HOT COWS: A hotter climate means that cows will eat less and produce less milk
CRAIG PACKER/USDA

In July 2006, a monthlong triple-digit heat wave scorched California, killing more than 25,000 cattle and reducing dairy production in the region. Land O'Lakes Creameries, which normally produces six million liters of milk daily, was short 1.5 million liters per day. All told, experts estimate that the high temperatures caused $1 billion worth of dairy shortfalls.

Extreme weather events and higher average temperatures are predicted to increase with global warming, and that's bad news for livestock producers in the U.S. and abroad. Warming will reduce grass, brush and other forage available in many areas, and it will also directly influence cows' physiology. Dairy production is optimal at cooler temperatures between 20 to 22 degrees Celsius, explains Terry Mader of the University of Nebraska–Lincoln. "For every degree above that," he says, "we'll have a decline of approximately 2 percent productivity."

Mader is one of many scientists studying the potential impacts of climate change on livestock, and how producers can mitigate them. He says that although climate models are not conclusive about whether the U.S. Midwest's average temperature will go up 2 degrees C or 5 degrees C over the next century, the physics of cows is pretty straightforward. "You have heat generated from metabolism and digestion, and then they have to cope with the environmental component," he explains, "How do they offset increased heat? They eat less." The decline in feeding results in a decline in output, whether that's meat, milk or fur. They also tend to have lower rates of conception during warmer months. "That's just physiology," Mader says.

In an upcoming issue of the journal Climatic Change, Mader and his colleagues combined his cow model with climate models to estimate milk production along three north-south transects in the Midwest under scenarios doubling and tripling climate-warming carbon dioxide. The researchers concluded that summer milk production in the U. S. would decrease by 16 to 30 percent, about double the normal summer decline. A drop in the supply could lead to an increase in the price of milk, and suppliers in some areas would also need to use electric fans to keep facilities cool for animals.

Other researchers have been looking to tap into the genetic diversity of livestock in tropical countries to create heat-tolerant breeds that can also pump out lots of milk. The problem is that a Holstein in the U.S. can produce up to 8,000 liters of milk annually, compared with lowland Brazilian breeds that are tick-resistant and heat-tolerant but are only producing just over a thousand liters of milk per annum.