2009 Proceedings of the Cornell Nutrition Conference For Feed ...

2002; Volden et al., 2002; Hedvquist and Uden, 2006; Reynal et al. 2007), thus, to
account for the AA profile of these peptides, we need to provide an AA profile for the
soluble pool. This is currently being done by mathematical manipulation of the pools
and rates but a more robust approach is needed to account for more variation in the
predicted AA flow.
Amino acid supply from protozoa
The CNCPS does not have a protozoa pool within the rumen submodel, however it
is now known that from 5% to at least 20% of the total AA flows from the rumen are
from the protozoa (Shabi et al., 2000; Sylvester, et al. 2005; Karnati et al., 2007). The
CNCPS currently calculates that the protozoa consume 20% of the estimated bacterial
yield, thus the Ymax estimation is reduced from 0.5 g of bacteria per g of carbohydrate
per hour to 0.4 g. However, there is really no provision for the ultimate fate of this
bacterial growth and results in an estimate of bacterial yield that is static and ignores
protozoa metabolism and any AA yield from the protozoa. There are enough data
available from the work of Jeff Firkins group and many studies prior to that to engage in
a remodeling of the rumen submodel to include the protozoa and develop a true
microbial growth model that then drives yield based on the liquid and solid passage
rates. This would be a very different rumen submodel than we currently have and would
allow for the development of a volatile fatty acid model which is needed if we are to
effectively predict the substrates available for milk production that differentiates how the
cow utilizes those nutrients. Some of this will be addressed by Recktenwald and Van
Amburgh in during this conference.
It is important to recognize that protozoa have a different AA profile, especially with
respect to methionine and lysine. The methionine content of protozoa is lower than that
of bacteria (24.0 vs 28.4 g/kg of total AA, whereas the lysine content of protozoa is
significantly greater than bacteria (121.4 vs 90.3 g/kg total AA) (Shabi et al., 2000). This
suggests that under certain formulation conditions, if protozoa were included in the
prediction of AA flow, lysine might not be as limiting an AA provided protozoal growth
and escape made up a significant portion of the MP supply. Protozoa are lower in
BCAA content, thus potentially creating conditions where those AA are more limiting.
Absorbed Amino Acids – Efficiency of Use
Coefficients for the efficiency of individual AA use for pregnancy and lactation have
been updated from the original values provided in O'Connor et al. (1993). The revised
coefficients for the efficiency of individual AA use for lactation were calculated from
summarized data for uptake/output of individual AA by the mammary gland in
experiments using dairy cattle (Cant et al., 1993; Clark et al., 1977; Erickson et al.,
1992; Guinard and Rulquin, 1995; Hanigan et al., 1992; Lykos and Varga, 1997; Mackle
et al., 2000; Metcalf et al., 1996; Spires et al., 1975).
Efficiency factors for use of individual essential AA that are utilized in CNCPS are
provided in Table 1. Of note is the substantial standard deviation associated with the
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