Growth and development of three subterranean clover cultivars and their tolerance of herbicide application : A dissertation submitted in partial fulfilment of the requirement for the Degree of Agricultural Science with Honours at Lincoln University

‘Whatawhata’ is a naturalised genotype of subterranean clover in New Zealand that has been identified as different to the parent of origin ‘Mt Barker’. A field experiment was conducted from the 10th of May 2021 to the 2nd of May 2022 to characterise and quantify key aspects of growth and development of the ecotype and compare it with the cultivars ‘Denmark’ and ‘Woogenellup’. Evaluation against ‘Denmark’ and ‘Woogenellup’ allowed recommendations to be made for the establishment and management of the cultivar for New Zealand grazing systems. ‘Whatawhata’ is a small leaved plant with a prostrate growth habit and exceedingly hairy petioles, trifoliate leaves, and runners. It is differentiated from ‘Denmark’ by the visible leaf mark and structural pubescence, while ‘Woogenellup’ has a large light green leaf with inconsistent hairiness on structural components. ‘Whatawhata’ can be classified as a ‘Late’ flowering cultivar with phenological development consistent with that of ‘Denmark’ at all stages. In comparison with ‘Woogenellup’, the ‘Whatawhata’ strain is slightly later to produce its first trifoliate leaf and reach flowering. ‘Whatawhata’ required 260±21.2 Cd to produce its first trifoliate with phyllochron thereafter 64.4±7.0 Cd/leaf. ‘Woogenellup’ thermal time requirement in the field (230±21.2 Cd) and controlled experiment (188±9.29 Cd) was consistent with published values. Thermal time requirement up to the fourth trifoliate leaf stage was between 417-489 Cd. Runner initiation occurred with a change in photoperiod. The first runner leaf appeared at the same time for all cultivars (683±39.5 Cd). Runner phyllochron was not different for ‘Whatawhata’ (83.3±9.45 Cd) and ‘Denmark’ (64.5±9.45 Cd) but was longer for ‘Woogenellup’ (96.1±9.45 Cd). ‘Whatawhata’ (952±10.9 Cd) and ‘Denmark’ (962±10.9 Cd) required the same amount of thermal time after the 21st of June to produce their first flower. This was longer than for ‘Woogenellup’ (883±10.9 Cd). Legume yield of ‘Whatawhata’ (1898±166 kg DM/ha) was consistent with that of ‘Denmark’ (1986±166 kg DM/ha) and ‘Woogenellup’ (2482±166 kg DM/ha). ‘Whatawhata’ herbicide tolerance is comparable to ‘Denmark’ and ‘Woogenellup’ with a similar response reported for pastures sprayed with flumetsulam and imazethapyr. However, the application of recommended herbicides for weed suppression did not increase total DM yield compared with unsprayed pasture. Phytotoxicity scores supported the lack of difference, with none of the pasture components reaching the commercial threshold (7) for high yield suppression. Cooler temperatures (<10 C at application) may have lowered the efficacy of herbicides utilised. Low moisture availability and a short growing season also reduced the growth potential of subterranean clover. It is predicted that under favourable growing conditions the effects of herbicide would have been consistent with that of previous research. Based on data from this experiment, ‘Whatawhata’ can be managed as a ‘Late’ cultivar in New Zealand with establishment and grazing management consistent with that outlined for other small leafed cultivars like ‘Denmark’.