Nitrogen Excretion

A dairy cow fed a well balanced ration will convert about 30% of the N consumed into milk protein. About 70% of the N consumed will be excreted as feces or urine, because of the inefficiencies in digestion and metabolism and maintenance cost of replacing body cells. As the total milk production for the lactation increases, feed and N intake will increase to meet the higher demand for nutrients. However, a higher proportion of the N consumed is converted to milk, because much of the maintenance N cost is fixed. The feces contains N in undigested fiber, bacterial cell wall, secretions into the intestinal tract, and sloughed intestinal tissue. An average of 20% of the ruminally undigested feed will be excreted in the feces, and 40% of the N in bacteria will be excreted in the feces (bacterial cell walls) or urine (bacterial nucleic acids). As the proportion of the N bound in fiber or in heat damaged protein increases, a higher proportion of the N will be lost in the feces as bound protein. The urine contains N from tissue protein that is mobilized for energy or turned over as cells are replaced, and from bacterial nucleic acids. A high proportion of the N in urine is from excess nitrogen in soluble protein or non protein nitrogen such as urea and amino acids not used for tissue or milk protein synthesis that are converted into glucose and urea. Thus as N in these various forms are consumed above the animals requirements for rumen fermentation, maintenance, growth, pregnancy or milk production, N excreted in the urine increases.

The absorbed amino acids are used for maintenance or milk protein synthesis with an average efficiency of 67%. They are used for pregnancy with an efficiency of about 33%, and are used for growth is used with an efficiency that declines with age (83% when first born and 28% when nearly mature). The efficiency of use of the absorbed amino acids is improved when they are in a proportion that is similar to the profile of amino acids in body tissue and milk; protein with this profile is called an ideal protein. However, the efficiency declines when the profile of absorbed amino acids provided by the ration do not match those required for tissue or milk synthesis. Under these conditions, the synthesis of milk protein will be limited to the availability of the most limiting amino acids, and some will have to be overfed to support the energy allowable milk production. Thus less protein is required in rations that provide an ideal pattern of absorbed amino acids while providing an ideal amount and mix of ammonia and peptides in the rumen to maximize rumen fermentation.

The table, below, contains estimates of the nitrogen consumption and excretion of an average lactating dairy cow in 46 commercial dairy herds in New York State. The Cornell Net Carbohydrate and Protein System (CNCPS) was used to predict the nitrogen excretion values, based on nitrogen content of feeds, dry matter intake, body weight, and milk production and milk protein content.

Estimated Nitrogen Excretion in 46 Commercial Dairy Herds in New York1
Item	Average	Minimum	Maximum
Milk, lb/day	85.5	54	118
DMI, lb/day	50.3	38	63.4
Ration CP	17.9	15.3	20.5
N intake, g.day	653	496	897
Milk N, g/day	188	1190	260
N intake converted to milk N, %	28.8	21.4	35.8
Total N excreted, g	460	336	624
N excretion, % of N intake	70.4	63.5	82
Fecal N, g	252	183	320
Urinary N, g	207	150	304
Urinary N, % of total	45	37	50

1. L. E. Chase, 2004.

This table indicates that the CNCPS model predicted that on average, 28.8% of the N consumed is converted to milk N by lactating dairy cows in these herds, which agrees with milk N efficiencies measured in nitrogen balance trials summarized by Jonker et al. (1998; 28%) and by Chase (2003; 27%). Thus we believe the CNCPS can predict total N excretion with an acceptable accuracy. This table shows the milk N efficiency ranged from 21 to 35% across these herds. The amount of N excreted varied with the level of milk production, because N intake rises as the demand for nutrients increases with higher milk production. About 37 to 50% of the N excreted by lactating dairy cows is in the urine. A high proportion of the urinary N is in the form of urea, which can be degraded to ammonia by the urease enzyme in the feces on the barn floor, and thus is the primary source of N emissions from a dairy barn. The percent of the total N that is excreted in the urine will increase as protein intake in excess of requirements increases, because most of the excess N is converted to urea during metabolism and is excreted in the kidneys.