Nonprotein Nitrogen and Protein Distribution in the Milk of Cows E. J. DePETERS and J. D. FERGUSON Department of Animal Science University of California Davis 95616-8521 and Department of Clinical Studies University of Pennsylvania School of Veterinary Medicine Kennett Square 19348 ABSTRACT

The NPN content of milk represents only 5 to 6% of the total N in milk. However, the significance of this milk N fraction to energy and N metabolism in the dairy cow has not been well characterized. The single largest contributor to the NPN fraction of milk NPN is urea. Urea equilibrates in body water, and blood urea is the primary source of milk urea. The urea in milk can be derived from at least two sources: the end product of digestion and amino acid catabolism. Blood urea N was positively associated with intakes of ruminally degradable and undegradable protein and negatively associated with intake of net energy. Consequently, it might be possible to develop a system to evaluate the dietary protein and energy status of the lactating dairy cow employing milk urea in conjunction with milk true protein. (Key words: milk nitrogen, nonprotein nitrogen, milk urea) Abbreviation key: BUN = blood urea N, EE = ether extract, DIP = ruminally degradable protein, MUN = milk urea N, P:E = protein to energy ratio, TP true protein, TPY true protein yield, UIP = ruminally undegradable protein.

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INTRODUCTION

Interest in the protein content of milk is increasing in the US. This is probably related to the increasing proportion of the milk produced for the fluid market but actually uti-

Received October 22. 1991. Accepted May 29. 1992. 1992 J Dairy Sci 75:3192-3209

lized for manufactured products. In addition, human health concerns about animal fats result in development of milk pricing systems that place less economic emphasis on milk fat. As milk pricing schemes that place more economic emphasis on milk protein evolve, questions arise about current methods used to estimate the protein content-or more accurately N content-of milk and about factors that control milk N content and yield. A brief review of factors influencing the total N fractions of milk is presented as an introduction. For more complete discussions of N components of milk the reader is referred to the literature (25, 59, 88, 90, 91). The focus of this paper is the NPN fraction of milk. The NPN fraction of milk N is gaining attention because of its impact on testing procedures. Other papers in this symposium discuss the implications of testing procedures that are based on either total protein (CP) or true protein content of milk. In addition, this paper addresses potential relationships between the diet of the cow and the NPN content of the milk. TOTAL MILK N

Some confusion exists as to what is meant by milk protein as a consequence of the various methods used for N determinations. Typically, the CP content (percentage) of milk is measured as total Kjeldahl percentage of N times 6.38 (2). The Kjeldahl procedure accurately measures the total N content of milk. However, to convert total N percentage to protein, one assumes incorrectly (50), that all proteins in milk contain 15.7% N. Some (50) have proposed that a conversion factor of 6.34 be used. Although this might be an improvement, any change in the relative proportions of proteins in milk will alter the appropriate fac-

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tor. The N-containing portions of milk can be TABLE 1. Casein content of milk by region within 1 divided into three broad fractions, including California. casein, whey protein, and NPN (84). Of the Casein content 2 total milk N across a limited number of breeds. Area Herds Mean SD Min Max approximately 78.5% was associated with the (n) ---(%)---casein fraction, 16.5% with the whey protein 3.13 .27 2.60 3.64 fraction, and 5.0% with the NPN fraction (84). Northwest Coast 55 Bay and which is similar to later observations (17). The North North San composition of each fraction varies. For exam2.71 .25 2.10 3.30 Joaquin 141 ple, the casein fraction is composed of proteins South San 2.61 .21 2.23 3.31 Joaquin 129 from four gene products. including

Nonprotein nitrogen and protein distribution in the milk of cows.

The NPN content of milk represents only 5 to 6% of the total N in milk. However, the significance of this milk N fraction to energy and N metabolism i...
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