Some effects of environment, age and growth regulating compounds on the growth, yield and quality of sugarcane in southern Africa.
An assessment has been made of the potential for increasing yields of well grown irrigated sugarcane crops beyond their present maxima. The effects of age at harvest and artificial growth regulators on yield have also been investigated. Measurement of the apparent maximum yield of cane fresh weight (tc/ha) and total dry matter (t.dm/ha) of three varieties grown on a specially prepared site, indicated that actual cane yields were 63-70% of the estimated potential maximum. Differences in morphology between two extreme varieties, NCo 376 and CB 36/14, were insufficient to affect growth and yield under good growing conditions. The average productivity over a period of one year of plant and first ratoon crops of three varieties was 65 t.dm/ha and 150 tc/ha. This is a crop growth rate of 17,0 g/m(2)/dy, representing an average conversion into plant dry matter of 1,9% of total incident radiation (ri.) or 4,3% of visible ri. This level of productivity is equivalent to rates reported for sugarcane growing in more favourable parts of the world and for other highly productive crops. In an experiment in which a series of crops were ratooned at different times of the year and harvested at ages ranging from 32 to 72 weeks, sugar yield and all measures of cane quality were closely correlated with cane yield for crops of all ages. Sugar yield varied with age at harvest, according to the cycle of weather conditions experienced by the crop. Crops ratooned in January and February produced the highest yields of 23 tons estimated recoverable sugar per hectare ( ters/ha) at 64-72 weeks of age. The average rate of sugar production at this time ranged from 1,3-1,5 ters/ha/month. At 12 months of age crops ratooned in July and September had the highest yields of 17,2 ters/ha, but they then made little further growth. High rates of sugar production of 1,4-1,6 ters/ha/month were obtained from crops ratooned between June and September and harvested between May and. August at 40-56 weeks of age. Considering data for all crops, cane yield was correlated with weather conditions only up to 40 weeks of age. After this, increments of cane yield were correlated better with the amount of growth already made than with either crop age or the average weather conditions experienced. Artificial chemical ripening was successful on young immature crops harvested in May at the beginning of the milling season, but there was only a small response under less favourable growing conditions and when the crop was older and more mature. Ethrel and Polaris were the most successful of several ripeners tested, Ethrel being more active than Polaris. Cane quality was improved and sucrose storage was increased despite reductions in rates of photosynthesis and sheath and lamina size. The ripening response varied with variety, condition of the crop at the time of spraying, rate of chemical application and the time interval between spraying and harvesting. From this work it is concluded that it will probably be easier to raise the sugar yield of existing high yielding varieties by altering the proportioning of photosynthate in favour of sucrose storage, rather than by increasing cane yield.