Management options for improved survival of perennial ryegrass (Lolium perenne L.) under restricted irrigation during summer

In northern Victoria, Australia, perennial pastures for the dairy industry have historically been based on perennial ryegrass. However, perennial ryegrass does not perform well under high summer temperatures and water stress. We investigated irrigation, renovation and grazing management strategies for improving the resilience and survival of perennial ryegrass over summer on two farms in northern Victoria over 2.5 years. At each farm, two irrigation bays were sown in March 2015. During summer (late December–mid-March), one bay was not irrigated and the other bay received a single mid-summer irrigation. Each bay was sown to 12 ryegrass cultures: five cultivars of perennial ryegrass either oversown or not oversown, and two oversown cultivars of short-lived ryegrass (SLRG). A mid-summer grazing treatment was applied to one-half of each of these plots. All plots at both sites received the same irrigation and grazing management at other times of the year. There were significant variations in dry matter (DM) production, nutritive characteristics and plant frequencies across sites and seasons, and across renovation, irrigation and grazing management strategies. Cumulative DM yields were higher in the perennial ryegrass than the SLRG cultivars. Summer DM removal was very low at both sites; the ungrazed plots recovered and produced more DM in the following cooler period than the grazed plots in the first year but not the second. Metabolisable energy concentration was higher for perennial ryegrass than SLRG cultivars and was also higher in plots that had been grazed over the summer. There were large differences in plant frequency between species, and there were some differences among perennial ryegrass cultivars. The effect of summer grazing on plant frequency varied at each site and was not significant within an irrigation treatment. This research confirms that perennial ryegrass is not well suited to the hot, dry summers of northern Victoria and suggests that, under conditions of limited summer irrigation, farmers should focus on plant survival rather than summer yield if they are to maximise annual DM production.