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

Physiological factors affecting sheep milk quality
Age and parity
Maiden ewes produce less milk than older ewes (Boyazoglu 1963; Casoli et al. 1989;
Hatziminaoglou et al. 1990; Ubertalle et al. 1990; Giaccone et al. 1992) and maximum yields
are generally achieved at the third or fourth lactation, after which total lactation yields tend to
decrease (Boyazoglu 1963; Ozcan and Kaimaz 1969; Casoli et al. 1989; Giaccone et al. 1992).
These order-of-parity factors are often confounded with the age of the ewes, so that it is not
possible to distinguish between the two. There are contrasting literature reports on the effect of
parity on milk composition. For European dairy ewes, the parity of the ewes affects milk composition:
with increasing lactation number the milk contains higher concentrations of fat and
protein (Olivetti et al. 1988; Casoli et al. 1989; Boylan and Sakul 1989; Pulina 1990; Giaccone
et al. 1993; Dell’Aquila et al. 1993), higher somatic cell counts (Olivetti et al. 1988; Pulina et al.
1990a; Bergonier et al. 1994) and lower concentrations of lactose (Casoli et al. 1989; Pulina
1990).
For the Australian Merino, (Corbett 1968) reported that the concentration of fat is higher in
older than younger ewes, but milk production does not differ with age. By contrast, Wohlt et al.
(1981) reported that the age of the ewes, sibling status and sex of lamb had no effect on the milk
composition.
Since the negative relationship between yield and milk quality has been confirmed to apply
to individual animals within a flock (Barillet et al. 1986) changes in milk yield brought about by
age and lactation number are likely to result in changes in the composition of the milk as reported
by Pulina (1990) and Pulina et al. (1992).
Stage of lactation
The stage of lactation markedly affects the amount of milk produced. Lactation begins at
parturition and daily yields increase rapidly for the first few weeks. Peak yields are achieved
around the third to fifth week of lactation (Horak 1964; Torres-Hernandez and Hohenboken
1980; Geenty 1980b; Cappio-Borlino et al. 1989; Bencini and Purvis 1990; Reynolds and Brown
1991; Bencini et al. 1992). After the peak, lactation declines more or less rapidly depending on
the breed and genotype and on individual dairy potential. The trends for the concentrations of
fat, protein, lactose and totals solids during the course of lactation are very similar to those
reported by Holmes and Wilson (1984) for dairy cows, by Pulina (1990), Pulina et al. (1992) for
Sarda ewes, by Fadel et al. (1989) for Awassi ewes and by Bencini and Purvis (1990) and
Bencini et al. (1992) for Merino and Awassi x Merino ewes in Australia. The concentrations of
fat, protein (both casein and whey protein) total solids and somatic cells are high at the beginning
and at the end of the lactation, and low at peak lactation, while the concentration of lactose
follows closely the lactation yield. The mineral content of milk also is affected by the stage of
lactation: throughout lactation there is an increase in chloride (Pauselli et al. 1992) and magnesium
and a reduction in potassium (Polychroniadou and Vafopolou 1984).
The processing performance of the milk also changes with the stage of lactation. As lactation
proceeds the clotting properties of the milk tend to worsen, with an increase in renneting
time and rate of curd formation and a decrease in the consistency of the curd (Ubertalle 1989;
1990).