When cows are happy, they produce more nutritious milk with higher levels of calcium, new research suggests.

The researchers found that daily infusions with a naturally-occurring chemical commonly associated with feelings of happiness increased calcium levels in the milk of Jersey cows that had just given birth.

The results, published in the Journal of Endocrinology, could lead to a better understanding of how to improve the health of dairy cows, and keep the milk flowing.

Demand is high for milk rich in calcium and dairy products such as milk, cheese and yoghurt are primary sources of the mineral.

But this demand can take its toll on milk-producing cows as evident from the fact that a large number of dairy cow population suffers from hypocalcaemia -- in which calcium levels are low.

A team of researchers led by Laura Hernandez from the University of Wisconsin-Madison in the US investigated the potential for serotonin (a naturally occurring chemical commonly associated with feelings of happiness) to increase calcium levels in both the milk and blood of dairy cows.

The team infused a chemical that converts to serotonin into 24 dairy cows, in the run up to giving birth.

Half the cows were Jersey and half were Holstein -- two of the most common breeds. Calcium levels in both the milk and circulating blood were measured throughout the experiment.

While serotonin improved the overall calcium status in both breeds, this was brought about in opposite ways.

Treated Holstein cows had higher levels of calcium in their blood, but lower calcium in their milk (compared to controls).

The reverse was true in treated Jersey cows and the higher milk calcium levels were particularly obvious in Jerseys at Day 30 of lactation -- suggesting a role for serotonin in maintaining levels throughout lactation.

"By studying two breeds we were able to see that regulation of calcium levels is different between the two," Hernandez said.

"Serotonin raised blood calcium in the Holsteins, and milk calcium in the Jerseys. We should also note that serotonin treatment had no effect on milk yield, feed intake or on levels of hormones required for lactation," she noted.