Examining perennial ryegrass (Lolium perenne L.) persistence through identifying genetic shifts within two cultivars after nine years in the field : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Plant Breeding at Massey University, Palmerston North, New Zealand

Abstract

Perennial ryegrass (Lolium perenne L.) is a commercially important forage species in New Zealand agriculture. Ryegrass persistence is important for farmers as it substantially decreases the costs associated with reseeding pastures. Breeding for ryegrass persistence is difficult because of the complex interaction between genotype and environment; and the short time of field trial assessment compared to the expected longevity of pasture. A nine year old cultivar comparison trial at Poukawa, Hawkes Bay, New Zealand was identified and plants surviving in the cultivar plots of ‘Grasslands Samson’ and ‘Commando’ were retrieved. These populations that had survived were termed Persistent. A sample of commercially sourced seed of these cultivars were also grown to represent the ‘Original’ genetic pool of the cultivars sown in the field. Persistent populations were compared to Original cultivar seed to characterise morphology and underlying genetics associated with persistence. Results were interpreted to determine if a genetic shift had occurred in Persistent populations due to advantageous phenotypes surviving. Three methodologies were used to compare populations: 1) In a glasshouse, eight morphological traits were measured after 10 weeks growth for Original and Persistent populations of ‘Grasslands Samson’; 2) Half-sibling families were generated from Persistent and Original populations for both cultivars and were assessed for additive genetic variation of seven traits as one metre rows in the field over 13 months; 3) Simple sequence repeat (SSR) markers were used to explore the genetic composition of Original and Persistent populations of each cultivar. Analysis and interpretation of data showed genetic shifts were cultivar specific. The greatest differences were identified between populations of ‘Grasslands Samson’. Compared to the Samson Original population, Samson Persistent plants had significantly greater means for four traits in the glasshouse and half-sibling families showed evidence of shifting population means of traits associated with animal grazing avoidance. SSR marker results were confounded by late detection of contamination in samples. Analysis of a reduced sample size showed no significant differences between any of the four populations using F statistics and genetic structure analysis. These results suggest future studies could reduce risk of contamination by collecting single tillers from the field of Persistent populations. Further investigation of the genetics of persistence should focus on the role of lamina sheath lengths in tiller production, and using the half-sibling families identified in this study for germplasm development and quantification of genotype-by-environment interactions.