The major source of cis-9, trans-11 conjugated linoleic acid (CLA) in bovine milk is from the post-ruminal conversion of trans-11 C18:1 (TVA) by D9-desaturase. Activity of D9-desaturase is predicted from the product/substrate ratios of fatty acids dependant on the enzyme. Data from three experiments was investigated. Cows selected for having soft or hard milk fat were fed ruminally protected oilseed in spring 1999, and autumn 2000 or crushed full-fat rapeseed in autumn 2001. Cows with harder milk fat had higher concentration of milk fat (P<0.001), and lower concentrations (P<0.001) of TVA and CLA. Feeding ruminally protected oilseed in spring had no effect on milk fat, TVA or CLA concentrations but in autumn increased (P<0.001) milk fat concentration. Feeding crushed rapeseed depressed (P<0.001) TVA and CLA concentrations. Ratios of fatty acid pairs dependent on D9-desaturase (C10:1/C10:0, C16:1/C16:0 cis-9 C18:1/C18:0 and CLA/TVA) were lower in milk from cows with harder milk fat and the C12:1/C12:0, C14:1/C14:0 ratios were similar for soft and hard milk fat cows. Feeding ruminally protected oilseed decreased (P<0.001) all ratios except for C12:1/C12:0 in autumn 2000. Feeding rapeseed had no effect on the product/substraight ratios except the cis-9 C18:1/C18:0 ratio. None of these ratios were associated with milk fat CLA concentrations in spring however in autumn C14:1/C14:0 and C16:1/C16:0 were positively associated (P<0.05). Significant associations (P<0.001) between the milk fat cis-D9 C18:1/C18:0 ratio and CLA concentration and between TVA and CLA concentrations were also evident. This suggests that milk fat CLA concentrations may be influenced more by the availability of TVA than by the variation in the activity of D9-desaturase.
Proceedings of the New Zealand Society of Animal Production, Volume 63, Queenstown, 25-30, 2003
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