall, the tanks and the catch system, and laid the mats.

Flood wash specification

DairyNZ good practice compared with Jersey Girl

Jersey Girl runs below DairyNZ's 2% guideline and still cleans in under five minutes. That is DairyNZ's own point: volume and slope work together, so a lower fall needs enough water behind it. DairyNZ also notes that dung left too long turns sticky and one flush may not shift it.

Three winters of composting first

These barns did not start life as flood wash barns. Jersey Girl is a wet farm, and cow comfort was the priority. The Erbs looked at herd homes, composting and free stalls, then built composting barns. They were specced for 660 cows at about 7 m² per cow, on deep sawdust bedding packs. Much of the infrastructure at Jersey Girl comes from converting those composting barns, not from what a flood wash itself needs.

The pack worked for the first few weeks of each winter. Once the cold and wet set in, the pack turned to soup. They could only fit about 230 cows per barn, and in the wettest spring the tractor got stuck trying to rip the pack. After three winters, they decided composting did not fit their farm.

The bedding bill made the decision easier.

Composting compared with flood wash at Jersey Girl

The rebuild

The turning point was a visit to another farm.

Converting the barns was not a light job. The concrete they had laid the year before was jackhammered up and used as fill. The site was then relevelled to give the floor more than a metre of fall, which is why the poles look shorter at the top end of each barn.

What the conversion involved

The payoff was capacity. The same two barns that held about 460 cows on compost now winter the whole herd.

Separation is what makes it work

A flood wash reuses its own water, so the quality of that water decides how clean the barn gets. If solids are not separated out, they come back down the barn on the next flush. The Erbs learned this in their first season.

The Erbs worked with Dairy Green on effluent storage.

DairyNZ guidance matches what the Erbs found. Where effluent is not irrigated to land regularly, DairyNZ strongly recommends solids separation, with mechanical separators generally preferred. Weeping walls can work, but need multiple beds and deeper bunkers to cope with the thicker effluent from off-paddock systems. Rain and yard wash water should go into storage to dilute the greenwash, and DairyNZ case study farmers linked their barn effluent to the cowshed system for the same reason.

How much effluent housed cows produce

Source: DairyNZ Dairy Cow Housing guide. Size storage with the DairyNZ Dairy Effluent Storage Calculator (DESC), and have the final numbers checked by an accredited effluent system designer.

Running the barns day to day

Feeding

A centre lane runs between the two barns, so the mixer wagon feeds both sides from one pass. The feed lane is deliberately left without a roof.

Feed gets wet, but if no more is offered than the cows eat in 24 hours, they clean it up.

In barns the risk is cows getting too fat rather than too light. Last winter the herd went in on ad lib grass silage for the first month, then the Erbs added hay made on the farm to bulk the diet out. They offer about 10 kg per cow, shifting to more hay later in the season.

Running mobs

Gates split each barn into bays, so different mobs can run on different diets. During calving, the colostrum mob comes in every night for feed with magnesium and lime flour mixed through. The partitions mean the barns can do more than one job at once.

Animal health

Calving

Cows do not calve on the mats at Jersey Girl. The farmer whose flood wash barn the Erbs visited advised against it. Springers move to a separate calving barn on a transition mix, which the Erbs keep to under about 200 cows.

That lines up with DairyNZ guidance. The Dairy Cattle Code of Welfare requires clean, dry bedding for cows calving on hard surfaces, and DairyNZ's good practice guide goes further: all hard surfaces, including rubber matting, should have a layer of bedding such as straw or woodchip on top for calving. DairyNZ recommends at least 10 m² per cow in calving areas, with cows moved there at least two weeks before calving.

On hoof health, DairyNZ notes that digital dermatitis is not common in New Zealand, and that continuous exposure to effluent raises the risk. That is why the cleaning and separation results above matter as much as the surface itself.

The surface: Siesta rubber mats

At Jersey Girl, the floor is 2,363 Siesta mats covering 4,687.6 m².

Jacob Judd from Numat explained where the mat comes from. Numat partnered with a German company about 20 years ago and spent time in Europe learning how cubicle barns deliver cow comfort. The product has been used in Europe for two decades. Numat took a version of it and adapted it for open loafing areas like these barns.

Siesta specification

Full specifications are on the Siesta product page.

What DairyNZ research says about lying surfaces

Dawn Dalley, DairyNZ Senior Manager, Farm Systems Science, put the barn in context. If cows are meant to lie down comfortably, the surface has to compress under them.

Four or five years ago, DairyNZ looked for alternatives to woodchip and sawdust for loafing areas. Part of the reason was supply: if more farms move to composting, there may not be enough organic material to go round, and the cost will follow. DairyNZ teamed up with Numat to test recycled rubber in a trial at DairyNZ's Lye Farm in Hamilton, looking at performance, cleaning, and whether softness changed how long cows lay down.

Measuring softness took some work. The old method was the knee drop test: stand over the surface, drop onto your knees and judge how much it hurt. It depended on how tall you were and how good your knees were. DairyNZ moved to a Clegg Hammer, the device used to test compaction on cricket pitches. It gives a Gmax reading, and the lower the reading, the softer the surface.

DairyNZ then measured rubber types, paddocks, sawdust and straw, and ran the trial at Lye Farm, its Hamilton research farm. Cows wore devices that recorded lying time over a couple of weeks.

That is about two hours a day of lying time linked to how soft the floor is. The sector now has guidelines on the compressibility a loafing surface needs. DairyNZ also tested traction, which proved harder to measure than softness.

The DairyNZ surface trial

The thresholds come from a trial at DairyNZ's Lye Farm in Hamilton, published by Beukes, Dela Rue, Dalley and McCall in 2022. Four loafing surfaces were compared:

The interlocking yard matting was the firmest and also the most slippery of the four. A yard mat is not a loafing mat. DairyNZ found that comfort in rubber matting depends mostly on density, thickness and the structure under the mat, which lets it deform rather than just compress. Siesta is built as a loafing mat with that in mind: 30 mm thick, with a waffle-shaped, air-cushioned underside.

Space matters as much as surface. In an earlier DairyNZ study across Southland wintering systems, lying time in loose-housed systems came down to square metres per cow. Crowding means competition for lying space, and the younger, more submissive cows lose out. Numat's off-paddock systems comparison and investment guide puts a covered, rubber-matted loafing pad at 3.5 to 5 m² per cow. DairyNZ's housing guide, written for loose-housed wintering in general, gives 9 to 11 m² per adult cow, or 6 to 8 m² where cows also spend time on the paddock each day. Jersey Girl works out at about 5.2 m² per cow. That figure is the matted loafing area only: 4,687.6 m² of Siesta mats shared across the 900 cows wintered, not counting the centre feed lane, and the barns are used for 6 to 10 weeks of winter loafing rather than full-time housing. Anyone planning a barn should size it for their own system.

Numat now owns a Clegg Hammer and uses it to test the rubber products it brings in.

Planning a flood wash barn: lessons from DairyNZ

DairyNZ partnered with six farmers who had built off-paddock infrastructure and documented how they did it. Dawn's main concern after the wet spring of 2026 is a knee-jerk reaction: farmers deciding they need infrastructure and buying the first thing off the shelf. The farmers who built well took their time.

What the case study farmers did

DairyNZ's published case studies add detail. Start planning 12 to 18 months ahead to manage consents, weather and construction delays. Case study farmers found a 10% budget contingency helpful, and kept extra feed on hand in case the build ran late. DairyNZ's housing guide warns that ancillary costs such as effluent, labour and machinery can add 30% to 100% to the build cost. The DairyNZ off-paddock infrastructure investment checklist works through each step. Winter weather will not wait. Start planning your flood wash barn with Numat today.

Consents

Three consent points came up on the day that are easy to miss.

DairyNZ's case study farmers found consenting a bigger task than expected, and some wished they had consented for more cows. Do not lodge until you know exactly what you want to build, or you may pay to change the consent later.

Roof or no roof

Open-air rubber matted pads are now going in across Southland. Whether to roof one depends on winter rainfall. Leaving the roof off often means building more effluent storage instead.

At Jersey Girl, the loafing areas are roofed and the centre feed lane is open. The cows stay dry where they lie, and no money goes on covering a lane they do not stand in.

Was it worth it

Nathan asked the Erbs the question every farmer in the crowd was thinking. They did not dress up the cost.

Standing in front of more than 100 farmers, three winters after the switch, Leanne summed it up: "It's a lot of money, but honestly, for us, it's been the best decision we've made for the farm."

The gains they described were practical. Production has gone up and become more consistent, because the cows can come off the paddocks before they do damage.

And there is the part that does not show up in a budget.

The barns are also about the next generation. Toby put the original proposal together, and the Erbs were clear he was the reason they went ahead.

If they started again

A farmer asked what they would build from a clean slate. The answer was honest.

The Erbs own another farm that came with a free stall wintering barn. They have not run it themselves, but see cubicles as keeping cows cleaner, because the cows lie on a raised bed rather than on the floor that gets washed. Either way, the flood wash and the matting stay.

The barns also have to fit their farm system. The Erbs run low input, use no urea, and do not use the barns to stretch the season at either end.

Watch the full Q&A

Every question from the day, unfiltered, with the Erbs, Nathan Nelson, Dawn Dalley and the Numat team.

What farmers ask about flood wash systems

Thank you

To Leanne, Jason and Toby Erb, thank you for opening your gates, your barns and your numbers, and for being so honest about what worked and what did not.

To the DairyNZ team, Nathan Nelson, Dawn Dalley, Jeana Close and Nic Blair, thank you for running the day with us and for bringing the research that puts a build like this in context.

And to every farmer who came along, thank you for the questions, the yarns and the laughs. You made it a great day.

References

Research and guidance
• Dawn Dalley, DairyNZ. Presentation at the Jersey Girl open day, 8 October 2026: lying surfaces, the Clegg Hammer and Gmax, and planning lessons from DairyNZ's off-paddock case studies.
• Beukes, P.C., Dela Rue, B., Dalley, D.E. and McCall, D. (2022). Evaluating loafing surfaces for off-paddock facilities on New Zealand dairy farms. New Zealand Journal of Animal Science and Production, 82, 22–27. nzsap.org/node/4136
• Neave, H.W., Schütz, K.E. and Dalley, D.E. (2022). Behavior of dairy cows managed outdoors in winter: Effects of weather and paddock soil conditions. Journal of Dairy Science, 105(10), 8298–8315. AgResearch and DairyNZ. doi.org/10.3168/jds.2022-21819
• DairyNZ (2025). Dairy cow housing: a good practice guide for dairy cow housing in New Zealand, version 3. Download the guide
• DairyNZ. Artificial bedding surfaces.
• DairyNZ. Off paddock infrastructure insights: case studies on planning, roofing, effluent, bedding, consents and feeding.
• DairyNZ. Off paddock infrastructure investment checklist.
• DairyNZ. Housed cow comfort.
• DairyNZ (2026). Composting Barn guide: a practical guide to planning, design and management for NZ dairy farmers. Download the guide.

Product data
• Numat, Siesta product specification.

Numat project records
• Jersey Girl, Northope, Southland. Flood wash and Siesta rubber matting, completed 30 May 2024.
• Wintering and feed barn with flood wash and rubber mats, Jersey Girl, 2025.
• Jersey Girl case study, Northope, Southland.