Studies on the properties of New Zealand butterfat: VIII. The fatty acid composition of the milk fat of cows grazing on ryegrass at two stages of maturity and the composition of the ryegrass lipids

SummaryThe lipid content of short rotation ryegrass at two stages of growth was followed at weekly intervals for several months in two spring-early summer seasons. Short succulent ryegrass consisting entirely of leaf tissue contained more lipid (mean, 8·1% of the dry weight) than mature ryegrass which contained appreciable stalk (mean, 5·1% of the dry weight). The lipid from the new growth contained fatty acids with appreciably higher proportions of linolenic acid, which was balanced mainly by lower proportions of linoleic and palmitic acids.When monozygotic twin milking cows were grazed on the short rotation ryegrass grown to two stages of maturity, it was found that the fatty acid composition of the milk fat from the two groups was different. The milk fat of cows grazed on the new growth contained higher proportions of oleic acid and other C18acids, whilst the proportions of myristic and palmitic acids were lower. The total proportions of the short-chain fatty acids were not greatly different, although in the group on new growth butyric acid was present in higher proportions and hexanoic and octanoic acids in lower proportions. The higher unsaturation of the fatty acids in the milk fat of this group of cows may be related to the higher levels of unsaturated fatty acids in the young ryegrass diet and to the extent to which these unsaturated fatty acids are hydrogenated in the rumen.