The inclusion of supplementary fat in the dairy cow diet is an effective and cheap method of increasing its metabolisable energy (ME) density. However, an increase in the fat content of the diet frequently causes a decrease in the protein concentration of milk, which, when the market requires an increase in the protein:fat ratio of milk, is not a desireable effect. This experiment was designed to compare die effect of dietary fat on die yield and concentrations of milk proteins in comparison to a carbohydrate ME source.

Twelve multiparous mid-lactation Holstein-Friesian cows were used in a 3x3 Latin square changeover experiment with 3 four-week periods. Cows were allocated to squares on the basis of their mean milk yield over the 7 days prior to the experiment. Within squares, treatments were randomly allocated to each cow. The first 21 days of each experimental period were used for adjustment, and the last 7 for sample collection.