The effects of summer moisture stress and its interaction with spring cutting managements on the production and persistence of a ryegrass (Lolium perenne L.) sward : a thesis ... for the degree of Master of Agricultural Science in Plant Science at Massey University

The effect of an increasing water deficit during summer, achieved by means of an automatic Rain-Out Shelter, on the production and persistence of a perennial ryegrass (Lolium perenne L. "Grasslands Nui") was compared to that under irrigation. In addition, the effect and interaction with water deficit, of 2 different cutting intensities (i.e. heights of 2.5 and 7.5 cms) during spring were also investigated. The responses of perennial ryegrass to an imposed drought, compared to those under irrigation, were related to measurements of soil and plant water status; where the soil and plant water status were measured by soil water deficit and leaf water potential respectively. It was found that the growth of the components of pasture production (particularly tiller and leaf dynamics) was reduced when the soil water deficit exceeded a critical point of 104-111 mm, which could be predicted for the Tokomaru silt loam soil type, according to the soil water balance of Scotter, Clothier and Turner (1979). At this point the soil water potential was approximately equal to -0.1 MPa and had not reached the permanent wilting point (-1.5 MPa). This critical point was characterised by a sharp 0.6 MPa decrease in dawn leaf water potential which contrasted with a more gradual decline in mid-afternoon leaf water potential. Such a change in leaf water potential (particularly at dawn) was found to be a very sensitive indicator showing that this change in water status had occurred. At deficits exceeding the critical point reductions in pasture yield were attributable to a reduction in the rate of tiller appearance, an increase in the rate of tiller death, and a reduction in the rate of leaf production (i.e. extension and appearance). Furthermore it was found that spring managements to increase tiller density will result in a greater tiller density at the end of drought, but will not give an advantage in dry matter yield during the drought. Any benefit which might result from a management system to prepare a sward for drought is unlikely to be in summer production but may be in the potential for faster recovery after drought.