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Protein in Pastures: Can It Be Too High?
At times the crude protein content of pasture
can be too high and can be detrimental to cow
performance. Strategic feeding of concentrate
supplements that contain the proper types and
amounts of rumen fermentable carbohydrates
can improve utilization of excess protein and
improve milk yield.
The nutrient quality of well-managed pastures is often higher
than the same plant material harvested as silage or hay.
Well-managed spring and autumn pastures may be 25% crude
protein or even higher. On some occasions, we have observed
30% crude protein in the spring. The protein content usually
decreases during the warmer months and fiber content tends to
decrease.
Is the Protein Content of Pasture
Too High?
The topic of high protein in pastures has been discussed at
various meetings. With high quality pastures often containing
25% protein or higher, even supplementation with low protein
supplements will not reduce the total ration crude protein (CP)
down to the suggested 16 to 18%. Not only is the total protein
too high, but the protein in pasture is highly degradable in the
rumen (70 to 80% of CP) and not efficiently utilized in the
rumen.
Grains and supplements that are high in readily fermentable
carbohydrates need to be fed to enable rumen microbes to
“capture” this protein and synthesize microbial protein, which
is ultimately utilized by the dairy cow.
In addition, the rumen undegradable protein (RUP), and the
protein and amino acids reaching the small intestine may be
inadequate to meet the requirements of early lactation, high
producing cows.
The consequences of this excess total and rapidly degradable
protein in pasture and inadequate amounts of supplemental
fermentable carbohydrates include:
•
•
•
•
fast nutrient passage through the rumen
loose manure
reduced milk fat percent
loss of body condition
• less than optimum milk production
When excess protein is fed, the protein is deaminated by
rumen microbes to ammonia in the rumen. When inadequate
dietary carbohydrates are available to “capture” the ammonia,
the ammonia is absorbed from the rumen. High degradability
of the protein in pasture can lead to losses of up to 50% of the
ammonia-nitrogen from the rumen at high pasture intake. This
ammonia is converted to urea in the liver to detoxify the
excess ammonia. The metabolic costs associated with
absorption of ammonia and detoxifying ammonia to urea
require energy. This is commonly referred to as urea cost, or
the energy that is used to excrete the excess ammonia from
high protein diets. In turn, this energy is not available for milk
production, and results in less than optimal animal
performance.
Urea Cost – Milk Loss
The energy required for this “urea cost” may result in 3 to 6 lb
less milk/cow/day. Pastures with 25% to 30% crude protein
instead of 20% can result in this amount of lost milk
production. The large milk production loss often seen when
cows move to pasture in the spring may be associated with this
urea cost. Using nutrition models, up to 9 lb/cow/day lost milk
yield may occur in some situations with high producing cows.
This process results in inefficient utilization of pasture protein,
with perhaps only 16 to 18% of the dietary nitrogen appearing
in milk. The excretion of over 80% of the dietary protein is a
loss of the most expensive nutrient and is harmful to the
environment. Strategic supplementation can increase dietary
protein utilization (feed to milk) to about 25%.
Milk Urea Nitrogen
The urea that is produced in the liver from excess rumen
ammonia eventually appears in milk as milk urea nitrogen
(MUN). Therefore, MUN testing can help monitor excess
dietary protein and inadequate dietary carbohydrates. When
cows graze pasture as the major forage, the MUN values are
generally higher than with a non-grazing feeding program.
However, studies at Cornell and Penn State Universities that
monitored numerous grazing herds, all of which were feeding
supplemental concentrates and forages, found MUN values of
14 to 15 mg/dl. These were not excessive since we consider
values above 16 to signal excess dietary protein in relation to
dietary carbohydrates.
In a recent study at Penn State, we found somewhat elevated
MUN in cows fed only pasture (Table 1). Milk yield was 20
lb/day higher when fed 19 lb of concentrate, indicating a
greater than 1 lb milk per 1 lb concentrate fed. Milk fat
percent was lower and milk protein percent was higher with
supplementation. The MUN was nearly 3 mg/dl lower (14.1
vs. 11.3) with supplementation suggesting improved
utilization of dietary protein.
a Bargo et al., 2002. J. Dairy Sci. 85:1777–1792. b Cows fed
19 lb concentrate supplement per day.
Item
Unsupplemented
Supplemented b
Milk, lb/cow/day
45.3
65.5
Milk fat, %
3.80
3.30
Milk protein, %
2.96
3.10
MUN, mg/dl
14.1
11.3
Table 1. Milk production of unsupplemented and supplemented
Holstein cows grazing cool season grasses with 20% crude
protein during 6-month grazing season. a
After fertilization with nitrogen (N) from urea, the total and
rumen degradable protein in pasture often increases. In turn,
the MUN increases because of this higher protein intake. In a
recent study at Penn State, we found elevated crude protein in
pasture (25 to 30%) about two weeks following each of four N
fertilizations (Figure 1). In turn, the MUN reached about 18 to
22 mg/dl, a significant increase compared to the MUN prior to
the N fertilization. Clearly, these high MUN values, which
occurred following application of 50 lb nitrogen fertilizer/A,
indicate a high loss of protein in the rumen and a high “energy
cost” to excrete the excess urea.
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Figure 1. The crude protein % of pasture and the weekly milk
urea nitrogen (MUN) content in relationship to four fertilization
times (indicated by arrows) with 50 lb N/acre from urea.
Milk production typically declined after each fertilizer
application. The use of N fertilizers other than urea may have
minimized this situation. These data suggest that N
fertilization should occur more than two weeks prior to the
cows grazing the pasture. Monitoring of MUN in milk can be
a useful tool to monitor the dietary protein and energy with a
pasture-based system.
Is Dietary Protein Supplement
Needed?
From this discussion, we could conclude that concentrate
supplements with proper amounts and types of nonfiber
carbohydrates will improve the utilization of the protein in
pasture and that the concentrate should contain little additional
protein. Grains which supply readily fermentable
carbohydrates to “capture” some of this ammonia are most
important. We generally recommend the concentrate
supplements for primarily pasture-based forage program
contain about 12 to 14% total protein. This will provide a total
ration of about 16 to 18% CP. This supplement may require
the addition of a small amount (0.4 to 0.8 lb) of a protein
supplement. This supplement can provide some peptides and
amino acids that are required by rumen microorganisms.
Protein in Pastures: Can It Be Too High?
Do we need to supply RUP to high
producing cows?
The National Research Council nutritional guidelines (2001)
indicate that rumen undegradable protein is often needed for
cows producing more than above 70 to 80 lb of milk/day.
Research studies and field experiences do not report a
consistent response to increasing the RUP in supplements. A
Penn State study found that small amounts (0.5 to 1.0 lb/day)
of RUP increased milk protein yield in multiparous cows
averaging 80 lb milk/day and fed pasture as the primary
forage. Protein sources such as brewers, distillers, corn gluten
meal, and roasted/cooked soybeans are good sources of RUP
and 0.5 to 1.5 lb of RUP should be considered with grazing
cows producing greater than 70 to 80 lb of milk. Costs need to
be considered. However, energy is the first limiting nutrient
and must be supplemented to the rumen to optimize the
capture of nitrogen from pasture and to optimize rumen
microbial production.
Summary
To answer the question in the title; yes, the crude protein
content of pasture can be too high and be detrimental to cow
performance due to the energy required to excrete the N that is
not utilized in the rumen. Strategic feeding of concentrate
supplements that contain the proper types and amounts of
rumen fermentable carbohydrates can improve the utilization
of this excess protein and improve milk yield. In addition,
improved utilization of the high protein in high quality pasture
may improve reproductive efficiency.
Published as pages 71-74 in proceedings from "Nutrition of
Dairy Cows on Pasture-Based Systems" held March 31, 2003
in Grantville, PA.
Contact Information
Lawrence D. Muller
Professor Emeritus
Penn State College of Agricultural Sciences research and extension programs
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Protein in Pastures: Can It Be Too High?