Bunching is one of the most visible—and disruptive—behavioral patterns on modern dairies. When cows cluster tightly together in a corner or shaded area, producers often describe it as “cows just standing around for no reason.” But published research shows there are clear reasons, and they’re almost always environmental. Understanding these triggers is essential for improving cow comfort, protecting milk production, and optimizing facility design.
This article summarizes the leading peer‑reviewed research on why dairy cows bunch, what environmental factors drive the behavior, and how dairies can reduce it.
Stable Flies: The Most Documented Cause of Bunching
The strongest body of research identifies stable flies (Stomoxys calcitrans) as the primary driver of bunching. Stable flies deliver painful bites, and cows respond with a suite of defensive behaviors—tail switching, foot stomping, head tossing, and skin twitching. When irritation becomes intense, cows escalate to bunching, a protective grouping behavior.
A 2021 PLOS One study confirms that stable flies are directly associated with bunching and that this behavior reduces grazing and feed consumption, ultimately lowering productivity and welfare.* When comparing milk produced by cows that experienced bunching vs. non-bunching, the average was 0.99 lb. of milk per cow, per day, lost to the bunching behavior.*
A 2023 study in Veterinary Sciences quantified the impact even further, documenting daily milk losses associated with bunching caused by stable flies.**
Why flies trigger bunching
- Cows group tightly to reduce the surface area exposed to biting flies.
- The cows on the outside of the bunch take the brunt of the flies, giving those inside relief.
- Fly pressure peaks in late spring and early summer, especially May and June.***
Environmental factors that increase fly activity include higher temperatures, higher humidity, trees or windbreaks that reduce airflow, and wet or poorly drained areas around feed lanes or loafing spaces.***
Heat Stress: A Major Secondary Driver
Heat stress is another well‑documented cause of bunching. When cows are too hot, they seek out areas that feel cooler or darker—even if those areas are crowded.
A 2024 Journal of Dairy Science study found that bunching increases at higher ambient temperatures, especially in housed cows.****
Why heat triggers bunching
- Cows perceive darker areas as cooler, even if the temperature difference is small.
- Uneven lighting or sunbeams create “hot zones” cows avoid.
- Cows cluster in shaded or darker areas, often leaving large parts of the pen empty.
Heat stress also compounds fly pressure, making bunching even more likely.
Facility Design and Lighting: Hidden Contributors
While flies and heat are the primary causes, facility design can amplify or reduce bunching behavior.
Research and extension publications highlight several design‑related triggers:
- Uneven lighting creates contrast cows avoid.
- East–west barn orientation allows deeper sunlight penetration.
- Open curtain walls can create bright, hot areas.
- Poor airflow increases both heat stress and fly landing rates.
- Dead‑end alleys or corners become natural bunching points.
- Poor ventilation is a documented stressor linked to bunching.*****
These factors don’t cause bunching on their own, but they make cows more sensitive to flies and heat.
What Bunching Costs Producers
Published research shows that bunching has measurable economic and welfare impacts:
- Reduced feed intake due to cows avoiding the bunk
- Lower milk production, documented in peer‑reviewed studies**
- Higher lameness risk from prolonged standing
- Greater heat stress, which compounds production losses
- Increased irritation and stress from stable flies
Bunching is not just an annoyance—it’s a production and welfare issue.
Research‑Supported Strategies to Reduce Bunching
The most effective solutions fall into three categories: fly control, heat abatement, and facility adjustments.
- Fly control
- Improve manure management and drainage
- Reduce wet bedding and organic buildup
- Increase airflow to reduce fly landing rates
- Use targeted fly control systems as part of an overall Integrated Pest Management (IPM) Program
- Manage vegetation and windbreaks that harbor flies
- Heat abatement
- Improve ventilation and airflow
- Add or optimize fans and misters
- Reduce light contrast by adjusting curtains
- Provide uniform shade
- Facility design improvements
- Eliminate bright sunbeams or high‑contrast lighting
- Avoid dead‑end alleys where cows naturally bunch
- Improve cow flow and reduce crowding points
- Enhance ventilation to reduce both heat and fly pressure
Bringing It All Together
Bunching is a behavioral signal that cows are uncomfortable—usually due to flies, heat, or environmental conditions. Published research consistently shows that:
- Stable flies are the leading cause of bunching.
- Heat stress is a major secondary driver.
- Lighting, airflow, and barn design can trigger or worsen the behavior.
- Bunching leads to lost milk, reduced intake, and lower cow comfort.
Understanding the science behind bunching helps producers take targeted action that improves cow welfare, increases production, and reduces stress on both cows and people.
References
*El Ashmawy, W. R., Ferreira, F. C., Williams, D. R., Gerry, A. C., & Aly, S. S. (2023). Daily milk losses associated with dairy cow bunching, cattle’s protective behavior against stable flies (Stomoxys calcitrans) in California. Veterinary Sciences, 12(11).
**Taylor, D. B., et al. (2021).Stable fly activity is associated with dairy management practices and seasonal weather conditions. PLOS One.
***Forbes, L. (2021). Environmental factors seem to influence dairy cow “bunching,” say UC scientists. University of California Agriculture & Natural Resources.
****Rao, T. K. S., Singh, A., Thakor, H. P., et al. (2025). Bunching behaviour: Signals stress in dairy herd. SR Publications.
*****UC Davis & UC Riverside Research Teams. (n.d.). Calculating the economic impact of biting stable flies to California dairies. University of California Division of Agriculture and Natural Resources.
