Great Lakes Dairy Sheep Symposium - the Department of Animal ...

Milk fat (%)
Milk fat (%)
14
12
10
8
6
4
2
14
12
10
8
6
4
2
y = 0.01x + 8.02
R 2 = 0.00
30 35 40 45 50 55
Dietary NDF (% DM)
y = 0.001x + 7.846
R 2 = 0.007
0 100 200 300 400 500
Milk yield (g/d)
Figure 5 - Relationship between milk fat concentration and dietary and animal characteristics for
ewes that produced less than 400 g/d of milk.
The concentration of urea in the milk as a nutritional indicator
As for dairy cows, milk urea (MU) is an excellent nutritional indicator of the protein
status of dairy sheep. Milk urea originates from the diffusion of blood urea (BU) into the
mammary gland. Indeed, MU is closely correlated with BU concentration. However, MU is more
stable (Baker et al., 1992) and easier to sample than BU.
In ruminants, urea is formed by the liver using ammonia as a major substrate. This process is
energetically very costly (e.g. the energy cost of eliminating 100 g of CP in excess of sheep
needs is the same as that of producing 200 g of sheep milk). In some cases, this process
remarkably affects total energy requirements. The two major sources of ammonia, and thus of
MU, are:
a) the ammonia formed in the rumen by bacteria fermentation of nitrogen sources (protein
and non protein nitrogen, NPN) that is not used by bacteria for protein synthesis. The
ammonia in excess is absorbed by the rumen wall and then carried to the liver, where it is
converted in urea; and
b) the ammonia formed as a by-product in the process of gluconeogenesis, when amino acids
are converted to glucose, or in processes of amino acids catabolization.
Despite the importance of MU as a nutritional indicator in dairy cows, systematic studies on
its use in dairy sheep are recent. In one of the first studies published on this topic, Cannas et al.
104
Milk fat (%)
Milk fat (%)
14
12
10
8
6
4
2
14
12
10
8
6
4
2
y = -0.96x + 9.27
R 2 = 0.04
0.5 1.0 1.5 2.0 2.5
NDF intake (% BW)
y = -1.14x + 8.59
R 2 = 0.06
-0.5 0.0 0.5 1.0 1.5
Energy balance (UFL/d)