It pays to keep your cows in the dark


Imagine a couple thousand research and extension types from all over the world in one place. No, not the plot for the latest horror movie, but a description of the Cincinnati Convention Center last week.
The Joint Annual Meetings of the American Dairy Science and American Animal Science Associations were “local” this year.
The purpose of the meetings is to share current research results and extension programs through oral presentations or written presentations (posters). There were 700+ of each.
Three categories. Research projects and extension programs range from what I consider: 1) Really neat (i.e. things that we are ready to apply at the farm or processing level); 2) Very basic research (background work for things that may eventually be applicable to production or processing); to 3) “Duh” (things we really didn’t need a research project to verify.)
Honestly, the first two days were a little slow in the “really neat” category. Fortunately days three and four kicked it up a gear.
For more than 20 years now, research has been tweaking the use of controlled lighting to increase milk production in dairy cattle. Initially the work focused on lactating cows.
Light and dark. Study after study have shown an increase in milk production when cows receive 16 hours of light alternating with eight hours of dark. Generally, milk yields increase 8 percent to 10 percent over the production of cows receiving shorter or longer light periods.
More recently, work has shifted to look at the impact of lighting (photoperiod) effects on dry cows.
Geoff Dahl, dairy scientist from Illinois found that a shorter light period for dry cows increased milk production in the following lactation. Cows exposed to eight hours of light and 16 hours of dark during the whole dry period responded with increased milk production in the following lactation.
This response was seen in cows that received long-day light or ambient (existing daylight) treatments after calving.
Heifers. What about heifers? Heifers showed the same response if they were exposed to short day photoperiods for 60 days before calving.
A study looking at short day photoperiods for the last 21 days of the dry or prefresh period did not show the same increased milk production response.
So, what causes this response? Researchers believe that greater mammary growth, improved immune response and increased dry matter intake is related to hormonal changes.
Prolactin levels circulating in the blood decrease and sensitivity increases in dry cows exposed to short photo periods compared to cows exposed to long periods of light.
Application. How practical is limiting light to eight hours a day? Shouldn’t be hard in the dead of winter, but can be challenging, if not impossible, in many facilities the rest of the year.
Dry cows stuck in a dingy corner of the old bank barn may be getting plenty of dark. How much light and air they get is usually questionable!
The dark must be contrasted with eight hours of light measuring 15 to 20 footcandles 3 to 4 feet off of the ground.
This is actually pretty darned bright. A few incandescent bulbs will not do the trick. When considering light sources, we need to be looking at energy efficient alternatives. Flourescent, metal halide or mercury, for example.
Tunnel ventilated barns are actually the most likely candidates for providing short day photoperiods. Dry cows and heifers on pasture? Forget the photoperiod thing.
Calving. So, if it is dark for 16 hours a day in the dry cow/transition cow barns, how do we keep track of cows that are calving?
Little red lightbulbs. Fifteen watt or less red lightbulbs will provide enough light to monitor cows while they do not perceive the red light as “light” that interferes with their prolactin response.
Putting red screens over existing, higher wattage lights (while creative) will interfere with the prolactin response.
Is this practical for all farms? No. Is it practical for your farm? Maybe, look around. Will it work where dry cows are currently housed? Will it work somewhere else on the farm if groups are shifted around a bit?
If you are planning to do some building, this is worth taking a serious look at while plans are being developed.
Online resource. Visit Dahl’s photoperiod website at
The site includes background information as well as examples of laying out lighting systems.
(The author is the northeast Ohio district dairy specialist with OSU Extension. Send comments or questions in care of Farm and Dairy, P.O. Box 38, Salem, OH 44460.)


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