The objective of this study was to investigate the effect of pre-grazing herbage mass (HM) and pasture allowance (PA) on sward characteristics, milk production performance, dry matter intake (DMI) and rumen fermentation of spring calving dairy cows. Sixty-four Holstein-Friesian dairy cows (mean calving date, February 11) were balanced and randomly assigned to one of four treatments (n = 16) in a 2 x 2 factorial design. Two swards, a low HM (L HM) sward (1600 kg dry matter (DM)/ha) and a high HM (H HM) sward (2400 kg DM/ha), were created by varying the regrowth interval before the study start date. with two PA, 15 and 20 kg DM/cow/day imposed on each sward. Animals fitted with rumen cannula (n = 4) moved in a complete Latin square to investigate the effect of HM and PA on rumen fermentation. The study lasted 30 weeks and was analysed as two periods; PI (April 9-July 20) and PII (July 21-October 31). Sward structural characteristics, grass dry matter intake (GDMI) and rumen fermentation were investigated during PI and PII. The mean HM was 1551 kg DM/ha and 2678 kg DM/ha for L HM and H HM respectively in PI, and 1753 kg DM/ha and 2281 kg DM/ha for the L HM and H HM respectively in PII. During PII, the leaf proportion (> and <40 mm) was higher in the L HM swards than the H HM swards. There was a lower dead proportion > 40 mm in the L HM swards compared to the H HM swards. In PI, the L HM treatments had significantly higher milk production (+0.9 kg), milk protein yield (+42.4 g/day) and milk lactose yield (+42.9 g/day). Increasing PA increased milk yield (+1.1 kg; +1.8 kg), solids-corrected milk (+0.9 kg: +1.3 kg), protein (+67.9 g/day; +63.5 g/day) and lactose (+55.2 g/day; +80.1 g/day) yield in PI and PII respectively. The milk response per kilogram of GDMI was 1.13 kg for the L HM animals compared to 0.65 kg for the H HM animals. Rumen pH was lower for animals grazing the L HM swards (pH 5.69) than for animals grazing the H HM swards (pH 5.88) with no adverse rumen conditions evident. The results of this study suggest that L HM (1600 kg DM/ha) and 20 kg PA had a positive effect on sward characteristics which resulted in increased milk production and sward quality. (C) 2009 Elsevier B.V. All rights reserved.