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
G. Jahreis et Gh. Richter, 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, Journal of animal physiology and animal nutrition, 72(2-3), 1994, pp. 71-79
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,