Dietary manipulation of the content and fatty acid composition of milk fat

The fat content of bovine milk shows considerable seasonal variation.For example, values ranging from 35.6 to 41.2 g/1 were recorded in manufacturing dairies in South-west Scotland (Muir et al. 1978), the samples being taken from silos containing up to 20 ooo 1 of milk.With individual cows, much more extreme values may be observed, e.g.20-60 g/l, depending on breed, age, stage of lactation, nutritional status and state of health.Rather more than 970 g milk fat/kg is composed of triglycerides.Whilst several hundred fatty acids have been identified in these triglycerides, the vast majority are present in such small quantities that for most practical purposes only about twelve need be considered.Again, pronounced seasonal effects in the relative proportions of these fatty acids are observed (Hall, 1970;Muir, 1982).This review deals with the dietary manipulation of the content, and consequently the fatty acid composition, of milk fat.Basically, this end may be achieved by two methods, namely (a) inclusion of fat in the diet of the cow and (b) changing the ruminal fermentation pattern.The experiments to which reference is made are of the changeover type and are applicable to the immediate post-peak period of lactation. Inclusion of fat in the diet of the cow Free fat in hay-based diets.There is a fairly voluminous literature in this area, stretching back some 50 years.The results that have emerged from this considerable input are somewhat contradictory and practical benefits of including fat in the diet have not materialized, despite the recognition that the energy density of fat makes it an appropriate addition to the feed of the high-yielding cow (Storry, 1980).For example, the average incorporation of added fat in dairy cake in the UK is only about 10 g/kg (Clapperton & Steele, 1983).Particular concern is expressed regarding the adverse effects of fat on the digestibility of the other constituents, especially fibre, of the diet (Steele & Moore, 1968).These interactions would be expected to have significant effects on the proportion and yield of milk fat, quantities of obvious economic importance to the farmer.The classical work of Virtanen (1966), in which synthetic diets were used, established that, if fatty acid intake fell below a limiting value, the output of milk and its constituents fell considerably.Stony et al. (1967) suggested that this limitation would not be encountered with natural diets, but in one of our earliest fat-feeding trials we found otherwise.Each component of the diet, by chance, had a fatty acid content at the lower end of the range associated with that component, so that the intake of fatty acid on this basal diet was only about 80 g/d (Banks et al.