Light and Darkness: Managing Photoperiod in Dairy Barns

Lucas González-Chappe and Anaclara Daudet, Graduate Research Associates, Department of Animal Sciences, The Ohio State University

As dairy cows spend more time in indoor housing, their exposure to light is increasingly determined by barn design and management rather than by the natural day–night cycle alone (Winsten et al., 2010; Cloutier et al., 2025). Light exposure has received limited attention despite its potential role regulating circadian rhythms and influencing physiology, metabolism, and behavior in mammals (Dauchy and Blask, 2023). Photoperiod-management programs have been increasingly adopted by dairy operations during both the transition period and lactation (Adamczyk et al., 2024).

Light suppresses melatonin synthesis, whereas darkness stimulates its secretion (Elsabagh et al., 2020). Melatonin therefore serves as an endocrine signal of the environmental light–dark cycle and contributes to the regulation of circadian rhythms, metabolism, physiology, and behavior (Mayo et al., 2017). In dairy cows, melatonin is a key hormone associated with increased antioxidant capacity, improved reproductive function, and enhanced overall health and performance (Afzal et al., 2024). Importantly, melatonin secretion is influenced not only by the duration of light exposure, or photoperiod, but also by light intensity and wavelength (Shen et al., 2026). Consequently, lighting conditions within confined dairy housing systems may alter circulating melatonin concentrations.

Matching the Photoperiod to the Physiological Stage

The effect of photoperiod management for dairy cows is not a new concept. In dairy facilities where lighting can be controlled, evidence supports different photoperiod management strategies depending on the physiological stage of the cow (Dahl et al., 2000). As summarized by Shoemaker (2005) in a previous issue of Buckeye Dairy News, for lactating dairy cows, long-day photoperiods consisting of approximately 16 h of light followed by 8 h of darkness are associated with 5-10 % increases in milk yield. For cows milked three times daily, at least 6 h of uninterrupted darkness was recommended, whereas approximately 8 h was considered optimal for cows milked twice daily.

Dry cows, in contrast, may benefit from a short-day photoperiod of approximately 8 h of light followed by 16 h of darkness (Miller et al., 2000). Short-day photoperiods during the dry period may support mammary development and cellular immune function. Moreover, cows exposed to short-day photoperiod have produced 6.83 lb/day more milk during the subsequent lactation (Dahl et al., 2000; Miller et al., 2000). According to Dahl et al. (2000), managing lighting during the dry period offers an opportunity to producers who find lighting programs impractical during lactation. According to a relatively recent study (McCabe et al., 2021), another important factor of lighting is consistency. These authors found that exposing cows to chronic light–dark phase shifts during late gestation negatively affected mammary gland development and added to insulin resistance development, which highlights the importance of keeping consistency in the lighting programming.

Light Intensity: Are the Cows Receiving Enough Light?

Beyond photoperiod, lighting is also characterized by its intensity and spectrum, both of which may influence cow physiology and productivity. Light intensity is commonly expressed as lux or foot-candles (fc). Lux describes the amount of light reaching a surface, with 1 lux equal to 1 lumen per square meter. In contrast, lumens describe the total amount of light emitted by a fixture, whereas watts describe the electrical power consumed. Because different lighting technologies vary in efficiency, wattage alone does not indicate how much light a fixture produces. When comparing fixtures, producers should focus on total lumen output and luminous efficacy (lumens per watt), rather than wattage alone (Table 1).

So, how can we determine whether our cows are receiving enough light? A reliable way to evaluate a lighting program is with a handheld light meter (Figure 1). Measurements should be taken at cow-eye level in feed alleys, stalls, holding areas, and other locations where cows spend time. Recording the minimum and maximum values helps identify dark areas that may reduce the effectiveness of the lighting program.

Figure 1. Measuring light intensity at cow-eye level using a handheld light meter in a freestall dairy barn. Photos were taken at the Krauss Dairy Center (Wooster, OH) on July 26 at 7:30 PM while the barn lighting system was operating. The barn has a north–south orientation, and natural daylight was still present.

For long-day photoperiod programs, a minimum of approximately 100 lux (9.3 fc) at cow-eye level has traditionally been recommended. However, designing for approximately 150 to 200 lux (14.0 to 18.6 fc) provides a practical safety margin for uneven light distribution, fixture aging, and dust accumulation (Janni, 2002; Lim et al., 2021). Importantly, the average barn illumination is often less informative than the darkest locations. Even if the average light intensity meets recommendations, poorly illuminated areas may reduce the effectiveness of the lighting program.

Table 1. Typical characteristics and practical considerations of common light sources used in dairy facilities.

Note: lm/W = lumens per watt. Values are approximate and vary among products. Fixtures should also be evaluated for light distribution, moisture and corrosion resistance, operating temperature, warranty, and compatibility with controls.

Does Light Color Matter?

Cattle perceive light differently from humans because species vary in the number and sensitivity of retinal photoreceptors (Li et al., 2025). Cattle have dichromatic vision and can distinguish long-wavelength red light from shorter-wavelength blue or green light (Phillips and Lomas, 2001). Therefore, red light is visible to cattle and should not automatically be considered equivalent to darkness. However, red light appears to stimulate some biological responses less strongly than blue or white light. In dairy cows, increasing blue- or white-light intensity caused greater pupillary constriction, whereas red light produced little or no additional pupillary response as intensity increased (Lindkvist et al., 2021). This supports the traditional use of dim red lighting for nighttime observation and animal handling.

For practical purposes, red light should be used at the lowest intensity needed for employees to work safely and only for the time required to complete nighttime tasks. A recent study compared cows exposed at night to darkness, red light, or yellow light at 350–500 lux. Red light produced responses similar to darkness for milk melatonin concentration, milk yield, and several milk-quality measurements (Shen et al., 2026). In contrast, yellow light increased milk melatonin concentration and milk yield and was associated with greater antioxidant capacity and lower concentrations of the inflammatory indicators TNF-α and IL-6 (Shen et al., 2026). These results suggest that different wavelengths may produce distinct physiological responses. However, the findings for yellow light should be interpreted cautiously because evidence remains limited. More research is needed to make recommendations for commercial dairy farms.

Putting it into Practice: Equipment and Consistency

Producers interested in adopting or refining a photoperiod program should think about three practical factors

1. Measure before you manage. Use a basic light meter, or a smartphone lux-meter app, held at cow-eye level in the resting and feeding areas.
2. Automate. Timers or programmable controllers help maintain the same on/off schedule every day which maintains consistency.
3. Match the light fixture to the purpose. LED fixtures allow more precise control of intensity, and in some systems of spectrum, and are more energy-efficiency than legacy fluorescent or incandescent fixtures. For nighttime handling in prepartum and maternity pens, a dedicated, dimmable red-light circuit, separate from the main barn lighting, makes it easier to keep the general dark period while still allowing staff to work when calving activity requires it.

Wrapping Up: What Should Dairy Producers do Today?

Effective light management involves not only how many hours the lights are on, but also when, how brightly, and with what type of light cows are exposed (Figure 2). Understanding these different dimensions of light exposure may help producers use lighting more strategically.

Figure 2. Key recommendations for lighting programs in dairy barns.

Acknowledgement

We thank Dr. Jason Hartschuh for reviewing this article and for sharing his valuable insights on the topic.

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