Using cumulative spring-summer water deficit to estimate perennial ryegrass pasture presistence

Persistence of perennial ryegrass (Lolium perenne L.) in grazed pastures is strongly influenced by climate, in particular summer water deficits. While there is evidence of perennial ryegrass tiller mortality in summer‐dry environments of southern Australia, there is little empirical analysis of the effects of summer water deficits on persistence. Such a metric could be used to predict perennial ryegrass persistence in response to climate variability and change. In the present study, a cumulative spring‐summer water deficit (rainfall minus potential evapotranspiration) of 400mm was adapted from research in the northern hemisphere and used to analyse climate in three dairy regions of south eastern Australia. While average annual rainfall was similar across the three sites, the 400mm threshold deficit for tiller mortality was exceeded much more regularly (24 of 53 years) at the summer‐dry site (Parawa, South Australia) than at the other two sites. Despite having similar summer rainfall, the threshold was exceeded more frequently at Moe (Gippsland, Victoria; 5 of 53 years) than Wynyard (north west Tasmania; 1 of 53 years) primarily due to higher potential evapotranspiration. While the results are consistent with the expected trend for perennial ryegrass persistence for the three regions, further validation of the appropriate spring‐summer water deficit threshold for prediction of perennial ryegrass persistence in southern Australia is required.