Decreasing the proportion of long forage in mixed diets from 400 to 100 g/kg at constant digestible energy intakes reduces milk fat content by approximately 5 g/kg for every 100 g/kg decrease in hay. This response varies widely and a safe minimum diet composition to maintain approximately 40 g fat/kg milk from Friesian cows in mid-lactation is approximately 450 g long forage/kg or 220 g acid-detergent fibre/kg dry matter. This, however, would reduce milk yields. With barley-based concentrates, milk yield increases as the proportion of hay in the diet is reduced, with the result that the reduction in the yield of fat is less than the fall in its concentration. Milk fat content is higher when ground maize, which is a slowly fermented starch source, or fodder beet or fibrous by-products replace rapidly fermented starch sources such as barley in low-roughage diets. Milk yield, however, is lower. Supplementary fats and oils generally increase milk yield but their effects on milk fat content and yield vary widely.
Increasing the intake of high-concentrate diets of fixed composition increases the yield of milk but reduces its fat content. Increasing the number of meals per 24 h reduces this milk fat depression without affecting milk yield. Thus, advice on milk fat production must take account of the level of intake, the pattern of feeding and the diet composition.
In most situations, the avoidance of low milk fat content requires control of rumen fermentation to prevent high proportions of propionic acid. However, with frequent feeding during the 24 h, high propionic acid in the rumen has less effect on milk fat. It appears that high plasma insulin concentration is the main factor reducing milk fat production.
The release of insulin is stimulated by the peaks of propionate, which are produced after large meals of concentrates but not by the steady supply of propionate associated with frequent feeding.
Available knowledge can permit wide variation in milk fat production by dietary manipulation with reasonable accuracy but the future aim should be for more direct intervention at metabolic control points.