THE EFFECT OF FEEDING RAPESEED ON THE FATTY-ACID COMPOSITION OF MILK LIPIDS AND ON THE CONCENTRATION OF METABOLITES AND HORMONES IN THE SERUM OF DAIRY-COWS

Rapeseed can be used to influence the fatty-acid composition of milk f at in a physiologically useful manner for the consumer, and to improve butter spreadability. This trial was undertaken to determine the inpu t/output relationship of rapeseed fatty acids in milk fat. Two groups of 10 cows were fed a corn-silage based diet with or without 1 kg of g round rapeseed in an initial 3-month investigation period. During the following 3-week period, the proportion of rapeseed was isonitrogenica lly increased to 1.5 kg/animal/day. Despite higher milk production in the rapeseed-fed cows, there were no significant differences in daily fat and protein yields between treatments. The secreted amount of shor t- and middle-chain fatty acids (C4:0-C17:0) per animal per day decrea sed by 148 and 174 g, respectively, whereas the production of long-cha in fatty acids increased by 138 and 183 g (1 kg and 1.5 kg rapeseed, r espectively). The butter fat oi cows fed with rapeseed contained one t hird less C16:0 and two thirds more C18:1. These beneficial changes al so significantly improve the spreadability of butter since the ratio o f C18:1 to C16:0 is an index of milk-fat hardness. The higher fatty-ac id transfer from rapeseed to milk fat seems to have a glucose-saving e ffect. Both glucose and fatty acids are known as stimulators of IGF-I synthesis, a process involved in milk production. The benefits of incr easing levels of dietary fat tended to be progressively decreased if t oo much fat was added to the diet since this caused a linear reduction in fat digestibility and lipid transfer to the udder. It can be concl uded that for the optimization of the input/output relationship, 1 kg of ground rapeseed per day, or 5 % total far, should be the upper limi t in the diet of lactating cows,