FEEDING AND NITROGEN LOSSES IN DAIRY HERDS
James D. Ferguson
Center of Animal Health and Productivity
University of Pennsylvania
School of Veterinary Medicine
Leaching of nitrogen from land disposal of animal manure contributes to
significant problems with eutrophication of estuaries, lakes, ponds, and streams and high
nitrate concentrations in ground water in agricultural counties. Volatilization of nitrogen
as ammonia from animal operations contributes to air particulate pollution and may also
contribute to eutrophication of water systems on deposition into water systems. Reducing
and controlling nitrogen losses to the environment from animal operations is essential to
minimize their environmental impact. The efficiency of rumen capture of feed nitrogen
into bacterial crude protein has a major impact on potential losses of nitrogen to the
environment.
Consider a Holstein dairy cow producing 40.8 kg of milk containing 3.7% fat and
3.1% true protein consuming 631 g of nitrogen in 23.3 kg of dry matter. The flows of
nitrogen in this cow would be as follows:
Intake N, g
631
% of intake
Milk protein, g
200
31.7
Milk NPN, g
6
1.0
Growth, g
2
.3
Fecal N, g
257
40.7
Urinary N, g
150
23.7
Scurf N, g
17
2.7
Urinary nitrogen would contain 90.3 g as urea, which would be rapidly degraded to
ammonia and potentially lost as ammonia. Milk urea nitrogen would be 9 to 10 mg/dl,
depending on partitioning between urine and milk. This situation represents a very
efficient feeding program. These losses may vary with the proportion of corn silage and
alfalfa haylage in the ration. Using fixed ration constraints to meet the energy and protein
needs of this dairy cow, changing forage base may have the following consequences:
Forage %
CP,% Intake N, g
CSG 100, HYLG 0 17.0
635
CSG 73, HYLG 27 18.5
691
CSG 50, HYLG 50 18.8
702
CSG 27, HYLG 73 20.1
750
CSG 0, HYLG 100 20.4
762
MilkNPN, g
6.8
8.8
9.1
11.1
11.4
Fecal N, g
249
254
258
257
260
Urine N, g
161
210
217
263
271
Forage program may have a significant impact on urinary losses at fixed production
levels. In addition, the increase and decrease in forage quality can alter urinary losses. At
the farm level, cattle grouping based on production and feeding appropriate rations can
further reduced urinary losses.