Grass/clover swards vary in nitrogen (N) content throughout the season. This is mainly due to the changing proportion of clover in the sward, being low in spring and increasing throughout the grazing season as shown by Søegaard (1993). This could have major implications for efficiency of the grazing animal as demonstrated by Beever et al(1986). Season specific supplementation could be necessary in order to maximise utilisation and N-efficiency.

Two grazing experiments were carried out with 30 multiparous Holstein Friesian cows on a spring (1) and summer (2) growth of a grass clover pasture containing 7.1 and 38.2% clover in the DM respectively. The three supplementation treatments (Al, Bl, CI) on the spring (1) sward consisted on 0.7, 3.1 and 3.1 kg DM day-1 of a concentrate with an analysis of ME and CP of 10.4, 12.2, 12.3 MJ kg DM-1 and 276, 230 and 204 g kg DM-1 respectively for treatment Al, Bl and CI aiming to supply 90, 100 and 90% of ERDP requirements.