Natural rhythms are key to milk production

Barn management practices still important, but researchers find evolution is critical to milk fat and protein production

It has long been recognized that there is an annual rhythm in the composition of milk from dairy cows with higher milk fat and protein in winter and lower levels in summer.

The thinking until now was that milk composition was influenced by environmental factors such as heat, cold and humidity.

But research out of the Pennsylvania State University’s College of Agricultural Sciences has shown that these biological rhythms are regulated internally by the cows.

“There is a difference between the northern and southern states we studied and I expect it extends into Canada,” said Kevin Harvatine, associate professor of nutritional physiology whose research group conducted the study. “In the northern states, there is a larger amplitude (the amount the highs and lows vary from the mean) to the biorhythms of fat and protein percent, meaning that there is more yearly variation than in southern states.

“The effect is actually the opposite for milk, fat and protein yield, which have smaller amplitudes in northern states compared to southern states.”

He said he expected that the pattern of greater amplitudes of fat and protein percent and lower amplitudes of milk, fat and protein yield to become more extreme in animals located further north in Canada.

The research team analyzed national milk composition statistics from 2000-15 provided by the Agricultural Marketing Service of the U.S. Department of Agriculture, as well as records collected from 1,684 cows in 11 Pennsylvania dairy herds from 2002-11. In general, among all herds studied, peak yield, fat concentration and protein concentration occurred in winter months, when days are shorter, and lowest in summer months, when days are longer.

According to lead researcher Isaac Salfer, a doctoral student in animal science working with Harvatine, protein concentrations peaked in December, fat concentrations peaked in January, fat and protein yields peaked in February and milk yield peaked in April. The rhythms of yield and concentration consistently occurred, regardless of region.

The results, while valuable for all dairy farmers, indicate not only an internal response but an evolutionary one.

“We think it is likely an evolutionary response to provide the most energy to the young calf,” said Harvatine. “In other species, these annual biorhythms are primarily used to improve reproduction and survivability of the offspring. So, we think these cows evolved to provide the most fat and protein to calves in the winter when they have the greatest need for energy. However, cows are still responsive to heat stress and other environmental factors in addition to the biorhythm.”

Harvatine said that other researchers have also monitored annual rhythms in Europe and found there is a yearly pattern of milk fat percentage there too.

“In the southern hemisphere, a group in New Zealand found that these rhythms of production occurred but followed patterns that were opposite than the ones in the U.S., which makes sense because they have opposite seasons. One of the challenges in these other datasets is the large influence of nutrition on milk composition and the high rates of grazing herds. It is just impossible to separate the animal effect from the seasonal changes in the nutrient composition of grass. The U.S. is highly based on TMR (total mixed ration) feeding, and diet composition varies much less over the year.”

These rhythms, Harvatine said, occur independent of management strategy. While management may have an influence on the levels of production to shift rhythms up and down, the general pattern stays the same over long periods of time.

“We expect that even under theoretical perfect conventional management conditions, yearly biorhythm of milk production would still occur,” he said. “Milk production is improved by constant long-day lighting (16 hours of light, eight hours of dark) during lactation and we recommend following that procedure. We are also interested in investigating if there are ways to influence the seasonal rhythms.”

Fundamental to dairy cow management is the fact that many in large commercial facilities are permanently housed in barns where they may not have the influence of natural daylight in the same way as cows turned out to pasture. The research was not able to look at housing types or the design of some barns that may let in more light than others.

“Cows may also be exposed to light when going to the parlour,” he said. “It only takes small episodes of light exposure to set these rhythms, and natural light and artificial light will differ in wavelengths. What is probably the bigger consideration is that many farms never turn off the lights. There is considerable discussion on the effect of light pollution on humans and a similar issue is probably occurring in cows.”

The findings were published December 2018 in the Journal of Dairy Science.

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