Using models to optimize the efficiency of nitrogen and amino acid use in
lactating cows
Thomas R. Overton, Michael E. Van Amburgh, and Larry E. Chase
Department of Animal Science
Cornell University, Ithaca NY
The importance of improving the efficiency of nitrogen use in rations for dairy cattle
continues to increase. Much of this interest has been driven by comparatively high
prices for purchased protein sources on a global basis, although improving the
efficiency of nitrogen use also is important for decreasing ammonia emissions and
other nitrogen losses from dairy farms, which is the next major environmental issue
that dairy farms likely will face. Use of models such as the Cornell Net Carbohydrate
and Protein System (CNCPS) will continue to enhance our ability to take the
nutritional margins out of diets for dairy cattle with potentially direct implications
for both ration cost and net milk income over feed cost. Recently, we released
CNCPS version 6.1 in beta form (downloadable from www.cncps.cornell.edu). This
new biology is also available in licensed software available from Agricultural
Modeling and Training Systems, LLC (AMTS; www.agmodelsystems.com) and two
Italian companies, one of which (RUM&N; www.rumen.it) has a license for the U.S.
market. This new version of CNCPS represents the first major changes to the core
biology of the CNCPS in a number of years. In this paper, we first will briefly
describe the updates to the CNCPS. Then we will provide an overview of nitrogen
metabolism in the cow and a discussion of the major opportunities to improve the
efficiency of nitrogen use
Updates to the CNCPS
The CNCPS is a combination of dynamic and static approaches to model both
ruminal and whole-body metabolism in the cow. The most dynamic part of the
CNCPS is the rumen submodel, which relies on the competition between rates of
degradation and rates of passage of different fractions in feeds. This results in
variable amounts of both carbohydrate and protein fractions degraded in the
rumen.
The major biological updates to the CNCPS are described in detail elsewhere
(Tylutki et al., 2008; Van Amburgh et al., 2007; Van Amburgh et al., 2009). Briefly,
these changes include expansion of the carbohydrate fractions, changes in passage
rate equations and assignments, changes in ruminal nitrogen accounting, several
corrections, and further updates to pool sizes, rates, and chemistries. The net result
of these changes is a model that is more robust in predicting on-farm responses and
more sensitive to predicted changes in metabolizable energy (ME) and
metabolizable protein (MP) allowable milk, especially at lower levels of dietary
protein.
Overview of nitrogen metabolism
Nitrogen transactions, both in the rumen and in the whole body of ruminants, are
complex. Figure 1 provides an overview of these pathways in the cow. Dietary
crude protein consists primarily of nonprotein nitrogen (NPN; major sources would
be silages and urea) and true protein. Nonprotein nitrogen typically is rapidly
degraded to ammonia in the rumen. True protein is degraded by rumen microbes at
varying rates to peptides, amino acids, and ammonia. Collectively, the sum of
dietary nitrogen that is degraded to peptides, amino acids, and ammonia is referred
to as Rumen Degradable Protein (RDP). Concurrent availability of ruminally
fermentable carbohydrate results in incorporation of these nitrogen sources into
microbial protein. True protein that is degraded at slower rates and thereby
escapes ruminal degradation because of passage to the lower gut is referred to as
Rumen Undegradable Protein (RUP).
At the level of the small intestine, the sum of microbial protein, RUP, and smaller
amounts of endogenous protein are referred to as Metabolizable Protein (MP). In
most lactating cow rations, microbial protein typically comprises 45 to 60% of MP
supply. Metabolizable protein is digested in the small intestine with absorption of
amino acids (AA) into the portal vein that supplies the liver. Metabolism of AA by
the gut tissues and by the liver is extensive (Lapierre et al., 2006). The liver takes up
AA both for its own protein synthetic needs and also catabolizes AA that are in
excess of those required for other synthetic processes, with the resulting conversion
of the nitrogen to urea and the carbon skeletons to either oxidative metabolism or
glucose. Circulating AA are used by the mammary gland for synthesis of milk
protein and by all tissues in the body for protein synthesis.
In the rumen, ammonia that is not captured as microbial protein is absorbed across
the rumen wall, extracted by the liver, and converted to urea. The urea that is
released by the liver is excreted in the urine and milk, and recycled back to the
gastrointestinal tract. The amount of nitrogen intake that is recycled back to the
gastrointestinal tract as urea is large (~40% of intake nitrogen) and has a wide
range (~10 to 60%; Van Amburgh, personal communication).
Quantitative aspects of nitrogen metabolism in dairy cows are well illustrated by a
study conducted by Raggio et al. (2004) that is described in Tables 1 and 2. They fed
increasing amounts of MP to cows, primarily through feeding increasing amounts of
an animal-marine protein blend, and measured nitrogen transactions across the gut
and liver. In this study, increasing MP supply increased both milk yield and milk
true protein yield. As expected, nitrogen (CP) intake was increased substantially;
however, fecal nitrogen excretion was increased only slightly as MP supply
increased. This corresponded to a large increase in digested nitrogen as MP supply
increased. Although some of the increased digestible nitrogen ended up in the milk,
the majority was excreted in the urine. The net release of ammonia absorbed from
the gut was essentially completely extracted by the liver and converted to urea, and
this amount was increased as MP supply increased. Accordingly, the amount of urea
released by the liver was increased as MP supply increased. Interestingly, the
amount of urea recycled back to the gut was decreased as MP supply increased.
Marini and Van Amburgh (2003) also reported that the proportion of nitrogen that
was recycled to the gut was decreased as protein intake in heifers was increased.
Opportunities to increase efficiency of N use
In the study conducted by Raggio et al. (2004), opportunities existed to take better
advantage of nitrogen recycling in the cow and improve the efficiency of nitrogen
use by feeding lower protein diets was clear. However, in that study milk protein
yield also was decreased as MP supply was decreased. Although full diet nutrient
composition was not reported in that study, we would speculate that the supply of
ruminally fermentable carbohydrate, and hence energy available to drive milk
protein synthesis, was low. Clearly, ways to maintain (or increase) milk protein
yield while taking advantage of opportunities to increase nitrogen recycling and
decrease urinary nitrogen loss by feeding less overall protein would be of interest
both from a profitability perspective in many cases and from an environmental
standpoint relative to decreased nitrogen excretion.
Recently, Cyriac et al. (2008) sought to determine whether feeding RDP at lower
than NRC (2001) recommended levels while maintaining RUP at similar levels
would affect lactational performance and efficiency of nitrogen use (Tables 3 and 4).
They determined that decreasing NRC-predicted RDP supply from 11.3% of DM to
8.8% of DM did not affect performance and increased the efficiency of nitrogen use
from 27.7 to 35.5%. Further decreasing the RDP level in diets to 7.6% of DM further
improved the efficiency of nitrogen use, but decreased yields of milk and milk
components. In contrast, Kalscheur et al. (2006) reported that milk and milk
component yield decreased as RDP content of the diet decreased from 11.0 to 6.8%
of DM; however, it appeared that RUP supply as predicted by NRC (2001) also
decreased as RDP decreased in that study, making it difficult to separate the effects
of RDP from RUP.
A number of other studies have been conducted during the past ten years that
indicate that we can reduce overall protein feeding through these and other
strategies to improve the efficiency of nitrogen and amino acid use. Leonardi et al.
(2003) determined that cows fed 16% CP diets had similar milk and milk protein
yields than cows fed 18.8% CP diets achieved through addition of soybean meal, but
better efficiency of nitrogen use. Noftsger and St. Pierre (2003) reported that
supplementing highly digestible RUP sources with both rumen-protected
methionine and 2-hydroxy-4-methylthiobutanoic acid (HMB) resulted in increased
milk and milk protein production and increased efficiency of nitrogen use in a 17%
CP diet compared to higher CP diets.
Over the past several years, our group (led by Larry Chase and Mike Van Amburgh
with an M.S. student named Ryan Higgs) has been working with commercial dairy
farms in New York to use these strategies and the CNCPS version 6.1 to decrease
overall protein feeding levels in herds. In general, the focus has been to decrease
overall levels of RDP in diets to the 8 to 9% of DM range and to use quality sources
of RUP and AA supplements in order to balance overall MP and AA needs. The net
result has been maintenance of milk and milk component yields and decreased feed
cost (~ $0.50/cow per day in several of these herds). We will continue working
with these herds and using this model to optimize the efficiency of nitrogen use on
farms with the goal of improved profitability and decreased environmental impact.
Summary
Improvements in the efficiency of nutrient use, particularly nitrogen use, will
continue to be important in the dairy industry both from the standpoint of input
cost and environmental impact. This means that integration of new understanding
of the biology of nutrient use into models such as the CNCPS will be important for
continued advances in these areas. The changes to CNCPS version 6.1 represent the
first major biological enhancement of this model in a number of years, and the result
is a model that has better sensitivity for prediction of ME and MP-allowable milk,
especially at lower ration total CP levels. Consequently, this provides opportunities
for nutritionists to more carefully evaluate nutrient inputs, with potentially
increased net milk income over feed cost.
References
Cyriac, J., A. G. Rius, M. L. McGilliard, R. E. Pearson, B. J. Bequette, and M. D. Hanigan.
2008. Lactation performance of mid-lactation dairy cows fed ruminally degradable
protein concentrations lower than National Research Council recommendations. J.
Dairy Sci. 91:4704-4713.
Kalscheur, K. F., R. L. Baldwin IV, B. P. Glenn, and R. A. Kohn. 2006. Milk production
of dairy cows fed differing concentrations of rumen-degraded protein. J. Dairy Sci.
89:249-259.
Lapierre, H., D. Pacheco, R. Berthiaume, D. R. Ouellet, C. G. Schwab, P. Dubreuil, G.
Holtrop, and G. E. Lobley. 2006. What is the true supply of amino acids for a dairy
cow? J. Dairy Sci. 89(E. Suppl.):E1-E14.
Leonardi, C., M. Stevenson, and L. E. Armentano. 2003. Effect of two levels of crude
protein and methionine supplementation on performance of dairy cows. J. Dairy Sci.
86:4033-4042.
Marini, J. C., and M. E. Van Amburgh. 2003. Nitrogen metabolism and recycling in
Holstein heifers. J. Anim. Sci. 81:545-552.
National Research Council. (2001). Nutrient Requirements of Dairy Cattle. 7th rev.
ed. Natl. Acad. Press, Washington, DC.
Noftsger, S., and N. R. St. Pierre. 2003. Supplementation of methionine and selection
of highly digestible rumen undegradable protein to improve nitrogen efficiency for
milk production. 86:958-969.
Raggio, G., D. Pacheco, R. Berthiaume, G. E. Lobley, D. Pellerin, G. Allard, P. Dubreuil,
and H. Lapierre. 2004. Effect of level of metabolizable protein on splanchnic flux of
amino acids in lactating dairy cows. J. Dairy Sci. 87:3461-3472.
Tylutki, T. P., D. G. Fox, V. M. Durbal, L. O. Tedeschi, J. B. Russell, M. E. Van Amburgh,
T. R. Overton, L. E. Chase, and A. N. Pell. 2008. Cornell Net Carbohydrate and Protein
System: A model for precision feeding of dairy cattle. Anim. Feed Sci. Tech. 143:174–
202.
Van Amburgh, M. E., T. R. Overton, L. E. Chase, D. A. Ross, and E. B. Recktenwald.
2009. The Cornell Net Carbohydrate and Protein System: Current and future
approaches for balancing of amino acids. Proc. Cornell Nutr. Conf. Feed Manuf.,
Syracuse, NY. pp 28-37.
Van Amburgh, M. E., E. B. Recktenwald, D. A. Ross, T. R. Overton, and L. E. Chase.
2007. Achieving better nitrogen efficiency in lactating dairy cattle: Updating field
usable tools to improve nitrogen efficiency. Proc. Cornell Nutr. Conf. Feed Manuf.
Syracuse, NY. pp 25-37.
Figure 1. Overview of protein metabolism in the gastrointestinal tract and whole
body of cows (Courtesy Dr. Charles G. Schwab).
Table 1. Composition of diets used to evaluate the effects of MP supply on wholebody nitrogen and AA metabolism (Raggio et al., 2004).
Treatments
Ingredients, % of DM
Low MP
Medium MP
High MP
Grass silage1
42.9
42.1
42.3
Soybean hulls
24.3
24.3
23.4
Corn grain, ground
14.8
14.8
14.3
Corn grain, cracked
7.8
7.5
7.5
Grass hay
3.2
3.1
3.1
Ca-PFAD
3.4
2.3
1.2
Animal-marine protein blend
0
2.8
5.4
Molasses, beet
1.3
1.3
1.3
Mineral/vitamin
2.1
1.7
1.5
NRC (2001) predictions2
NEL (Mcal/d)
36.4
36.4
36.4
CP, % of DM
12.7
14.7
16.6
RDP supply, g/d
2125
2307
2501
RDP balance, g/d
-162
4
182
RUP supply, g/d
907
1257
1602
RUP balance, g/d
-396
-58
317
Metabolizable protein, g/d
1922
2264
2517
MP – microbial, g/d
1156
1251
1261
MP – RUP, g/d
654
899
1140
MP – endogenous, g/d
112
114
116
1 Contained (DM basis) 17.4% CP, 33.1% ADF, 41.4% NDF, 5.2% lignin
2 Based upon actual study intakes (~ 24 kg/d) and production
Table 2. Effects of MP supply on production, nitrogen utilization, and dynamics of
ammonia and urea-N metabolism (Raggio et al., 2004).
Treatments
Item
Low MP
Medium MP High MP
Milk yield, kg/d *
33.9
35.7
36.2
Milk true protein yield, kg/d *
0.80
0.85
0.90
Nitrogen
Intake, g/d *
475
550
624
Feces, g/d *
222
226
246
Digested, g/d *
253
324
378
Urine, g/d *
79
122
165
Urine, % of intake (no stats)
16.6
22.2
26.4
Milk, g/d *
131
141
151
Milk, % of intake (no stats)
27.6
25.6
24.2
Tissue retain, g/d
41
59
59
Ammonia flux (mmol/h)
Gut (portal drained viscera) *
Liver *
Gut + Liver (total splanchnic)
265
-249
16
383
-361
22
396
-390
6
Urea flux (mmol/h)
Gut (portal drained viscera) *
Liver *
Gut + Liver (total splanchnic)
-404
881
476
-333
1202
869
-174
1258
1083
* denotes significant (P < 0.05) linear or quadratic effect of treatment
Table 3. Composition of diets used to evaluate effects of decreasing RDP supply
while maintaining RUP supply on performance and nitrogen utilization (Cyriac et al.,
2008).
NRC predicted RDP, % of diet DM
Item
11.3
10.1
8.8
7.6
Ingredient, % of DM
Corn silage
39.7
39.7
39.7
39.7
Mixed grass-legume silage
7.8
7.8
7.8
7.8
Whole cottonseed
2.9
2.9
2.9
2.9
Rolled high moisture shelled corn
15.5
15.5
15.5
15.5
Soybean hulls
9.7
11.4
13.0
14.7
Soybean meal
20.4
13.6
6.8
0
Protected soybean meal
0
4.1
8.3
12.4
Corn grain, ground
0.6
1.3
2.0
2.7
Tallow
0.9
1.2
1.5
1.8
Mineral/vitamin
2.6
2.6
2.7
2.7
Composition and NRC estimate
CP, % of DM
18.4
16.8
15.2
13.6
NDF, % of DM
31.4
32.8
34.1
35.4
ADF, % of DM
18.6
20.0
21.4
22.8
NEL, Mcal/kg
1.6
1.6
1.6
1.6
RDP supply, g/d
2611
2328
2045
1762
RDP balance, g/d
301
9
-282
-574
RUP supply, g/d
1646
1648
1649
1651
Table 4. Performance and nitrogen utilization in cows fed diets to decrease RDP supply
while maintaining RUP supply on performance and nitrogen utilization (Cyriac et al., 2008).
NRC predicted RDP, % of diet DM
Item
11.3
10.1
8.8
7.6
DMI, kg/d *
24.1
23.9
23.2
20.4
Milk yield, kg/d *
41.2
42.1
40.3
36.6
Milk CP, %
2.98
3.00
3.01
2.92
Milk CP, kg/d *
1.23
1.26
1.21
1.07
MUN, mg/dL *
20.2
17.6
14.2
12.4
NRC-predicted NEL allowable milk, kg/d
42.0
43.2
41.3
35.4
NRC-predicted MP allowable milk, kg/d
46.0
44.4
37.8
29.3
Nitrogen
Intake N, g/d *
719
613
Milk N, g/d *
197
191
Predicted urine N, g/d *
350
304
N efficiency (Milk/intake), % *
27.7
30.9
* denotes significant (P < 0.05) linear or quadratic effect of treatment
544
193
248
35.5
453
169
210
38.6