The effects of autumn closing date on sward leaf area index and herbage mass during the winter period

The provision of grass for early spring grazing in Ireland is critical for spring calving grass-based milk production systems. This experiment investigated the effect of a range of autumn closing dates (CD), on herbage mass (kg DM ha^-1), leaf area index (LAI) and tiller density (m^-2) during winter and early spring. Thirty-six grazing paddocks, closed from 23 September to 1 December 2007, were grouped to create five mean CD treatments - 29 September, 13 October, 27 October, 10 November, 24 November. Herbage mass, tiller density and LAI were measured every 3 weeks from 28 November 2007 to 20 February 2008; additionally, herbage mass was measured prior to initial spring grazing and tiller density was measured intermittently until September 2008. Delaying CD until November significantly (P < 0·05) reduced herbage mass (by approximately 500 kg DM ha^-1) and LAI (by approximately 0·86 units) in mid-February. On average, 35% of herbage mass present on swards on 20 February was grown between 28 November and 30 January. LAI was positively correlated with herbage mass (R² = 0·78). Herbage mass increased by approximately 1000 kg DM ha^-1 as spring grazing was delayed from February to April. Tiller density increased from November to February, although it did fluctuate, and it was greatest in April (9930 m^-2). This experiment concludes that in the south of Ireland adequate herbage mass for grazing in early spring can be achieved by delaying closing to early mid-October; swards required for grazing after mid-March can be closed during November.