The response of perennial ryegrass (Lolium perenne L.) seed yield to irrigation

A first year stand of perennial ryegrass (Lolium perenne L) cultivar GA66 (AR1Endophyte) established under a mobile rainshelter which excluded rainfall during crop growth was used to investigate the response of perennial ryegrass seed yield to the timing and severity of drought. The main research objective was to develop a relationship between seed yield and potential soil moisture deficit (PSMD). The experimental crop was established on a deep (>1.6m) Templeton silt loam near Lincoln, New Zealand during the 2000/2001 season, with twelve irrigation treatments being imposed to expose the crop to drought of varying duration at different stages of development. There was no clear advantage in irrigating a perennial ryegrass seed crop on this soil. Although there was a strong response of total biomass (straw plus seed) to variations in water supply, variations in the seed yield in the experiment were unrelated to the treatments imposed. Irrigation amounts varied from nil to nearly 460mm. These produced potential soil moisture deficits ranging from 110mm to 496mm. There was no systematic response of seed yield to the maximum potential soil moisture deficit (Dpmax) as it increased from 110mm to 496mm. Yield components were not affected by Dpmax, but there was an increase in ear length with decreasing Dpmax. However, this increase in length was not associated with an increase in seed yield. Above ground biomass produced by the crop decreased linearly with increasing Dpmax above a value of 300mm (P<0.001). It is therefore suggested that the limiting deficit for biomass production of perennial ryegrass on this soil type was 300mm. The response of biomass to drought was related to variations in both vegetative and reproductive tiller dry weights, changes in the leaf area of the crop and a decrease in the Harvest Index (HI) with increasing biomass. It is suggested that the changes in stem and ear length may both be linked to compensatory translocation, or the ability to compensate seed development under stress. The harvest index was unaffected by water stress until a critical deficit of 300mm was exceeded at which point the total weight of the vegetative tillers declined. From the results it appears that the perennial ryegrass seed crop may be able to compensate for the substantial reductions in biomass yield that were associated with the high potential deficits achieved, and that the reproductive stem and vegetative tillers may contain sufficient assimilate to support seed development. The major conclusion drawn was that perennial ryegrass seed crops that are grown on deep (<1.6m) soils may not need irrigation if the soil is at field capacity at sowing. On shallower soils with a much reduced available water holding capacity a seed yield response is much more likely. The difficulties associated with applying basal treatments to plots with widely varying seed yield potentials are also discussed.