Seasonal changes in the crude protein concentration of mixed swards of white clover/perennial ryegrass grown without fertilizer N in an organic farming system in the United Kingdom

Changes in the crude protein (CP) concentration of white clover and perennial ryegrass herbage from a mixed sward were determined on six sampling dates from May to October in each of 2 years. The swards were grown without fertilizer N in an organic farming system and continuously grazed by dairy cows during the grazing season. The annual mean contents of white clover in the dry matter (DM) of the sward were 272.3 and 307.0 g kg ^-1 in Years 1 and 2. The mean CP concentrations of the while clover and perennial ryegrass herbage were 251.6 and 151.9 g kg ^-1 DM in Year 1 and 271.9 and 174.0 g kg ^-1 DM in Year 2 respectively. The CP concentration of the white clover increased significantly during the grazing season from 220.0 to 284.1 g kg ^-1 DM in Year 1 and from 269.0 to 315.5 g kg ^-1 DM in Year 2. In the perennial ryegrass herbage the CP concentration increased from 112.2 to 172.6 g kg ^-1 DM in Year 1 and from 142.7 to 239.5 g kg ^-1 DM in Year 2. The rate of increase during the season in the CP concentration of the perennial ryegrass herbage was similar to the rate of increase recorded in the white clover herbage.