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

Milk fat and protein concentrations as nutritional indicators
Among the several factors that affect milk fat concentration, some of the most important
ones are:
- dietary NDF concentration, which is positively associated with milk fat concentration
(Emery, 1988; Bencini and Pulina, 1997);
- milk yield, which is negatively associated with milk fat concentration (Emery, 1988);
- energy balance, which is negatively associated with milk fat concentration in both cows
(Grieve et al., 1986; Palmquist et al., 1993) and sheep (Bocquier and Caja; 2001).
Among the above listed factors, energy balance (EB) has probably the largest effect on
milk fat concentration. In fact, when mobilization of body reserves is high, such as at the
beginning of the lactation, the blood concentration of long chain fatty acids (FA) derived from
the mobilized body fat triglycerides increases. Part of these FA are used by the mammary gland
to produce milk fat, whose concentration increases. Another effect of body reserves mobilization
is an increase in the proportion of long chain FA in milk fat. In dairy cows, it was suggested that
the variations in milk fat concentration during the lactation could be used to identify across-herd
differences in EB (de Vries and Veerkamp, 2000). In dairy ewes, a high negative association
between milk fat concentration and EB was reported by Bocquier and Caja (2001):
[4] milk fat concentration (%) = 9.65 - 1.22 EB (UFL/d) r 2 = 0.76
where UFL = 1700 kcal of NEL.
Cannas and Avondo (2002) tested the findings of Bocquier and Caja (2001) by using the
data of 6 feeding trials (58 experimental treatments) conducted in Sicily in various dairy sheep
farms over a period of eight years (Avondo et al., 1998). Multiparous Comisana lactating ewes
were used in all trials. The ewes were fed on pasture and the diet was supplemented with hay and
concentrates. Individual herbage, concentrate and hay intake of 440 ewes were measured. All the
experimental measurements were divided in 4 classes based on the milk yield of the ewes (< 400
g/d, n = 63; 400-799 g/d, n = 260; 800-1199 g/d, n = 97; 1200-1600 g/d, n = 20).
In the highest milk yield class (1200-1600 g/d), milk fat concentration was inversely
associated with dietary NDF concentration, NDF intake, and energy balance (Table 9 and Figure
4). Going from the highest to the lowest milk yield class, these relationships became weaker and, in
some cases, not significant (Table 9 and Figure 5). For all classes, the association between milk fat
concentration and milk yield was either weak or not significant (Table 9).
101