Effect of supplernentation of grazing dairy ewes with a cereal concer¡trate on rnllk fatty acid profile

Abstract. This work was conducted to investrgate the effect of supplementing grazing ewes on pasture with a cereal concentrate on the milk fatty acid (FA) prof i le. Ninety Assaf ewes in mid-lactai ion were disir ibuted in g lots of 10 animals each and al located to three feeding regimes: ( i) P- ewes were only al lowed to graze pasture (an irrigated sward of Lolium perenne, Trifolium pratense and Dactylis glomerata); (ii) SP- grazing ewes were supplemented r¡ ¡ i th oat grain (700 g/animaf per day); and ( i i i) TMR- ewes were fed ad l ibi tum a total mixed ration (TMR; 80:20 concentrate/forage rat io). Milk yield and composit ion were recorded after 3 weeks. There vvere no signif icant dif ferences in milk yield between P and SP; the highest production being observed for TMR. Milk fat from SP and TMR ewes presented similar concentrat¡ons of rumenjc (cis-9 trans-1i C1B:2) and vaccentc acids ( lrans-11 C18:1, the rumen-derived precursor for endogenous synthesis of CLA). Total conjugated l inoleic acid (CLA) was sl ightly higher ¡n SP than in TMR but far from the results observed in grazing ewes rece¡ving no supplement (2.96, 1.71and 1.35 for P, SP and TMR, respeci ively). The atherogenity index in SP milk fat was not dif ferent from that observed in milk from animals on pasture (1.45for P. 1.61 for SP and 3.25 for TMR). l t is concluded that, when paslure qual i ty and avai labi l i ty are not l imit ing for dairy production, suppiementation of grazing ewes with oat grain compromises the milk FA profi le without any signif icant pos¡t ive effect on milk production