A study of the vegetative and reproductive morphology of Grasslands Huia white clover (Trifolium repens L.) with emphasis on the effects of defoliation and paraquat on seed yield and quality

Abstract

The present study examined the effect of management systems, particularly closing date and paraquat application, on the vegetative and reproductive morphology of 'Grasslands Huia' white clover (Trifolium repens L.) over two successive years in a field mixed (grass and clover) sward situation. More detailed studies involving two different genotypes of 'Grasslands Huia' white clover grown in monoculture were also carried out to examine the effects of cutting and paraquat application. The mixed sward studies clearly showed that November closing dates resulted in highest seed yields. In grass/clover swards the closing of crops in November accompanied by paraquat spraying to remove grass competition either in mid October or at closing, enhanced seed yield. Later grazing and spraying was deleterious to seed yield unless climatic conditions allowed continued vegetative growth into December. In this latter case spraying in November and closing in December gave high seed yields. Treatments involving closing in September and October and spraying 30 days before, at closing or 30 days after closing, always gave less seed yield than November treatments sprayed 30 days before or at closing time. Closing time and spraying time had a marked effect on seed yield components. The most consistent and major effect of closing and spraying treatments in the two mixed sward experiments was on inflorescence numbers. Other components such as seed set, seed weight, and floret numbers were not consistently influenced by management but did vary according to environmental conditions. Studies on the effects of cuttings and paraquat application on plant structure and on seeding potential and yield of 'Grasslands Huia' white clover clearly showed that there is a partitioning between vegetative growth and reproductive development. The vegetative process was characterised by a high percentage of the nodes on main stolons forming lateral stolons in the winter and early spring. Reproductive development in late spring and summer showed that approximately 80% of inflorescences were formed on main stolons. Highest inflorescence numbers were produced from nodes formed in October and November, although floral initiation started during late winter (August). The two genotypes used in this study showed considerable variation in relation to reproductive development. Differences of 47% in inflorescence numbers, 25% in seeds per floret and 13% in seed weight were observed between genotypes. The effects of defoliation by cutting involved a reduction in stolon elongation and a general increase in lateral stolon production, particularly when terminal buds were also removed. Both light and heavy cutting treatments resulted in seed yields which were 70 to 90 kg/ha less than the 657 kg/ha produced by uncut plants. The effect of paraquat application was also detrimental to seed yield, mainly through a direct effect on white clover morphology. Although paraquat reduced the amount of lateral branching on the main stolon the destruction of stolon tissue by the herbicide reduced plant recovery and resulted in a seed yield of only 392 kg/ha compared with 657 kg/ha from unsprayed plants. This effect was most pronounced in pure swards of white clover and was less obvious when the grass component of a mixed sward provided some protection for the white clover by reducing the extent of direct contact with the herbicide. Some potential areas for future research and the potential for white clover seed production in Colombia are also discussed.