The following answer is provided by Michelle Aguilar and Mark Hanigan of the Department of Dairy Science at Virginia Tech.
Q: How can milk urea nitrogen be used to improve nitrogen efficiency and reduce the environmental impact of dairy cows?
A: The global human population is projected to increase from its current estimate of 7.1 billion to 9.4 billion by the year 2050. These projections are alarming because food production will have to double, and meeting such demand will be a challenge as arable land is limited. Meeting this demand will require increased efficiency of production in all facets of the food production system. Additionally, gains in productivity cannot come at the expense of environmental health, or the gains will not be sustainable. The use of management tools, such as milk urea nitrogen (MUN), can help improve the efficiency of milk production, reduce feed costs, and reduce environmental problems associated with dairy production.
Impact of nitrogen excretion on the environment
Excess nitrogen fed to dairy cattle and other animals is excreted as urea in manure, much of which is converted into ammonia, and volatilized into the atmosphere. Ammonia emissions to the atmosphere are a concern as they can form particles less than 2.5 microns in size (PM2.5), which cause haze and contribute to lung and asthma problems in humans (WHO, 2005). Excess soil nitrogen can result in high levels of nitrate in drinking water or the leaching of nitrogen into surface water. Consumption of water with nitrates causes severe health problems in infants (methemoglobinemia), while nitrogen in surface water results in eutrophication and other serious environmental problems. Thus, the use of management practices that improve nitrogen efficiency of lactating dairy cattle may aid in the reduction of environmental and health risks.
Use of MUN to achieve optimum return
High-producing dairy cows have an overall average nitrogen efficiency of 25 percent, which is less than half the post-absorptive efficiencies of precision-fed growing pigs. Higher efficiencies can be achieved in pigs because they are fed diets that perfectly match their amino acid requirements (precision feeding). Unfortunately, we do not currently possess the same level of knowledge of amino acid requirements in ruminants. However, because nitrogen efficiency is related to blood and MUN concentrations in dairy cattle, we can use MUN values as a management tool to monitor and improve nitrogen efficiency.