The amount and composition of milk produced by dairy cows appears to be more regulated by internal, annual biological rhythms than by environmental factors such as heat and humidity, according to Penn State researchers who studied more than a decade of production records from herds across the country.

Although researchers have long recognized an annual pattern of milk composition in dairy cattle — with higher milk fat and protein concentrations observed in winter and lower levels occurring in summer — the rhythms of milk yield and composition had not been well quantified. 

The findings of this research are important because they better-inform producers of what to expect from their cows, according to Kevin Harvatine, associate professor of nutritional physiology, whose research group in the Penn State College of Agricultural Sciences conducted the study. He noted that being more precisely aware of their cows’ rhythms allows dairy farmers to better judge the effectiveness of management strategies.

Researchers used a statistical method to demonstrate predictable cycles to reveal the annual rhythms of milk yield and milk fat and protein concentration in two large datasets. They analyzed national milk composition records from 2000-2015, obtained from USDA’s Agricultural Marketing Service, along with records from 1,684 cows in 11 Pennsylvania dairy herds from 2002-2011.

More yield, protein during winter
In general, among all herds studied, peak yield, fat concentration and protein concentration occurred during the winter months, when days are shorter, and lowest during the summer months, when days are longer. And the amplitude of the rhythms — the amount that the highs and lows varied from the mean — were greater in herds in more northern areas and declined depending how far south herds were located. 

"On average, milk yield peaked in April, fat and protein yield peaked in February, fat concentration peaked in January, and protein concentration peaked in December," says lead researcher Isaac Salfer, Ph.D. student in animal science. "And the yearly rhythms of milk yield and fat and protein concentration consistently occur, regardless of region." 

The long-standing opinion among dairy farmers and professionals was that milk production was regulated by seasonal conditions, according to Salfer; 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 says. "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."

Day length matters most
Better quantification of the annual rhythms shows that fluctuations in milk yield and composition are mainly driven by photoperiod ­— seasonal changes in day length — and not strictly by environmental conditions such as heat stress, Salfer points out.

"But that remains a hot topic, no pun intended, in the dairy industry," he says.

That milk production occurs in rhythms dictated by a physiological response should not be unexpected, he says. Other studies have shown that milk production is responsive to day length. 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 in 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 farmers from being misled by seasonal milk fluctuations, Salfer says, adding that the 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% milkfat may indicate suboptimal milk fat in January but normal milk fat in July," he says. "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."

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