Compared to U.S. dairy production in 1944, the environmental impact of modern day dairy systems is significantly less; there are fewer resources used and less waste output, although some industry characteristics remain the same. The carbon footprint per billion kilograms of milk produced in 2007 was 37% of equivalent milk production in 1944.

To conduct this study of the environmental impact of the U.S. dairy system in 1944 versus the system in 2007, researchers used a model based on the metabolism and nutrient requirements of the U.S. dairy cow herd. Both systems were modeled using relevant management practices and U.S. dairy farm production data.

The 1944 dairy herd totaled 25.6 million cows and produced 53.0 billion kilograms of milk per year. The 2007 dairy herd was considerably smaller, with 9.2 million cows and an annual milk production of 84.2 billion kilograms. The 2007 herd produced 59% more milk from 64% fewer cows, reducing the carbon footprint per gallon of milk by 63%.

Contemporary dairy systems require fewer resources than dairy systems did in 1944. To produce one billion kilograms of milk in 2007 required 21% of the animals, 23% of the feedstuffs, 35% of the water, and 10% of the land used in 1944. Waste output is also dramatically lower. Contemporary systems emit only 24% of the manure, 43% of the CH4, and 56% of the N2O output per billion kilograms of milk from previous levels.

The article does not address the welfare of cows used to produce milk, nor does it address the welfare implications of increasing the milk yield per cow through the use of hormones such as rBST.