Dairy Sheep Symposium - the Department of Animal Sciences ...

An important aspect of dietary fibre is related to its dimension as it affects both chewing time
and rate of passage through the rumen: Cannas (1995) observed that in sheep fed hay and concentrates,
for a given concentration of NDF, a reduction in the particle size of hay resulted in a
reduction in chewing time and an increase in feed intake. As a result the digestibility of DM and
NDF decreased, but the amount digested per day was not affected. The composition of volatile
fatty acids in the rumen showed increased concentrations of propionate and butyrate and a stable
concentration of acetate. As a consequence there was an increase in milk and milk protein yield.
Excessively high doses of concentrates can reduce the intake of fibre and therefore reduce
chewing times and rumen pH. This can depress milk production and reduce the concentration of
fat in milk (Oddy 1978; Chiofalo et al. 1993) probably because they cause rumen acidosis (Rossi
et al. 1988).
Various workers (Perez Hernandez et al. 1986; Rossi et al. 1991; Horton et al. 1992; Sklan
1992; Chiofalo et al. 1993) have shown that feeding protected fat to ewes increases the fat
concentration in their milk, confirming previous findings in the dairy cow (Palmquist and
Jenkins 1980; Dell’Orto and Savoini 1989). This is accompanied by a reduction in milk protein
due to a reduced capacity of the mammary gland to utilize amino acids (Cant et al. 1993). However,
protected fat added to the diet has no effect on the relative proportion of nitrogenous
compounds (Campus et al. 1990) and on the clotting properties of the milk (Campus et al. 1990;
Chiofalo et al. 1993). Bertoni (1992) cautioned against the inclusion of protected fat in the diet
of dairy ewes because it is expensive and fat concentration in sheep milk is already high, so there
appears to be no need of increasing it further.
The protein content of the diet affects the quantity and the partition of nitrogenous substances
in the milk: Calderon-Cortes et al. (1977) reported that milk protein was significantly reduced if
ewes were fed a protein deficient diet. Robinson et al. (1974), Calderon-Cortes et al. (1977),
Robinson et al. (1979), Cowan et al. (1981) and Pulina et al. (1995) all have shown that milk
yield and concentration of milk fat can be increased by increasing the protein content of the diet.
However, in the study of Pulina et al. (1995) the increase in milk fat was accompanied by a
decrease in milk protein, which worsened the processing performance of the milk. By contrast
Sinclair and Gooden (1990), Lynh et al. (1991) and Rossi et al. (1991) reported that high concentrations
of protein in the diet can increase the concentration of protein in milk, together with
non protein nitrogen (Pulina et al. 1990a), and especially urea (Cannas et al. 1995) which results
in a poorer processing performance of the milk.
Increases in dietary protein was generally accompanied by increases in food intake. The
consequent increase in milk production masks the increased synthesis of protein in the mammary
gland, and the concentration of protein in the milk does not change (Robinson et al. 1974;
Cowan et al. 1981; Penning et al. 1988; Frey et al. 1991).
McHattie et al. (1978) showed that supplementing lactating ewes with fishmeal increased
both their milk yield and the protein concentration in the milk. This finding was later confirmed
by Gonzales et al. (1982; 1984), but contradicted by Hadjipanayiotou (1992), Vincent et al.
(1988), Ngongoni et al. (1989) and Limandis et al. (1992). These contradictory results may have
been due to the variable rumen degradability of the fishmeals used by different researchers.
Recently Chiofalo et al. (1993) found a significant reduction in milk protein of sheep fed high
doses of starch and sugars, in agreement with similar findings of Murphy and O’Hara (1993) for
dairy cows.