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

number of problems connected to the out-of-season breeding of their dairy flocks. First, only a
proportion of ewes will mate out-of-season, so most of the lambings are concentrated in winter.
Second, the availability of good pasture during summer is low, especially in Australia, and those
sheep that have lambed tend to produce less milk. As a consequence severe shortages of milk
occur during the summer months, when the demand for fresh sheep milk yoghurt is highest.
The differential nutrition also changes the processing performance of the milk and consequently
the quality of the derived dairy products. So, it is difficult to maintain a constant and
consistent production throughout the year. These problems have not been addressed so far and
are a source of uncertainty in the Australian and New Zealand sheep dairy industries.
Use of hormones
Growth hormone increases milk production in dairy cows (Peel and Bauman 1987; Zinn and
Bravo-Ureta 1996). This occurs also in dairy sheep, where the increase in milk production is
accompanied by increases in milk fat content and reductions in milk protein, which is not desirable
for cheese making (Fleet et al. 1986; 1988; Holcombe et al. 1988; Pell et al. 1989; Sandles
et al. 1988; Stelwagen et al. 1993). In any case, the use of hormones to increase milk production
is not allowed in Europe and Australia.
Nutrition
Since nutrition affects both the yield and the composition of the milk produced, it also affects
the quantity and quality of cheese. Kervina et al. (1981), Treacher (1983), Robinson (1988)
Mills (1989) and Gilbert (1992b) have written practical recommendations on the nutritional
requirements of dairy sheep.
Early work was concentrated on the effect of nutrition on milk yield, but effects on milk
composition were not examined (Wallace 1948; Thomson and Thomson 1953; McCance and
Alexander 1959).
With the exception of Peart (1967) who reported no effects of supplementation in late pregnancy
on future milk yield, most authors agree that ewes supplemented in late pregnancy produce
heavier lambs (Treacher 1970; Stern et al. 1978), and have greater production of milk
(Treacher 1970; Egan 1984; Hossamo and Farid 1986; Bencini and Purvis 1990). Mellor and
Murray (1985) reported that ewes severely underfed in late pregnancy had a reduced development
of the udder and lowered prenatal and postnatal accumulation of colostrum. Most authors
also reported that underfeeding ewes in early lactation impaired milk secretion and the growth
rate of the lambs (Coop et al. 1972; Jagush et al. 1972; Maxwell et al. 1979), but did not report
on the effects on the quality of the milk.
Poulton and Ashton (1972) examined the effect of the quality of diet on milk yield and
composition and demonstrated that diets rich in carbohydrates and poor in fibre disturbed the
function of the rumen and resulted in lower milk yields and lower concentrations of fat in the
milk.
The concentration of fat in the milk is correlated positively with the concentration of fibre in
the diet. Pulina and Rassu (1991a) have calculated the relationship between milk fat and Neutral
Detergent Fibre (NDF) in the diet (Fat % = 4.59 + 0.05 NDF; r = 0.48). However, this relationship
is difficult to interpret because a higher concentration of NDF provokes a reduction in the
digestibility of the diet and a reduction in feed intake which in turn results in a reduction in milk
production and a consequent increase in milk fat concentration.