Persistence and productivity of perennial ryegrass in sheep pastures in south-western Victoria: a review

Loss of perennial ryegrass (Lolium perenne L.) from the pasture within seve ral years of sowing is a common problem in the higher rainfall (550-750 mm annual rainfall), summer-dry regions of south-eastern Australia. This pastu re grass came to Australia from northern Europe, where it mostly grows from spring to autumn under mild climatic conditions. In contrast, the summers are generally much drier and hotter in this region of south-eastern Austral ia. This 'mismatch' between genotype and environment may be the fundamental reason for the poor persistence. There is hope that the recently released cultivars, Fitzroy and Avalon, selected and developed from naturalised ryeg rass pastures in south-eastern Australia for improved winter growth and per sistence will improve the performance of perennial ryegrass in the region. Soon-to-be released cultivars, developed from Mediterranean germplasm, may also bridge the climatic gap between where perennial ryegrass originated an d where it is grown in south-eastern Australia. Other factors that influence perennial ryegrass persistence and productivit y can be managed to some extent by the landholder. Nutrient status of the s oil is important since perennial ryegrass performance improves relative to many other pasture species with increasing nitrogen and phosphorus supply. It appears that high soil exchangeable aluminium levels are also reducing r yegrass performance in parts of the region. The use of lime may resolve pro blems with high aluminium levels. Weeds that compete with perennial ryegras s become prevalent where bare patches occur in the pasture; they have the o pportunity to invade pastures at the opening rains each year. Maintaining s ome herbage cover over summer and autumn should reduce weed establishment. Diseases of ryegrass are best managed by using resistant cultivars. Insect pests may be best managed by understanding and monitoring their biology to ensure timely application of pesticides and by manipulating herbage mass to alter feed sources and habitat. Grazing management has potential to improve perennial ryegrass performance as frequency and intensity of defoliation affect dry matter production and have been linked to ryegrass persistence, particularly under moisture defic it and high temperature stress. There is some disagreement as to the merit of rotational stocking with sheep, since the results of grazing experiments vary markedly depending on the rotational strategy used, climate, timing o f the opening rains, stock class and supplementary feeding policy. We concl ude that flexibility of grazing management strategies is important. These s trategies should be able to be varied during the year depending on climatic conditions, herbage mass, and plant physiology and stock requirements. Two grazing strategies that show potential are a short rest from grazing the p asture at the opening rains until the pasture has gained some leaf area, in years when the opening rains are late. The second strategy is to allow rye grass to flower late in the season, preventing new vegetative growth, and p erhaps allowing for tiller buds to be preserved in a dormant state over the summer. An extension of this strategy would be to delay grazing until afte r the ryegrass seed heads have matured and seed has shed from the infloresc ences. This has the potential to increase ryegrass density in the following growing season from seedling recruitment. A number of research opportunities have been identified from this review fo r improving ryegrass persistence. One area would be to investigate the pote ntial for using grazing management to allow late development of ryegrass se ed heads to preserve tiller buds in a dormant state over the summer. Anothe r option is to investigate the potential, and subsequently develop grazing procedures, to allow seed maturation and recruitment of ryegrass seedlings after the autumn rains.