Defoliation ofLolium perenneandAgrostisspp. tillers, andTrifolium repensstolons in set-stocked and rotationally grazed hill pastures

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Abstract
Defoliation components of Lolium perenne (perennial ryegrass), Agrostis spp., (browntop), and Trifolium repens (white clover) were compared for one year under set stocking and rotational grazing with sheep on hill pastures which had received low and high rates of fertiliser input. All species were defoliated more frequently under set stocking than rotational grazing when rotation lengths exceeded 21 days. Mean minimum and maximum defoliation intervals for set stocking were 11.4 and 23.5, 16.2 and 52.7, and 19.9 and 88.5 days for ryegrass, browntop, and white clover respectively. Defoliation intervals were longest in winter and shortest in spring when both ewes and lambs were grazing. Grazing severity (total leaf length grazed per tiller) for ryegrass and browntop was greater under rotational grazing than set stocking. About twice as much leaf was removed per grazing for ryegrass as for browntop. However, grazing management probably had little effect on total leaf length grazed per unit area because of the compensatory effects of decreased defoliation interval and increased tiller density in set stocking, and greater defoliation severity in rotational grazing. When seasonal effects were omitted, tiller length was the main influence on total leaf length removed per ryegrass tiller under set stocking, and tiller length and the number ofleaves per tiller were the main influences under rotational grazing. Tiller length was the only important variable for browntop leaf removal under both managements. Leaves less than 2 weeks old of all species had low defoliation risks as did grass leaves in the last month of their life. Except under high grazing pressure in spring, leaves were able to develop fully before being grazed.