Until recently, dairy producers regarded milk production as governed by seasonal influences, Salfer said. They thought that cows just reacted to their environment and conditions.
"But our research is leading us to believe that cows have predictable changes in their physiology that leads to regular variation in milk production," he said. "It is shifting the way we are thinking about the seasonal changes in milk production from being a response to the environment to actually being a physiological element of the cow."
Better quantification of the annual rhythms shows that fluctuations in milk yield and composition are mainly driven by photoperiod and not strictly by environmental conditions such as heat stress, Salfer pointed out.
"But that remains a hot topic, no pun intended, in the dairy industry," he said.
That milk production occurs in rhythms dictated by a physiological response should not be unexpected, Salfer said. Other studies have revealed that milk production of cows is responsive to photoperiod. So, any change in the amount of daylight per day will affect these kinds of rhythms, similar to the way photoperiod changes trigger annual rhythms among wildlife that account for such regular behavior as hibernation, breeding and migration.
The findings, published this month in the Journal of Dairy Science, should help prevent dairy producers from being misled by seasonal milk fluctuations, Salfer explained. This information will allow them to interpret the effects of diet changes or implementation of new technologies on herd performance within the context of the annual rhythm.
"For example, 3.6 percent milk fat may indicate suboptimal milk fat in January but normal milk fat in July," he said. "In addition, feeding a dietary supplement in July may appear to improve milk fat percent in the following months, but the increase may be merely a consequence of the annual rhythm of production."
Also involved in the research was Chad Dechow, Penn State associate professor of dairy cattle genetics.
The U.S. Department of Agriculture's National Institute of Food and Agriculture and the National Institutes of Health partially supported this work.