The aim of the experiment was to study the response in milk production and composition of substituting barley with either saturated or unsaturated fat in mixed rations (MR) for dairy cows. The experiment included 35 Danish Holstein (DH), 39 Danish Red (DR), and 31 Danish Jersey (DJ) cows from parturition until week 30 of lactation. Cows were ad libitum fed one of the three MR based on maize and grass/clover silage, barley, soybean meal, and dried sugar beet pulp. In the saturated ration (SFA), C16 rich fat substituted barley on dry matter (DM) basis; in the unsaturated ration (UFA), ground rape and linseed (0.74:0.26 ratio) substituted barley on DM basis; and in the control ration (CO), no barley was substituted. Concentrations of crude fat (CF) of total ration DM was 3.03% in CO, and in diets supplemented to intended similar fat levels 5.65% (SFA) and 5.81% (UFA), respectively. Statistical analyses were performed within breed and random regression was used to test for differences between treatment responses throughout lactations.

For all breeds, SFA increased milk fat concentration of C16 whereas UFA increased milk fat concentration of unsaturated as well as saturated C18 fatty acids (FA). For each of the three breeds the following responses were obtained compared to CO. DR: UFA increased the net energy intake (NEI) (P=0.04), but energy corrected milk (ECM) yield was reduced to 30.9 kg/d compared to 31.7 kg/d for CO (ns). SFA increased milk fat concentration to 4.42% compared to 3.87% for CO (P<0.0001). DH: SFA decreased DM intake (DMI) (P=0.05), but ECM was slightly increased to 34.5 kg/d compared to 34.0 kg/d for CO (ns). Both SFA and UFA decreased the milk protein concentration to 3.30% compared to 3.49% for CO (P<0.05). Sensory analysis of milk from DH showed only minor differences between treatments. DJ: SFA (P=0.2) and UFA (P=0.07) tended to decrease DMI, and decreased ECM to 28.6 kg/d (SFA, P=0.02) and 28.0 kg/d (UFA, P=0.01) compared to 31.6 kg/d (CO). Furthermore, SFA and UFA tended to increase milk fat.

In conclusion, SFA caused a lower protein:fat ratio in milk for all breeds, while UFA only reduced protein:fat ratio for DJ. The minor increased (DH), and the tendency to decreased (DR, DJ) milk yield response to feeding SFA was unexpected, and may be due to a general negative effect of feeding fat in early lactation.