The Effect of Short-Wavelength White LED Illumination throughout the Night on the Milk Fatty Acid Profile of High-Yielding Dairy Cows

SIMPLE SUMMARY: Exposure to artificial white light at night (LAN) disrupts circadian rhythms, yet many dairy cows continue to be exposed to LAN for historical and practical reasons. We explored the effect of whole-night illumination, using white light-emitting diode (LED) bulbs, on the production and composition of milk collected during the day (day milk) and night (night milk). Compared to a natural light–dark cycle, white LED reduced the differences in milk composition between milking hours, yet did not affect milk production. Specifically, white LED increased the percentage of saturated fatty acids in day milk at the expense of unsaturated fatty acids. Therefore, we conclude that natural light is preferable to white LED for cows’ well-being and for preserving the natural milk fat profile. Maintaining the natural milk fat profile results in obtaining day milk fat with higher levels of unsaturated fatty acids, mainly oleic acid, which are known for their health-promoting characteristics. In conclusion, reducing the exposure of dairy cows to white LED during the night will reduce energy use in dairy farms, save electricity costs, and yield day milk with a healthier fat composition, and possibly a higher health-promoting market value. The welfare conditions of the cows will also improve. ABSTRACT: Fatty acid levels in milk vary between day and night milking. Many dairy cows are still kept under white light-emitting diode (W-LED) illumination throughout the night, although it is known to disrupt endogenous circadian rhythms. We investigated the effects of whole-night W-LED illumination (125 lux) on milk yield and circadian composition, compared to a natural light–dark (LD) cycle of 10 h light. Mid–late lactation cows (n = 34) that were exposed to natural LD cycle showed circadian variation in milk fat composition, characterized by higher health-promoting monounsaturated fatty acid (MUFA; 24.2 ± 0.4 vs. 23.2 ± 0.4 g/100 g fat, p < 0.001) and lower saturated fatty acid levels (71.2 ± 0.4 vs. 72.5 ± 0.4, p < 0.001) at 13:30 h (day milk) than at 03:30 h (night milk). Compared to natural LD (n = 16), W-LED (n = 18) did not affect milk production or milk fat yields, yet abolished the milking time variation in milk fat composition towards a less healthy fatty acid profile. This lowered MUFA levels of day milk (23.8 ± 0.4 vs. 26.7 ± 0.4, p < 0.01). Therefore, W-LED has no commercial advantage over the tested natural LD cycle, and conversely, even shows circadian disruption. Accordingly, a natural LD cycle of 10 h light is preferable over W-LED from the perspective of cost savings, the cows’ well-being, and preserving the natural milk fat profile, as the nutritional value of the day milk is slightly higher.