The effect of dietary forage to concentrate ratio and forage type on milk fatty acid composition and milk fat globule size of lactating cows

摘要

We examined the effects of 2 grass silage-based dietsdiffering in forage: concentrate (FC) ratio and those of ared clover silage-based diet on intake, milk production,ruminal fatty acid (FA) biohydrogenation, milk FAcomposition, and milk fat globule (MFG) size distribution.Ten multiparous Nordic Red cows received thefollowing treatments: grass silage-based diets containinghigh (70:30, HG) or low (30:70, LG) FC ratio or ared clover silage-based diet with an FC ratio of 50:50(RC) on a dry matter basis. Determinations of MFGwere performed from fresh milk samples without additionof EDTA so the results of fat globules >1 μm indiameter are emphasized instead of the entire globulepopulation. Lower FC ratio in grass silage-based dietsincreased milk production with no effect on daily fatyield, leading to 13% lower milk fat concentration. Theeffect of FC ratio on MFG size was moderate. It didnot affect the volume-weighted diameter in grass silagebaseddiets, although LG lowered the volume-surfacediameter of MFG in the size class >1 μm comparedwith HG. Compared with HG, feeding LG moderatelydecreased the biohydrogenation of 18:2n-6, leading to ahigher level of polyunsaturated fatty acids in milk fat.Feeding RC lowered milk fat concentration and dailymilk fat yield compared with grass silage-based diets.The volume-weighted diameter of MFG in the size class>1 μm was smaller in RC milk compared with grasssilage-based diets. Feeding RC increased the flow of18:3n-3 at the omasum by 2.4-fold and decreased theapparent ruminal 18:3n-3 biohydrogenation comparedwith grass silage-based diets despite similar intake of18:3n-3. It also resulted in the lowest amount of saturatedFA and the highest amounts of cis-9 18:1, 18:3n-3, and polyunsaturated FA in milk. In conclusion, LGdecreased milk fat content and induced minor changesin MFG size distribution compared with HG, whereasRC lowered milk fat production, altered milk FA compositionto nutritionally more beneficial direction, andled to smaller MFG compared with grass silage-baseddiets.