Effect of milking frequency and pasture intake on milk yield and composition of late lactation cows

Twenty-four monozygous twinsets in late lactation (>210 d in milk) were use d to examine the effects of feed restriction and milking frequency prior to drying off on milk yield and composition in a pastoral dairying system. Co ws were assigned to one of four treatment groups for 26 d and were milked e ither twice or once daily and given either unrestricted or restricted acces s to feed. Dry matter intakes averaged 16 or 8 kg per cow per day, and diet s comprised ryegrass and white clover pasture supplemented with 15% pasture silage. Feed restriction and once daily milking reduced milk yield and increased co ncentrations of milk fat and protein. Somatic cell count was increased by f eed restriction only. Production losses caused by feed restriction were nea rly threefold higher than were those for once daily milking. Yields of comp onents that were mammary synthesized and serum derived were reduced by feed restriction, in accordance with milk volume reduction. Plasma lactose conc entration increased with once daily milking only and indicated enhanced per meability of mammary tight junctions. Both feed restriction and once daily milking compromised milk quality, but increased leakage of serum components into milk via mammary tight junctions was deemed to occur only for once da ily milking.