Influence of the diet structure on ruminal biohydrogenation and milk fatty acid composition of cows fed extruded linseed

This experiment studied the influence of the diet structure value (SV) on ruminal biohydrogenation and milk fatty acid (FA) responses in cows fed heterogeneous basal diets equally supplemented with FA. Eight lactating Holstein cows were used in a replicated 4×4 Latin square design with four dietary treatments and four 21-day periods. The iso-fat, iso-18:2 n−6 and iso-18:3 n−3 diets were formulated to display three different SV, using different sources and proportions of forages, energy and nitrogen concentrates. The four diets contained maize silage as the main forage (SV1.2 diet), grass hay as the main forage (SV2.0 diet), maize silage and grass hay in a 4:1 ratio (SV1.6M diet) or maize silage and grass hay in a 1:1 ratio (SV1.6H diet). The diets also contained soya bean meal and/or urea as additional sources of nitrogen, sugar beet pulp and barley in a 1:1 ratio as additional source of energy, extruded linseed as supplemental 18:3 n−3, a mineral and vitamin mix and a vitamin E preparation. Wheat straw was added to the diets as additional structure source, except for the SV2.0 diet. Soya bean oil was added to the diets as supplemental 18:2 n−6 to adjust the diets for this FA, except for the SV1.2 diet. The diets were distributed as a restricted total mixed ration. The various C18 FA expressed as 100g of total C18 FA in milk fat are relevant indicators of ruminal biohydrogenation since duodenal concentrations of C18 FA follow similar changes as those in milk fat, and since these ratios only take into account FA involved in ruminal biohydrogenation. All the various C18 FA to total C18 FA in milk fat differed among diets (P<0.05). Milk 18:2 n−6+18:3 n−3/total C18 FA and total trans-C18 FA/total C18 FA decreased from SV1.2 to SV2.0 diets, whereas 18:0/total C18 FA increased from SV1.2 to SV2.0 diets. Subsequently, transfer efficiencies of 18:2 n−6 and 18:3 n−3 from diet to milk were higher for the SV1.2 diet than for the other diets (P