Kenaf (Hibiscus cannabinus L.) fibre yield and quality as affected by water, nitrogen, plant population and row spacing

Kenaf (Hibiscus cannabinus L.) is a highly productive crop that is cultivated worldwide for its fibre content which may be used to produce various commodities. The kenaf crop was commercially cultivated in South Africa in the 1950’s, but production was discontinued from the 1960’s up to the mid 2000’s. Production commenced again and kenaf emreged as a “new” fibre crop with the first kenaf processing factory in the country going into production in 2006 in KwaZulu-Natal. Due to the importance of kenaf in manufacturing of various commodities, there was a need to investigate the agronomic practices thereof to ensure sustainable yield. Therefore a two year study (2008/09 and 2009/10 summers) was conducted in Pretoria to investigate the influence of nitrogen, plant population, row spacing and water treatments on kenaf growth, yield, chemical quality and microscopic analysis of the fibre. In total, four field trials were conducted at the Hatfield Experimental Farm of the University of Pretoria. In 2008/09 a trial was conducted to investigate effects of plant population (200,000; 300,000 and 400,000 plants ha-1), nitrogen level (0, 50, 100 and 150 kg ha-1) and row spacing (0.17, 0.34 and 0.50 m) under rainfed conditions. Sampling for growth parameters were done at 85, 113 and 126 days after planting (DAP). The biomass and chemical analysis of bark fibre were conducted only at or after the final harvest, at 126 DAP. In general, no clear effect of different treatment was observed on either parameter studied. During 2009/10 three experiments were conducted. The first two had the same nitrogen levels as in the previous season, but were grown either under rainfed or irrigated conditions. The nitrogen was applied as two dressings of 0 and 50 kg ha-1 at planting and 0, 50 and 100 kg ha-1 at thinning (35 DAP). The third experiment investigated combinations of plant population (main plots) and row spacing (sub plots) under rainfed conditions. Due to increasing stem yield with increasing plant population during 2008/09, the lowest population of 200,000 plants ha-1 was left out and 500,000 and 600,000 plants ha-1 were added. The same three row spacings as in 2008/09 were used. Nitrogen was applied at 150 kg ha-1, with 50 kg ha-1 at planting and 100 kg ha-1 at thinning. Growth and biomass parameters, water use efficiency (WUE) (nitrogen trial only) were subsequently measured up to the end of the growth cycle. The chemical characteristics of bark fibre and nutrient removal (nitrogen trial only), nutrient use efficiency as well as the nitrogen contents of leaves and stems were determined only once at final harvest. The number of fibre rings and fibre bundles were assessed only once during the growth cycle. Growth and biomass parameters, WUE and both nutrient removal and nutrient use efficiency generally tended to increase with increase in nitrogen level under both rainfed and irrigated conditions. On the other hand, increasing plant population tended to result in a decrease in all growth parameters, while it increased biomass yield per hectare. Finally, the effect of row spacing was inconsistent for the same parameter from one sampling to another one, and from one parameter to another. The chemical characteristics of bark fibre showed inconsistent responses to all agronomic practices. The number of fibre rings and fibre bundles increased with increasing nitrogen level, decreased as plant population increased, but did not show clear trends with regard to row spacing. In general the plants grown under irrigated conditions performed better than those grown under rainfed conditions. The results of this study revealed that under the environmental conditions of Pretoria, nitrogen levels above 100 kg ha-1 applied in two dressings should result in best plant performance, but most benefit could be obtained under irrigated conditions. A plant population of 500,000 plants ha-1 or higher and row spacing wider than 0.34 m proved to be most suitable for both growth and biomass parameters.