Tactical versus continuous stocking for persistence of perennial ryegrass (Lolium perenne L.) in pastures grazed by sheep in south-western Victoria

The survival of perennial ryegrass (Lolium perenne L.) plants was studied i n sheep pastures in south-western Victoria during the dry summer of 1996-97 . Recruitment of perennial ryegrass seedlings into the pasture sward was al so monitored in the autumn-winter periods in 1997 and 1998. The objective w as to investigate whether a tactical stocking strategy, consisting of varia ble length summer, autumn and winter rotations and continuous stocking in s pring, might increase perennial ryegrass tiller survival and seedling recru itment in the autumn, compared with continuous stocking all year. The grazi ng strategies were compared on 2 contrasting pastures: an upgraded pasture [sown with newer cultivars of perennial ryegrass and subterranean clover (T rifolium subterraneum L.) with 26 kg phosphorus/ha. year as applied fertili ser] and a naturalised perennial ryegrass pasture receiving 6 kg P/ha. year . Paddocks were grazed by Border Leicester x Merino ewes, mated to a termin al sire so as to lamb in September. Perennial ryegrass tiller density was higher on the upgraded pasture with a mean density of 7750 tillers/m(2) in early summer which declined to zero l ive tillers by mid summer. Live tillers began to reappear before the openin g rains and then increased after the rain. Mean tiller density in the upgra ded pasture declined over the 2 summers, with only 2050 tillers/m(2) being present 2 months after the opening rains in 1998. There were no effects (P> 0.05) of pasture type or grazing strategy on the number of tagged tillers that survived the summer period. Only 12% of the vegetative tillers, random ly tagged in December 1996, survived to May 1997. More than half of the til lers (56%) that produced a seedhead produced daughter tillers which survive d the dry summer-autumn period. A significant (P< 0.05) interaction between grazing strategy and pasture ty pe occurred with the number of perennial ryegrass seedlings that had establ ished 4 weeks after the opening rains in 1997. There was a 5-11-fold increa se in seedling numbers which regenerated in the tactically stocked, upgrade d pasture compared with the other treatments. Seedling recruitment was cons iderably lower in the autumn of 1998, due presumably to an overall decline in perennial ryegrass density relative to annual grasses in 1997. A second experiment investigated the effect of excluding sheep from grazing at anthesis until seedhead maturation or until the opening rains, together with a mechanical seed dislodgment treatment at seed maturity. All exclusi on treatments increased seedling recruitment 4-7-fold, compared with contin uous stocking. The results suggest a possible mechanism by which perennial ryegrass density can be increased without expensive reseeding.