Regrowth dynamics and grazing decision rules: further analysis for dairy production systems based on perennial ryegrass (Lolium perenne L.) pastures

To support the further development of grazing practices for dairy production systems based on perennial ryegrass (Lolium perenne L.), allometric relationships among leaf-stage categories and pseudostem were derived for perennial ryegrass tillers sampled from swards each month, from July 2008 to January 2010, within a dairy grazing-system experiment in south-west Victoria, Australia. The relative lamina mass of the first leaf that emerged on tillers following grazing (denoted L3) and the subsequent leaf to emerge (L2) was used as an indicator of the trajectory of regrowth. L2 was consistently 3040% heavier than L3 during the period JulySeptember (mid-winter to early spring), but thereafter the difference between leaf stages lessened, and disappeared altogether in late spring. No substantial lag was observed in the rate of herbage accumulation during the early stages of regrowth of perennial ryegrass swards from 1500 kg DM ha-1 post-grazing. Therefore, grazing at any time in the period between emergence of the second and third leaves after the previous defoliation event should lead to high efficiency of pasture harvest under most conditions. The dry-matter digestibility (DMD) and crude protein (CP) content of the most recently emerged leaf (denoted L1) declined sharply during spring, whereas the DMD and CP content of older leaves were more consistent. Decision rules for grazing management should include sufficient flexibility to account for interactions between leaf stage and time of year in relative lamina mass and nutritive value.