Scientific American (link no longer available)
By Katherine Harmon

Like produce, milk quality can vary with the season and year. Dairy cows‘ daily diet, much of which comes from plant forage, determines the nutritional makeup of their milk, so when their food lags in quality, so too does their output. And a new study shows that conventionally produced milk is more prone to these unfavorable seasonal shifts than organic milk.

Earlier research had shown slight benefits in organic milk’s nutritional profile when it was tested on the farm. “Whereas on the farms the benefits of organic milk were proven in the summer but not in the winter, in the supermarkets it is significantly better quality year round,” Gillian Butler, of the Nafferton Ecological Farming Group at Newcastle University and study co-author, said in a prepared statement.

Butler and her team sampled 22 brands of U.K. milk four times between 2006 and 2008 and found that the organic milk brands had far less seasonal and annual variability. Some of the conventional milk brands, for instance, tended to have higher concentrations of saturated fats and lower levels of beneficial ones, such as omega-3 and polyunsaturated fats after poor growing seasons.

“We didn’t expect to find differences between the sampling periods,” Butler said. “But this is likely to be down to the impact of the weather on availability of quality forage.”

The researchers suggested that the nutritional profiles reflect a higher quality and more consistent diet for cows in organic milk production. In the U.K., organic production disallows use of nitrogen-based fertilizers, which in turn often leads to more red and white clover in the grass that the cows eat. These plants help cows produce milk that can make for a healthier fatty acid profile. Conventional management, however, might include giving the cows more fat-based supplements that could in turn raise the levels of saturated fats in the milk, according to the researchers.

Butler and her colleagues noted that the new findings have implications for nourishment in a changing climate as well. After the summers that were especially cool and damp in the U.K., such as 2007, the fat profile of conventionally produced milk took an especially noticeable hit. “If these weather patterns continue, both forage and dairy management will have to adapt to maintain current milk quality,” Butler said. But, she noted, “The higher levels of beneficial fats in organic milk would more than compensate for the depression brought about by relatively poor weather conditions in the wet year.”

The findings were published in the January issue of Journal of Dairy Science.

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