Fate of fatty acids during ensiling: relationship with milk fat composition of dairy cows

Transition of dairy cows from grazing to silage based rations significantly increases the saturated: unsaturated fatty acids (FA) ratio and decreases the content of beneficial C18:1 cis-9, C18:1 trans-11, C18:2 cis-9,trans-11 and C18:3n-3 in milk fat. This is partly related to a lower polyunsaturated FA (PUFA) supply from ensiled forages. The aim of this thesis was to investigate the scope of increasing the content PUFA in grass and maize silages, and to establish relationships between silage quality on the one hand and the FA content and composition, post-ensiling stability of PUFA, and milk FA composition of dairy cows on the other hand. The first focus of this thesis was to quantify the variation in FA content and composition in grass (n = 101) and maize (n = 96) silages, randomly sampled from commercial dairy farms in the Netherlands, and use multivariate analysis to identify the causes of this variation. The FA content and composition of grass and maize silages were highly variable, and this variation was primarily caused by differences in plant maturity at harvest. Silages made from younger grass and maize have higher contents of C18:3n-3.Most of the variation in FA content in the ensiled forages was caused by differences in plant maturity at harvest. Changes in FA content and composition were investigated in stover (leaves and stem) and ears (cob, shank and husks) in a set of maize genotypes, grown on sandy and clay soils and harvested at 14, 42, 56, 70, and 84 days after flowering (DAF). The contents of C18:3n-3 and total FAs in the stover dry matter (DM) declined at a slow rate up to 56 DAF and then decreased rapidly during 56–84 DAF. On the other hand the content of C18:2n-6 and total FAs in the ears DM increased up to 56 DAF and thereafter remained more or less constant. The maximum amount of PUFA in silage maize can be harvested around 56 DAF. Identifying pre and post-ensiling processes that optimize the stability of PUFA was the next goal. The stability of FA were investigated in untreated and mechanically bruised perennial ryegrass (Lolium perenne L.), wilted under field conditions for 0, 12, 24, 36, and 48 h, or wilted under controlled climate conditions at three temperatures (15, 25 or 35 °C) and two light (dark or light) regimes to DM contents of 425, 525 or 625 g/kg. The oxidation of FAs during wilting of grass was mainly caused by the duration of the wilting, wilting temperature only provoked small differences, whereas mechanical bruising of grass and light intensity did not affect the changes in FA contents. The highly esterified lipids of forages are extensively hydrolysed in the silo. Therefore, the post-ensiling stability of FAs was investigated in grass and maize silages, with a wide range in qualities, exposed to air for 0, 12, and 24 h. Exposure of grass and maize silages to air results in a quantitatively small, but consistent decline in the contents of major unsaturated FAs with a concomitant increase in the proportion of C16:0. The final study evaluated the effects of feeding maize silages, ensiled at different maturities, in combination with a high or low degradable carbohydrate concentrate on nutrient intake, milk production, and composition of milk and milk fatinearly lactating dairy cows. Maize maturity at harvest at a DM content of 300-420 g/kg fresh weight, did not affect the production performance of dairy cows, but resulted in decreased contents of C18:3n-3 and total n-3 and a decreased n-6:n-3 ratio in the milk fat of dairy cows.