Photoperiod Management of Dairy Cattle

Reprinted with the permission of the author. Presented at Ontario Dairy Symposium 7, Ontario Large Herd Operators.

Dairy producers are constantly searching out new management techniques to improve production efficiency and cash flow. Photoperiod management has received interest lately as a cost-effective method to increase production in lactating cows. That is because in cows exposed to long days, i.e. 16 to 18 hours of light and a 6 to 8 hour period of darkness, daily milk production increases an average of 2 liters/cow, relative to those on natural photoperiods (5). The purpose of this paper is to review the evidence for a response to long days, describe the physiologic basis for the response, and discuss the economic justification for implementing photoperiod management.

While almost all animals respond to photoperiod in some way, it is usually associated with reproductive events (17). Indeed, poultry producers use lighting to stimulate, layers and sheep and horse breeders manipulate the breeding season with light exposure. Though not seasonal breeders, photoperiod can affect reproduction in cattle. For example, long days hasten puberty in heifers relative to natural daylength. Long days are also thought to reduce the delay in return to cyclicity following parturition, particularly in the winter (8). However, reproductive changes in response to photoperiod are subtle in cattle in comparison with the effect on lactation.

The impact of long days on milk production was first observed in 1978 by researchers at Michigan State University (13). Cows were placed on 16 hours of light, 8 hours of darkness (16L:8D) or left on natural photoperiod at calving. The study was conducted between September and March, when natural light was each day. Over the first 100 days postpartum, cows on long days produced 2.0 L/d more milk than those on natural photoperiod. At 100 days, the treatments were switched and the cows previously on natural photoperiod increased milk production, whereas the cows previously on 16:8D decreased milk yield. Those results suggested that exposure to long days increased milk yield and did so across production levels. Since the first study, at least nine other experiments at seven laboratories across North America and Europe have confirmed the response (summarized in Figure 1). Based on those studies, it is expected that cows on long days will produce an average of 2 litres ore than control animals on natural photoperiod.

Figure 1. Summary of nine studies reporting the effect of long day photoperiod on milk yield in lactating cows.
Figure 1. Summary of nine studies reporting the effect of long day photoperiod on milk yield in lactating cows.

What is the basis of the response? Recent studies have shed light on the potential physiologic mechanism for the increase in yield from cows on long days. Differences in light exposure alter secretion of a number of hormones. Indeed, those hormonal shifts drive the commonly observed changes in reproductive activity in other species. The first hormone impacted by photoperiod is melatonin, which is secreted in response to darkness. Thus, in cows and other animals, a long day actually reduces the duration of elevated melatonin (5). Animals use this pattern of melatonin to track daylength, and then alter secretion of other hormones. In cows, a long day pattern is associated with higher secretion of the hormone insulin-like growth factor-I (IGF-I; 6). Higher IGF-I, in turn, is thought to increase milk yield. It is of interest that bovine somatotropin (bST), which also increases milk yield, stimulates IGF-I release as well (2). This similarity of mechanism led to a test of the effect of bST and long days on milk yield.

Given that long days and bST act through similar pathways, it was of interest to determine if the two management techniques could be combined to elicit additive responses in milk yield. Miller et al. (11) treated cows with either long days or natural photoperiod, and half of each of those groups received bST as well. As expected, milk yield increased 1.9 L/d in the cows on long days relative to those on natural photoperiod. Treatment with bST alone increased production 5.7 L/d relative to the cows on natural photoperiod. The combination of long days and bST improved production 7.7 L /d, clearly an additive response. An additional positive effect of the combined treatment was observed in dry matter intake. It is known that cows on long days eventually increase intake to meet the increased demand for energy for milk production. Cows on bST and long days increased intake sooner than cows on bST only. Therefore, the combination may be beneficial to maintenance of energy balance in lactating cows.

Lactation, however, is not the only time during the annual milk production cycle when photoperiod treatment is recommended. Recent studies suggest that appropriate photoperiod treatment of the dry cow can markedly enhance milk yield in the subsequent lactation. We treated cows with either long or short (8L: 160) days during the entire 60-d dry period (10). Cows were then exposed to natural photoperiod after parturition, which occurred between November and January. Surprisingly, over the first 120 days of lactation, cows treated with short days when dry produced 3.2 L/d more milk than the cows on long days. It is important to note that all cows were housed together, fed the same diet, and managed identically following parturition. Our data are consistent with other recent reports of the impact of photoperiod manipulation during the dry period (1,14). Thus, in contrast to the benefits of long day photoperiod for lactating cows, treatment with short days is recommended for dry cows.

From the preceding discussion it is clear that photoperiod manipulation is an effective method to improve milk yield. But any new management tool must be economically beneficial as well. Tables 1 through 3 detail the expected returns and costs for implementing photoperiod management on a typical large dairy. All calculations are in Canadian dollars (CDN$). Assuming a response of 2 L/d from each cow, income in response to long days yields a gross return of $1.16/cow/d. It is expected that an increase of 1 kg/d in dry matter would be needed to support the higher milk yield ($0.19/cow/d). A cost unique to Canada relative to the United States is purchase of additional quota; this is estimated at $0.46/cow/d ($0.23/L x 2 L/d). The electrical cost has been calculated in Table 2 based on a lighting design appropriate (4) for a 250 cow freestall barn. (Technical details of design criteria are presented elsewhere in this symposium by Harold House). Additional non-cash costs associated with capital investment (i.e. lights) total $0.03/cow/d. Subtraction of costs from milk income yields a net return of $0.43/cow/d, or $107.50/farm/d. This means that an investment of $116.08/cow for lights (Table 3) would be recovered in 9 months. An additional column has been provided in each table to assist producers in determining the economic justification unique to their farm.

In summary, photoperiod management offers dairy producers a novel tool to improve the efficiency of milk production. It is cost effective on dairies of all sizes, but economies of scale on larger dairies enhance the returns. Treatment to increase daylength should be considered during lactation and decrease daylength during the dry period to increase milk yield.

Table 1. Net daily income per lactating cow, net farm income and payoff time for fixed costs (CDN$) associated with supplemental lighting for a 250 cow freestall barn.
  Example Your farm
Response, L 2  
Quota Cost1, L $0.23  
Feed Cost, kg DM $0.19  
Electrical Cost2 $0.05  
Other non-cash Cost3 $0.03  
Milk Income4 $1.16  
Net Income/Cow $0.43  
Net Farm Income/day $107.50  
Net Farm Income/year5 $32,250  
Pay-off time for capital costs, days 270  

1 Cost of financing increased quota over 15 years at current interest rate.
2 Table 2.
3 Interest on investment and depreciation.
4 Two liters extra milk at $0.58/L.
5 Calculated for a response 10 months of the year.
6 Total capital cost (Table 3) divided by net farm income/day.

Table 2. Average daily electrical costs (CDN$) of operating supplemental lighting for a 250 cow freestall barn1
  Example Your barn
Lighting type Metal halide  
Lamp size in watts 250  
Lamps/fixture 1  
Electrical demand/fixture, kW 0.3  
Total number of fixtures 72  
Hours of operation / day 8  
Energy Use, kW/day 172.8  
Daily energy cost2 12.10  
Rated lamp life, hours 15,000  
Lamp life, days 2542  
Cost of lamp replacement, per day $0.03  
Average operating cost $12.13  
Daily operating cost/cow $0.05  

1 Six row barn approximately 62 metres long x 35 m wide.
2 $0.07/kWh


Table 3. Cost (CDN$) of supplemental lighting for a 250 cow freestall barn1
  Example Your barn
Lighting type Metal halide  
Lamp size in watts 250  
Fixture cost1 $300  
Lamps/fixture 1  
Installation cost/fixture $100  
Total cost/fixture $400  
Total number of fixtures 72  
Timer cost and installation3 $220  
Total capital cost $29,020  
Capital cost/cow $116.08  

1 Lamp included with fixture – replacement cost is $65/bulb
2 Labour @ $65/hour
3 Automatic timer $125; installation $95


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Author: Geoffrey E. Dahl - Ph.D. Associate Professor/University of Illinois
Creation Date: February 2001
Last Reviewed: 03 July 2012