The dairy industry in Ireland is based on a spring-calving grazing system with the use of concentrate supplementation in the spring and autumn when there is a scarcity of grass. Problems with milk quality are more pronounced in such a system when the majority of the national herd reach late lactation at the same time. Therefore, the objective of this research was to investigate the effect of concentrate supplementation strategy on milk yield and composition and rumen fermentation parameters in late lactation dairy cows. Thirty-six Holsten Friesian dairy cows were blocked on days in milk (+185DIM) and balanced for parity, pre-experimental milk yield and milk composition, predicted 305-day milk yield and BCS. Cows were randomly assigned to one of 3 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 (T2): grass + 2.6kg DM maize based concentrate (T3). The diets were fed for a 14-day acclimatization period and then for a further 63days. Cows offered T1 had a lower milk yield (14.54 kg) than T2 (17.15kg, P < 0.001) and T3 (16.73, P < 0.001). Similarly, T1 had lower milk solids (kg fat and protein; 1.47 kg) than T2 and T3 (1.51 and 1.48 kg, respectively; P < 0.001).The response to concentrates averaged 0.8:1 kg of milk per kg of concentrate (range of 0.5 to 1.2). Milk urea was higher in T1 (0.041%) than both T2 (0.038%, P < 0.001) and T3 (0.039%, P = 0.004) and rumen ammonia was significantly higher in T1 (5.63 mmol/L, P = 0.006) and T3 (5.77 mmol/L, P < 0.001) than T2 (5.28 mmol/L). Mean rumen pH of cows offered T2 (6.32) was lower than T1 (6.42; P = 0.002) and T3 (6.42, P = 0.003). In conclusion, concentrate supplementation increased milk yield and kilograms of milk solids and altered rumen fermentation parameters. Differences between barley and maize based concentrates were seen for rumen pH and ammonia.