When autumn-calving dairy cows are turred out to grass, the sudden change in diet composition, particularly the drop in fibre content, can frequently result in a depression in the fat content of milk. This depression can be minimised by feeding forages such as hay or silage to cows at grass but this will usually result in a decrease in the overall energy concentration of the ration, thus decreasing potential performance. In rations fed to dairy cows during the winter, milk fat content can be increased by the use of supplements containing fibrous sources of carbohydrate or fat. Feeding high levels of fat can be counter productive, due to its effect on fibre digestion, but calcium soaps have been found to have less effect on runen fermentation than free fatty acids.