Future pastures: exploring options for improving perennial ryegrass water-use efficiency and drought tolerance

Context Our planet is experiencing climate warming estimated at 1.1°C, on average, in the past 30 years and this warming is expected increase summer moisture deficit stress in pastures. Aims Here, we record ryegrass improvement research on the ecophysiology of summer water-deficit tolerance conducted at Massey University between 2003 and 2021 and briefly compare and contrast that research with work elsewhere. Key results Among other effects, warmer temperatures will increase pasture evapotranspiration in summer and breeding pasture cultivars for water-use efficiency could help offset global warming. We first explored prospects for introgression of current ryegrass cultivars with germplasm from North Africa, but made no breakthrough following this approach. We noted that the cultivar Grasslands Samson contained elite genotypes exhibiting a water-saving behaviour. Further research showed this water saving to come from a trait association linking increased shoot dry weight on regrowth in water deficit with more negative leaf osmotic potential, deeper rooting, and retention of soil moisture in the 20–50 cm soil depth. In a test using half-sib families these component traits of water-use efficiency were found to be highly heritable. A collaboration with a commercial ryegrass breeder was established to produce a high water-use efficiency (HWUE) selection for testing. Conclusions The initial HWUE selection exceeded performance of a ‘control’ selection by 7% in summer and 3% on an annual basis, but was not superior to the company’s best conventional selection. Implications These results showed both theoretical promise and practical difficulty in breeding for HWUE in ryegrass. The authors believe that there is sufficient promise to merit continued pursuit of breeding for HWUE, although it does appear that current commercial breeding methods have improved water-use efficiency of recent cultivars, relative to older cultivars. This may relate to a progression from Hawkes Bay ecotypes used in Grasslands Ruanui to the Mangere ecotype (Grasslands Nui and Ellett) and, more recently, use of germplasm from north-west Spain, or there may also have been spontaneous selection for water-use efficiency in field testing.