Effect of oil supplementation of a diet containing a high concentration ofstarch on levels of trans fatty acids and conjugated linoleic acids in bovine milk

Sixteen Holstein Friesian cows were offered ad libitum a forage mixture of three parts (dry-matter (DM) basis) maize silage (starch and neutral-deterg ent fibre (NDF) 324 and 395 g/kg DM) and one part grass silage (NDF 518 g/k g DM) in a 4 x 4 Latin-square. change-over design experiment using four per iods each of 4 weeks. Cows were housed in a cubicle house, bedded on sand a nd given food through Calan gates. The four supplement treatments were cont rol (no oil supplement, C); 1.5 kg/day processed crushed linseed (PL); 1.5 kg/day crushed linseed (L) and 0.6 kg/day marine algae (A) providing 0, 440 , 423 and 109 g/day of oil respectively. Cows also received a pelleted conc entrate in three equal meals (12.0, 10.5, 10.5 and 11.2 kg/day for C, PL, L and A respectively) containing (g/kg DM) 260, 203, 288 and 74 of crude pro tein, NDF, starch and water-soluble carbohydrate respectively. Oil suppleme ntation depressed (P < 0.05) forage intake (11.2, 10.3, 10.1 and 10.1 kg DM per day) but milk yield was unaffected (P > 0.05) by treatment (mean 35.9 kg/day). Milkfat concentrations were low and further depressed (P < 0.05) b y algal supplementation (33.5, 32.3, 32.3 and 25.6 g/kg). Algal supplementa tion caused a three-fold increase in the concentrations of (n-3) long chain (> C20) polyunsaturated fatty acids in milk fat (to 0.51 g/100 g fat) repr esenting a transfer efficiency from diet to milk of ca. 5%. Oil Supplementa tion increased levels of all trans monoenes in milk but the effect was much greater for treatment A (P < 0.05) and for trans-10 C18 : 1 (1.52, 1.94, 1 .72 and 6.12 g/100 g milk fat for C, PL, L and A respectively). Trans-10 C1 8 : 1 was the predominant trans monoene in milk fat for all treatments (47. 7, 45.2, 45.6 and 67.4% of total). Treatment A also caused the greatest inc reases (P < 0.05) in conjugated linoleic acid (CLA to 0.54, 0.69, 0.65 and 0.97 g/100 g milk fat). Although mainly cis-9, trans-11, a small proportion (4-8 to 5.5%) of the CLA was identified as the trans-10, cis-12 isomer for all treatments. This pattern of isomers of trans monoenes and CLA, which m ay have implications for the health properties of the milk, nay be related to effects on rumen function caused by the high starch intakes (5.75, 5.09, 5.11 and 5.27 kg/day).