Responding to the Practical Implications of Dry Cow Photoperiod Research

Over the last 20 years, numerous studies with milking cows have investigated their response to changes in the number of daily hours of light or "daylength". These studies have consistently shown an increase in milk yield of 2 to 3 kilograms when supplementary light is provided to extend the "daylength" in winter to 16 hours. The main outcome of these studies has been a general recommendation that cows need 16 hours of daylight and 8 hours of darkness each day to produce a maximum amount of milk. On the basis of this advice, many dairy producers have added timers to barn lights to provide this light regimen 365 days a year.

This dry cow study and two others like it, cast serious doubt on the validity of this practice. In the current study, cows in the university herd were dried off in the fall and winter, 60 days before expected calving date, paired by dry off dates and randomly assigned to one of two "daylength" treatments during the entire dry period. One of each pair was housed in a barn in which natural daylight was supplemented with fluorescent lighting to provide a daylength of 16 hours followed by 8 hours of darkness. The other group was housed in an identical barn, but windows were covered and lights were shut off in mid-afternoon so that these cows were exposed to 8 hours of light and 16 hours of darkness. Cows were fed identical rations during the dry period and after calving. After calving all cows were housed and milked in the same group, in a barn receiving natural winter daylight for Maryland or roughly 10 hours of light and 14 hours of darkness.

Did the longer daylength during the dry period result in a production response??…..It certainly did, but perhaps not the response most of us would expect. Milk production was monitored for the first 120 days of lactation. Cows given 16-hour days and 8-hour nights during the dry period produced an average of 34.9 kilograms of milk per day. This was 3.2 kilograms LESS than the average of 38.1 kilograms produced by cows kept in the dark for 16 hours per day during the dry period.

Although more research is needed to understand this response, the authors suggest that the higher milk yield in these cows is a response to an increase in daylength rather than to any absolute length of the light period. In earlier trials with milking cows, the treatment always involved extending the daylength of the treatment group, during the fall and winter period when the natural daylength was getting shorter. The positive response in milk production happened regardless of stage of lactation. In those trials daylength was increased by adding supplementary light for milking cows. In the Maryland trial, daylength during the milking period was increased at calving, but in this case, it was done by restricting light during the dry period.

As with most research this trial leaves a number of questions unanswered. Were dry cows kept in the dark stimulated to produce more milk, or did those in the light produce less than normal? The trial was not designed to address this, but production in previous and subsequent lactations suggests they did produce more. How dark is dark, or in other words how much light can filter into this area without altering the response? Neither this trial nor any of the others define the amount of light needed to establish a period as "light" or "dark". How long a period of short daylength is needed to trigger the response? In this trial cows were kept in the dark for the entire 58 days dry, but perhaps a shorter period would suffice.

The practical implications of this trial are quite important. It would appear that keeping the lights on for 16 hours and off for 8 on a year round basis in a barn housing both milking and dry cows, probably does not result in a production response and is likely a waste of electricity. Since the production increase results from an increase in daylength, an effective strategy will house dry cows in a separate area and manipulate the lighting in either this area or the milking cow barn, or both. The goal may be to provide a daylength in the milking barn that is always significantly longer than in the dry cow area. Either darkening the dry cow barn, or using lights to lengthen the day in the milking barn, may be equally effective. While darkening the dry cow area might cost less than supplementary lighting for the milking cows, eliminating the light from a modern naturally ventilated barn by 4 p.m. in July may not be easy. On many farms, finding separate housing for the dry cows without creating new bottlenecks in feeding, manure-handling etc. will be a challenge. On the farm, the economics of making housing and management changes always needs to be considered.

The graph of weekly production of energy corrected milk in this trial, (black squares plot the short daylength group and white squares the long) suggests that the production difference may extend beyond the 100 days. At 3.2 Kg per day extra milk in the first 120 days, a conservative estimate of the difference over a complete lactation might be 650 Kg per cow. If the net returns on extra milk after feed, quota and other variable expenses are deducted is roughly $.20 per Kg the benefit is $130 per cow. On this basis the average Ontario dairy herd (58 cows) can invest up to $7500 per year in additional labour, housing or lighting and break even on photoperiod manipulation. Further studies may better define exactly when and how, but the concept of artificially increasing daylength at calving appears to have real potential.

Practical guidelines for implementing the photoperiod management suggestions from this study do mean that you will need to make some assumptions that go beyond the research. If you are trying to make an area for dry cows dark, work with other species suggests that it needs to be dark, so that it is almost impossible to see, rather than just dim. The poultry breeder industry even uses special fans that block light entry through the fan. With current technology these conditions cannot be created in a naturally ventilated barn in summer. Where this is the only type of housing available, the best solution would appear to be housing dry cows in a barn with natural daylength and housing the milking herd in another barn where the daylength period is extended by 3 to 4 hours at all times of the year. Both daylight sensing switches and timed switches are readily available but devices that turn barn lights on at dusk, and off four hours later would be most appropriate for this strategy. Outdoor Christmas light controls that do this are available, but may not be able to deal with adequate amperage. Unless a manufacturer can be found the best choice may be manually adjusting a timer switch to correspond to the changing time of sunset.

Rennovating an exisiting building or area to shorten the daylength for dry cows may be possible on some farms. Assuming that the barn has to be quite dark, it will have to have opaque walls and if there are windows, these have to be covered completely. Automatic controls to close opaque window panels will probably cost more than covering the windows permanently and using artificial light throughout the desired daylength period.

Ventilation of such a building would have to be with fans and in summer this means tunnel ventilation. Fans and inlets would have to be designed to block out incoming light. Since tunnel ventilation involves large inlets special baffles may be needed to bend incoming air back and forth so that light follwing a straight path does not enter. When designing this inlet, make it even bigger so it does not restrict air flow, and ensure that no spot is more restrictive than the cross sectional area required for the fans being used. Exhaust fans can probably be darkened enough with opaque hoods and baffles on the fan.

Since the days are naturally lengthening from Dec 22 to June 22, it may not be all that beneficial to further manipulate day length during this period. The payback may be limited to tricking the cow into thinking longer days at calving in the second half of the year. It is noteworthy that Canadian BCA standards for cows always predict higher milk yield for cows calving in January to June. Could this be a daylength response?

How long do cows have to be housed in shorter daylength? The research does not offer a clear answer, but studies on the rate of response suggest cows react to lengthened days quite quickly after the change occurs. Assuming this implies only a short period is needed, it may be acceptable to house only the close up dry cows in the darkened area for the last 2 to 3 weeks before calving. The best bet will be to keep a close eye on further research in this area, which will hopefully address some of the many unanswered questions.