Effects of Neotyphodium lolii infection and sowing rate of perennial ryegrass (Lolium perenne) on the dynamics of ryegrass/subterranean clover (Trifolium subterraneum) swards

Abstract

There is no clear rationale for use of endophyte-infected or endophyte-free seed of perennial ryegrass when sowing pasture in Australia. This research was undertaken to clarify the importance of ryegrass endophyte in temperate pasture systems. A field experiment was conducted to study the dynamics of perennial ryegrass/subterranean clover swards in response to (a) endophyte infection in perennial ryegrass seed, (b) the sowing rate of this seed, and (c) the cultivar of subterranean clover. A higher density of perennial ryegrass seedlings established from seed infected with the endophyte Neotyphodium lolii (E+) than with endophyte-free (E−) seed and there was a significant interaction with the sowing rate. At the highest sowing rate of 16 kg/ha, about 25% more seedlings emerged in the E+ than the E− treatment. The size of individual ryegrass plants at establishment was reduced as sowing rate increased, but endophyte had no effect on their size. The slopes of the curves describing ryegrass tiller density relationships with sowing rate were similar for both endophyte treatments in the first 2 years but were different in the third year. Between the first and third years, tiller density in the E+ plots sown at the lowest rate increased by up to 1000/m², whereas in E− plots the corresponding increase was only 250/m². Such changes have not previously been demonstrated. This difference is believed to be due to the superior ability of E+ plants to recover after extended dry periods. At higher sowing rates, neither endophyte treatment led to increased tiller densities between the first and third years. The presence of N. lolii had no effect on plant density of subterranean clover during the 3 years of the experiment. In the second year, the 3 clover cultivars tested had similar patterns of reductions in plant density in response to increased sowing rate of the ryegrass in the previous year, but these patterns did not persist into the third year. Although dry matter (DM) yield of herbage in the third year was not responsive to initial sowing rate, the E+ treatment was still significantly higher than the E−. The cultivar of subterranean clover had little effect on companion ryegrass, and total DM production was insensitive to cultivar of clover. The use of endophyte-infected seed is desirable in order to maintain long-term density of perennial ryegrass in sheep-production systems in Australia.