What is the metabolisable energy (ME) of fat for ruminants

The ME of a feed ingredient represents the energy available to the animal and is primarily determined by digestibility of the ingredient. Digestibility of Megalac (calcium salt) has been determined in several studies, facilitating determination of the ME value of the product, specifically, at 33.3 MJ/kg dry matter. This ME is around 2.5-times the value of typical cereals, so Megalac is particularly effective at increasing energy density of diets (Table 1). 

Table 1      Metabolisable energy concentration of Megalac and common feed ingredients

                        Metabolisable energy
Feed              MJ/kg fresh                 MJ/kg DM
Grass silage                       variable                           9 - 12
Sugarbeet pulp                          11.1                           12.5
Wheat                           11.7                           13.6
Maize                          11.9                           13.8
Molasses                           9.5                           12.7
Megalac                           31.6                           33.3

The ME of most other brands of fat supplement has not been determined as there is no laboratory-based method of determining the energy value of fat supplements. Furthermore, due to the complexity of the rumen and its effects on ingested fats, it is particularly difficult to determine specific digestibility of individual fatty acids and fat supplements.

Hence, digestibility of supplements is often estimated using different methods, including comparison to that of similar fat types published in the literature, using estimates based on published digestibility of individual fatty acids, or by applying estimates of digestibility from particular published ‘standards’.

Literature on 99% fat products commonly quote ME values of 36‐39 MJ/kg, though this assumes near 100% digestibility which is not feasible for these product-types. Furthermore, ME can be misleading for some product-types, as energy from high-C16 supplements is expected to be partitioned more toward milk fat stimulation, making it unavailable to contribute to the energy requirements of the dairy cow.

Recognising the difficulty in accurately determining ME, Volac Wilmar do not list ME values for their non-Calcium salt products as the potential values differ depending on the method used to determine them.