The 1989 NRC says that a 1400 lb. cow producing 90 lbs. milk per day needs 9.032 lbs. of crude protein.
| Maintenance | .932 lbs. | |
| Production | 8.100 lbs. | |
| Total required | 9.032 lbs. | |
If this cow produces milk that has 3.2% protein, that is 2.88
lbs. of protein produced per day or 31.9% of the required protein.
Therefore at higher productions, approximately 1/3 of the required
protein is excreted in the milk. That leaves 2/3's that is available
for other biological processes or excreted in the feces as indigestible.
Of the 9 lbs. of protein required it is estimated that 3 to 3.5
lbs. can be supplied by rumen microbes that are digested in the
small intestine. A common guideline for lactating cows is for
60 to 65% of the protein to be in rumen degradable form with about
1/2 of that soluble. That means 35 to 40% would be resistant to
degradation.
Supplying rumen resistant protein to lactating dairy cows has
met with varying degrees of response. Some of the reasons for
lack of response are:
1. Excessive resistance to breakdown limits microbial protein production,
2. resistant protein is relatively indigestible and passes in manure, and
3. the resistant protein has a poor essential amino acid profile.
If there is not enough degradable and soluble protein or nitrogen
in the rumen, microbial protein production can be limiting. This
is something we want to avoid. We want to maximize the amount
of microbial protein produced. That means the proper balance of
rumen available energy and protein. Also, rumen resistant protein
may tend to be less digestible and therefore not supply amino
acids for metabolic use. Finally, if the essential amino acids
are low in the protein that is available for absorption this can
limit the ability of that protein to support greater levels of
milk production.
The amino acids that are usually most limiting are lysine, methionine,
and arginine. The ratio of needed lysine to methionine is approximately
3:1. The Cornell Model indicates that for every 10 lbs. of milk,
12 grams of lysine are needed and 4 grams of methionine. Remember,
we do not have good guidelines for actual requirements and these
are only estimates.
One common way to look at the quality of protein sources is to
compare amino acid content relative to milk. The following compares
lysine, methionine, and arginine in some common protein sources
relative to what is contained in milk. Calculations are from Chandler,
1989 (Feedstuffs 61(26):14).
| LYS | MET | ARG | |
| Milk | 100 | 100 | 100 |
| Rumen microbes | 100 | 97 | 79 |
| Blood meal | 91 | 45 | 33 |
| Brewers grains | 34 | 78 | 53 |
| Corn gluten meal | 18 | 100 | 36 |
| Distiller grains with solubles | 24 | 81 | 42 |
| Feather meal | 13 | 23 | 32 |
| Fish meal | 80 | 100 | 59 |
| Meat & bone meal | 55 | 49 | 76 |
| Soybean meal | 70 | 56 | 89 |
Relative to the other sources rumen microbes are an excellent
source of essential amino acids and contains both lysine and methionine.
Feeds such as feather meal have a poor amino acid profile. Other
feeds are low in certain ones and high in others. Corn gluten
meal is very low in lysine but has a lot of methionine. Blood
meal is low in methionine but high in lysine. Fish meal has a
good balance between lysine and methionine.
Looking at selected protein sources without putting them into
rations can be misleading. Schingoethe in 1991 (Feedstuffs, March
18 p. 11 &12.) formulated rations that contained 25% alfalfa
silage, 25% corn silage, and corn as the energy source. He considered
the degradability of each of the feeds in the diet and calculated
the amount of the first limiting amino acid relative to what is
in milk.
| Protein Score | First | Second | Third limiting | |
| Blood meal | 70 | ISO | MET | VAL |
| Brewers grain | 75 | HIS | MET | LYS |
| Corn gluten meal | 70 | LYS | HIS | VAL |
| Distillers grains w/ solubles | 80 | HIS | LYS | MET |
| Feather meal | 56 | HIS | MET | LYS |
| Fish meal | 81 | LEU | VAL | HIS |
| Meat & bone meal | 75 | HIS | MET | LEU |
| Soybean meal | 79 | MET | HIS | VAL |
Notice that the first limiting amino acid relative to milk (Protein
Score) is much higher than when individual protein sources are
evaluated. That is because the rumen microbes, forages, and corn
are all figured in the calculations. In other words, there is
a buffering effect of the microbial protein plus other ingredients
in the formulation. This method considers only one protein source
at a time and does not consider combinations of the above sources.
However, it is possible to look at the table and see which sources
result in similar limiting amino acids. Notice that several have
methionine as one of the first 3 limiting amino acids (blood meal,
brewers grains, distillers grains, feather meal, meat & bone
meal, and soybean meal). Brewers grain, corn gluten meal, distillers
grains, and feather meal have lysine as one of the first 3 limiting.
Fish meal has neither methionine or lysine as limiting.
I think this exercise points out the importance of having blends
of proteins in feeds rather than relying on one feed to supply
the majority of the supplemental protein. We then should consider
type of feed. Avoid using large amounts of corn based protein
sources (corn gluten feed or meal and distillers grains) when
corn silage is the main forage source and corn is the energy source.
Soybean meal is first limiting in methionine and this should be
considered when using other protein sources.
Finally, most of the feeds listed above have ration maximums.
The following are some commonly accepted restrictions.
| % of Concentrate | lbs./cow/day | |
| Blood meal | 3 | 1 |
| Brewers grainsdry | 25 | 7 |
| Corn gluten feed | 30 | 10 |
| Corn gluten meal | 3 | 1.5 |
| Distillers grains with solubles | 25 | 7 |
| Feather meal | 6 | 2 |
| Fish meal | 3 | 1 |
| Meat and bone meal | 6 | 2 |