Symposium summary: How periods of light and dark affect dairy cows
Cows, like almost all animals, have evolved internal “clocks” to respond physically to the cycles of daytime and nighttime. In humans, periods of lightness and darkness, called photoperiods, regulate when we wake up or feel tired. In dairy cows, longer exposure to daylight increases milk production.
At the Joint Annual Meeting, held this past July in New Orleans, scientists gathered to discuss how photoperiod affects dairy cow biology. The symposium was titled “Lactation Biology Symposium: Circadian Clocks and Photoperiod in Mammary Development and Lactation.” The goal of the symposium was to share research and better understand how photoperiods affect dairy cow lactation and mammary gland development.
For years, researchers thought photoperiods mostly affected hormone secretion from an animal’s pituitary gland and pineal gland, but today, many are researching the affects of photoperiods throughout the body.
“We’ve known for a long time that photoperiod regulates milk production,” said symposium chair Darryl Hadsell, an associate professor of pediatrics and molecular and cellular biology at Baylor College of Medicine. Hadsell said the symposium was important because researchers only recently realized that “there are actually circadian clocks in the mammary cells themselves.”
Speakers at the symposium tackled photoperiod research from several angles.
Paul Hardin, a distinguished professor of biology at Texas A&M University, presented research on how photoperiod affects cellular responses in many animals. When an animal perceives light, its brain sends signals to cells throughout the body, activating certain genes. In fruit flies, “gene clocks” can prompt the insects to hatch or moult at certain times of day. Hardin’s presentation, titled “Circadian timekeeping mechanisms,” supported research showing that certain genes in dairy cow mammary glands depend on photoperiod signals.
When organs other than the brain respond to photoperiod changes, scientists like Hadsell say the organs have “peripheral clocks.” These clocks can regulate gene expression and affect organs like the liver, heart and kidneys during the course of the day. During the symposium, Weston Porter, an associate professor of veterinary anatomy and public health at Texas A&M, explained how mammary glands in dairy cows have peripheral clocks that regulate cell growth. Porter’s research suggests that the signals given by peripheral clocks in the mammary gland are important during gland development in cows that have never lactated before. Porter’s presentation was titled “Circadian clocks in mammary gland development and differentiation.” His research could also be important in understanding how normal-growing mammary cells become cancer cells.
Teresa Casey and Karen Plaut of Purdue University then explained how photoperiod can affect mammary gland development as a cow transitions from pregnancy to lactation. Their studies of rats and humans indicate that exposure to different photoperiods can regulate the production of hormones needed for lactation. In dairy cows, Casey and Plaut found that the composition of a cow’s milk actually changes during daylight, suggesting changes in hormones and cow metabolism on the cellular level. Their talk was titled “Circadian clocks as mediators of the homeorhetic response to lactation.”
To wrap up the symposium, Geoff Dahl, Izabella Thompson and Sha Tao from University of Florida, Gainseville discussed their research on how exposure to light affects the production of a hormone called melatonin. Their talk, titled “Effects of photoperiod on mammary gland development and lactation,” provided information for dairy farmers looking to increase the amount of milk per cow. They explained that exposing cows to shorter period of daylight decreased milk yield but increase mammary growth. Cows that experienced these shorter days produced 3 to 4 more kilograms of milk when later exposed to longer periods of daylight. By manipulating light exposure, dairy producers could make their cows more efficient.
Overall, said Hadsell, research presentedat the symposium showed that mammary cells in dairy cows have their own biological clocks that respond to the change from day to night. Hadsell said he is curious to see future studies on how dairy cow diets, feeding times, and times of day for milking could affect mammary development. Researchers will present new research on lactation biology at the 2012 Joint Annual Meeting in Phoenix, Arizona.
The full articles can be found at journalofanimalscience.org.
Dr. Darryl Hadsell, Baylor College of Medicine
Madeline McCurry-Schmidt, American Society of Animal Science