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

Table 1. Efficiency factors for use of amino acids for maintenance and lactation with
indices of variation provided for mammary gland efficiencies of use.
Maintenance
efficiency of
Lactation (mammary gland utilization)
Standard
Deviation
Coefficient of
Variation
AA
Efficiency of
use
use
Met 0.85 1.00 0.10 10%
Lys 0.85 0.86 0.13 16%
His 0.85 0.94 0.13 14%
Phe 0.85 0.98 0.08 8%
Trp 0.85 0.85 NA NA
Thr 0.85 0.79 0.14 18%
Leu 0.66 0.71 0.07 10%
Ile 0.66 0.66 0.07 11%
Val 0.66 0.62 0.09 14%
Arg 0.85 0.34 0.06 17%
Source of Energy and Metabolic Regulation
Data generated over the last 15 years demonstrate that the site of absorption of
starch and sugars, and the form of substrate, either glucose in the small intestine or
propionate, might impact the animal response to the nutrient and potentially the
endocrine signaling in a manner that could affect the efficiency of use of absorbed AA
(Knowlton et al., 1998, Rulquin et al., 2004; Raggio et al., 2006, Reynolds, 2006;
Lemosquet et al. 2009). These data suggest that as starch is digested in the small
intestine, the transfer of glucose into the circulation increases, but the efficiency of
transfer of the glucose carbon into lactose production decreases, thus decreasing the
efficiency of use of absorbed AA. Reynolds (2006) summarized several studies and
concluded that although there is an increase in glucose availability, the tissues drained
by the portal vein appear to preferentially utilize the glucose and in addition retain
slightly greater body protein. There is an apparent increase in milk volume, but milk fat
content decreases so that the energy output of the cow is not increased. With
increased post-ruminal starch digestion there is a change in insulin status, but under the
conditions study to date, it does not translate into greater milk protein output. Currently
in the CNCPS, although the model calculates the amount of starch digested in the
rumen and small intestine, there is no provision for changes in how the model treats
energy absorbed as glucose in the small intestine versus energy yield from ruminal
digestion. To enhance the sensitivity of the model, we need to make adjustments for
post-ruminal starch digestion that describes the fate of that energy in a more
mechanistic approach – this should allow us to enhance the prediction of efficiency of
use of absorbed AA.
Efficiency of use of AA is a combination of the energy allowable production and then
the ability to balance the profile and supply of AA to meet the energy driven demand. It
is well known that particular hormones such as insulin, IGF-I, and cortisol to name a few
have a profound impact on protein synthesis in the mammary gland, thus can alter the
33