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

In dairy cows high concentrations of MU and BU have been associated with reductions in
reproductive efficiency (Ferguson and Chalupa, 1989). Similar findings have been reported in ewes
by Bishonga et al. (1994), who found markedly negative effects on sheep embryos cultivated in vitro
when MU was equal to or higher than 43 mg/dl of urea, and by Molle et al. (2001), who reported
decreased fertility when BU was above 45 mg/dl. High concentrations of MU have been also
associated with increased incidence of other disturbances (Table 11).
Milk sampling to measure milk urea
Since MU concentration shows a considerable variability among animals fed the same
diet, it should always be measured in pooled samples of milk taken from at least 8 to 10 sheep in
the same stage of lactation.
First lactation ewes, due to their higher protein turnover and higher consumption of
amino acids as energetic fuels to sustain growth, have 5-10 mg/dl higher MU concentration
compared to mature ewes (Cannas, 2004).
The differences in MU concentrations obtained at different moments of milking are small
and non-significant in practical terms. Thus, if individual milk samples need to be collected for
MU analyses, the first milk produced by the ewes can be used.
Milk urea can be measured in the laboratory by various very accurate analytical methods.
It is important to pay attention to the units of measurements used. In some countries milk urea is
expressed as "milk urea nitrogen". Since nitrogen makes up 46.65% of the urea molecule, when
the analyses are reported as "milk urea nitrogen" the values are about one half than when they are
expressed as "milk urea", as it is done in this publication.
Milk urea can be accurately measured on farm using accurate portable analyzers available
in the market. Tests based on strips for cow milk may bring to very erroneous estimates when
applied to sheep milk (Cannas, 2004).
Reference values of milk urea in sheep
Based on the previous discussion on milk urea and optimal dietary protein concentration,
it is likely that MU higher than 40-50 mg/dl is associated with excess dietary protein and lower
reproductive efficiency, while values lower than 25 mg/dl are associated with insufficient dietary
protein and low milk production. These values are higher than those suggested for dairy cattle.
Indeed, when the CP concentration of the diet was the same, MU concentrations were 5-15 mg/dl
higher for dairy ewes than for dairy cows, probably because the level of intake in sheep is
generally higher than in cattle (Cannas, 2004). At the same time, based on reproductive studies
mentioned above, dairy sheep seem to be more resistant to negative effects of N excesses than
dairy cattle.
When the sward is young and its CP concentrations are high, MU concentrations higher
than 60 mg/dl are often found in sheep. These levels are certainly associated with a large excess
of protein intake, poor health status and reproductive efficiency, and increases in energy
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