Glen Broderick, a dairy scientist at the Agricultural Research Services (ARS), a division of the US Department of Agriculture, found that the optimum percentage of protein in dairy herd diets was 16.5 per cent, although US farmers regularly formulate diets with protein content as high as 18 to 19 per cent.
Dairy farmers favour the use of supplementary dietary protein as microbial activity in the rumen - one of the cow's four stomach compartments - can often degrade high-quality protein, which means it must be replaced by protein supplements - such as soybean meal - added to cattle feed.
Protein contains nitrogen that is used in the production of amino acids, essential for the growth and maintenance processes in dairy cattle and which in turn can lead to increased milk yields.
Too much protein, however, can cause cattle to excrete more urinary nitrogen, which, in the farm environment, may be converted to ammonia - a component of acid rain, which poses contamination threats to farms' surface and ground-water.
Speaking to DairyReporter.com, Professor Sandra Edwards, a professor of agriculture at the University of Newcastle-upon-Tyne in the UK, claimed that "the majority of dairy farmers share the same ambitions as environmentalists, in that urinary nitrogen not only contaminates farms' own water supplies, but also harms the environment".
According to the US-based Federation of Animal Science Societies (FASS), many US farmers do not routinely formulate and evaluate the dietary requirements of their dairy herds, and a number have misconceptions about the quantities of minerals and supplements - including calcium, phosphorous and protein - that should be included in the diets of their cattle.
However, Professor Edwards noted that "in the case of British dairy farmers, the majority are self-qualified nutritionists in their own right, so the incidence of over-feeding is fairly uncommon".
"In fact, there is probably a higher incidence of under-feeding protein to cattle in the British dairy sector, due to the high price of supplement feeds, which can have even more serious effects on health than over-feeding," she added.
Broderick claims that if US farmers observed a 1 per cent decrease in protein in the diets of their dairy cattle, urinary nitrogen - which can contribute to global warming - could be reduced by up to 60,000 tonnes a year.