Farm Animal Metabolism and Nutrition

hepatocytes from fasted sheep using^15 N-
labelled substrates have shown, however,
that amino acid N is not essential for urea-
genesis (Luo et al., 1995). The mechanism
for the interaction between NH 3 and amino
acid deamination is not proven and
requires further investigation.

Amino Acid Supply and Utilization

by the Mammary Gland

Lactation imposes increased demands for
protein synthesis, and the onset of lacta-
tion is accompanied by a shift in protein
synthesis away from non-mammary tissues
towards the mammary gland (Champredon
et al., 1990; Baracos et al., 1991). This
increase in protein synthesis is balanced by
an increase in the supply of amino acids to
the gland of between 20 and 40% (average
across all amino acids, 25%; Bequette et
al., 1997). The major shift in protein
metabolism during lactation is emphasized
from studies in high-yielding dairy cows in
which it was demonstrated that milk
protein production may account for >80%
of digestible protein intake (Clark et al.,
1978). The rate of milk production by the
mammary gland is dependent on the com-
bined influence of changes in blood flow
and the concentration of specific nutrients
in the blood. The fact that in some instances
these may be inadequate to maintain maxi-
mal rates of milk synthesis has been demon-
strated in several studies in which either
peripheral intravenous or post-ruminal infu-
sion of amino acids has resulted in
increased milk protein output (Clark, 1975;
Bequette and Backwell, 1997). Lysine and
methionine are the first limiting amino
acids for milk protein synthesis under nor-
mal conditions, and it is provision of these
amino acids which may result in the
observed increase in milk protein output in
these studies. These data are not consistent,
however, and many studies have shown
either a small or no response to infusion of
individual amino acids (see, for example,
Mepham and Linzell, 1974; Metcalf et al.,
1991; Bequette et al., 1996; Vanhatalo et al.,
1999; Varvikko et al., 1999).

The control of nutrient supply, and in

particular amino acid supply, to the

mammary gland in non-ruminant animals

is poorly understood in comparison with

that in ruminant species, although it is

recognized that the imbalances between the

supply of individual amino acids to the

gland and those found in milk in

ruminants also exist in the pig (for a review,

see Trottier, 1997). Measurements of milk

output in non-ruminants is not easy, and

this has been a major factor in limiting

research in this area. However, there is con-

siderable need for improved knowledge on

the relationship between physiological and

nutritional factors influencing lactational

performance in non-ruminant farm animals

in order to maximize performance and

hence productivity.

The importance of low-molecular weight

peptides in amino acid supply to the

mammary gland

Several studies have demonstrated that on

a net basis the extraction of non-essential

amino acids by mammary tissue is

insufficient to account for their output in

milk protein. This suggests either the de

novosynthesis of amino acids within the

gland, or the utilization of the amino acid

components of low-molecular weight

peptides. There is no convincing evidence

that synthesis of individual amino acids

can account for this difference, since the

exchange of C, N and S components

between amino acids (for example, S from

methionine and C from serine for cysteine

synthesis) involves amino acids which may

themselves become limiting or are already

not extracted in adequate amounts in the

free form. Backwell et al. (1996) used a

dual-labelled tracer technique to demon-

strate that the arterially infused peptides

glycyl-phenylalanine and glycyl-leucine

were used as substrates for casein synthesis

in goats. In a further series of tracer

studies, they have shown that naturally

occurring low-molecular peptides can be

used in milk protein synthesis and that

there is some degree of substrate selectivity

56 C.J. Seal and D.S. Parker