The effect of light on dairy farm income

By now we have forgotten the hot and long days of last months… climate is definitely milder and light hours at disposal are constantly diminishing. Cows are positively affected by the temperature drop of this period. But what we know about the effects that this decrease of daylight hours have on our animals? In nature, the amount of daylight hours is an important piece of information for a lot of species of mammals to program births in a favourable period.
But cows are able to get pregnant all the year long and to have a constant production during this time. So, why light management is becoming more and more important in the daily management of dairy farm?

Photoperiod: what is it?

Photoperiod is defined as the light duration at which animals are exposed every day. For convention, we can distinguish two types of photoperiod:

  • LDPP long day photoperiod or long photoperiod with 16 hours of light and 8 hours of dark per day;
  • SDPP short day photoperiod or brief photoperiod with 8 hours of light and 16 hours of dark per day.

The impulse of light or dark goes through the ocular bulb and along the optic nerve it modifies the activity of the pineal gland or epiphysis. This gland is a sort of internal clock and it is sensitive to the light’s durability and intensity. The pineal gland secretes melatonin, the hormone, which regulate the cycle sleep/wakefulness and influences the immune system, reproductive system and lactation. Exposition to light does not allow the secretion of melatonin for the majority of the day and favours the secretion of prolactin and IGF-1, two hormones, which influence the mammary gland’s activity.

Gestione dell'illuminazione delle stalle
Photoperiod: what are the effects on our animals?

Researches have demonstrated that there is an increasing in the production of dairy cows exposed to long photoperiod, compared to animals in natural conditions. Picture 1 (from Geoffrey and Dahl, 2001) sums up 9 studies, which have registered important positive effects on production after a constant exposure to 16 hours of light every day.

Necessary light to obtain positive effects on production must have a minimum intensity of 150-200 lux at about 80 cm from the ground. It is not enough that this intensity is reached only along the feeding line, but it is necessary that the dispersion of light is as much homogeneous as possible in all directions of the barn, so that the light stimulus can reach animals’ eyes constantly.
The answer of animal is progressive, in 2-4 weeks. Studies have reported a production increase between 5 and 16%, but it would be more realistic to consider a 8/10% growth.
It is important to underline cows tend to produce more milk and then increase ingestion, so the growth of feed intake has to be considered as a consequence and not as a cause of the production’s increase. Some studies have reported also data regarding the improvement of reproductive performances, above all regarding the early heat return after birth. Until now, we have talked about the effects of long photoperiod’s exposition on dairy cattle only, but some researches reports positive effects of light management even on accretion herd:

  • An increase in growth, an advanced puberty and mammary development in heifers have been registered, with a consequent reduction of one month of the first birth age (Tucker et al.,1984);
  • No differences were registered regarding ingestion, but heifers exposed to long photoperiod had a minor BCS but a better skeletal development, from which we can deduce a better feed efficiency.

In contrast to dairy cows and heifers, the groups of dry and pre-calving heifers seems to obtain better results after the exposure to short photoperiod with only 8 hours of daily light (Miller et al., 2000). Picture 2 (from Geoffrey e Dahl, 2001) reports the difference of production in the consequent lactation of 2 dry cows’ groups (production equivalent in previous lactation) exposed to LDPP (yellow line) and SDPP (red line). The exposure for 2-3 weeks in pre-calving to short photoperiod, for these animals seems to be a sort of reset of their ability to react to exposure at a major amount of light hours after birth.

Picture 1 - Effect of the regulation of photoperiod in nine studies conducted on cows in lactation
Picture 2 - Differences of production in the following lactation of dry cows exposed to LDPP and SDPP
For what has been written until now, you could think to expose your animals to 24 hours of light a day. Studies conducted in this sense tell us that all animals have to be exposed at least to 8 hours of dark a day.Animals, which are exposed to constant light, modify their circadian rhythm, as if they were exposed to brief photoperiod with a lack of milk production’s increase (Marcek and Swanson, 1984). For cows, dark means a light intensity under 40-50 lux. For this reason, the use of red light during the night is very useful for humans, which has to move around the barn and it seems not to affect dark perception of animals. Night exposure to a light intensity higher than 40-50 lux completely suppresses the secretion of melatonin. The nocturnal peak of this hormone is very important for the proper functioning of the immune and reproductive systems. Furthermore, in absence of melatonin, heart rate increases with consequent expenditure of energy and less energy for milk production. Even lactating calves, if exposed to continuous light, have demonstrated a greater feed intake without improvements on growth performance.
After more than 35 year from the first studies on the effects of light on milk production, the research is still investigating on various aspects to further clarify the role of photoperiod within the dairy cow farm. At the moment, we know that to achieve positive results from light management, it would be desirable to expose to long photoperiod growing heifers and cows in production, respectively obtaining improved performance of growth and milk production. On the other hand, pre-birth heifers and dry cows would only need 8 hours of light a day to maximize their ability to respond to the increase in the amount of light during production phase.