Effect of spring grazing management on perennial ryegrass and ryegrass white clover pastures .1. Tissue turnover and herbage accumulation

The aim of this study was to investigate the effects of timing and duration of lax spring grazing on rates of leaf growth, botanical composition, and pasture production in perennial ryegrass swards, with and without white clover. In the first field experiment (September 1992-March 1993), swards of perennial ryegrass with and without white clover were rotationally grazed by sheep every 21 days to residual heights of 70-100 mm (lax) and every 14 days to 30-50 mm (hard). Two periods of lax grazing-short release (SR) from 26 October to 8 December and long release (LR) from 15 September to 8 December-were compared with conventional hard grazing throughout-early control (EC). All treatments were grazed to 30-50 mm every 14 days from 8 December until the end of March. Swards without white clover received 28 kg nitrogen (N)/ha every 2 weeks as urea. Treatments were arranged in a factorial design with three replicates. Overall, herbage accumulation in the SR and LR treatments was increased by 20 and 30%, respectively, compared to the EC treatment (P less than or equal to 0.001). Leaf growth in ryegrass and white clover was also increased following lax grazing. In the second field experiment (September 1993-April 1994) plots were subjected to similar grazing managements, though all spring treatments were imposed at a consistent grazing interval of 21 days and N use on non-clover swards was reduced to 14 kg/ha every 2 weeks. SR and LR treatments increased herbage production during spring by 24 and 28%, respectively, by increasing tiller weight, and during summer and autumn by 16 and 26% by enhancing tiller population density and leaf growth per tiller and/or stolen. Spring management effects were similar for swards with and without white clover. It is concluded that lax spring grazing management of ryegrass-white clover swards followed by hard grazing at the time of anthesis enhances pasture production, particularly during the summer-autumn period, by increasing both tiller population density and net leaf growth per tiller. Effects were larger following an extended spell of lax grazing. Contribution of tiller population density differences to these responses is discussed in the second paper in this series (Hernandez et al. 1997, this issue).