Grass based dairy production systems use concentrate supplementation primarily when there is a scarcity of grass.There is however, a paucity of information regarding the influence of supplement type on production and DMI, particularly in late lactation. Oat based supplements have shown improvements in milk yield and composition in early lactation, while Vitamin E (VE) plays a role in cell membrane structure and has potential to prevent deterioration of mammary cells. Therefore, the objective of this research was to investigate the effect of concentrate type and VE supplementation on milk yield, milk composition and rumen fermentation in late lactation dairy cows. Forty 8 Holstein Friesian dairy cows were blocked on days in milk (+185DIM) and balanced for parity, pre-experimental milk yield, milk composition and BCS. Cows were assigned to one of 4 dietary treatments in a randomized complete block design (n = 12). The dietary treatments (T) were: grass only (T1); grass + 2.6kg DM barley based concentrate + 350IU VE/kg (T2); grass + 2.6kg DM oat based concentrate + 350IU VE/kg (T3) and grass + 2.6kg DM oat + 1050IU VE/kg based concentrate (T4). The diets were fed for a 14day (d) acclimatization period and then for a further 49d. Concentrate supplementation increased milk yield and fat and protein kgs (T2 20.74kg, 1.74kg/d; T3 20.56kg, 1.81kg/d; and T4 20.56kg, 1.77kg/d respectively; P < 0.0001) in comparison to pasture only (T1 17.78kgs, 1.60kg/d). Cows offered T2 had lower milk fat (4.60%) concentration than T1 (5.00%, P < 0.0001) and T3 (4.90%, P = 0.001). Rumen pH of cows offered T1 (6.70) was higher than T2 (6.55, P < 0.0001), T3 (6.50, P < 0.0001) and T4 (6.58, P = 0.001). T4 reduced rumen ammonia (5.28mmol/l, P = 0.002) in comparison to T1 (5.83mmol/l). In conclusion, concentrate supplementation increased milk yield and kgs of fat and protein. The oat based concentrate had a higher milk fat concentration in comparison to barley. Increasing the level of VE reduced rumen ammonia concentration but did not alter milk yield or milk composition parameters for cows fed oat based supplements.