Comparison of Sucrose Yields Third Leaf N % Dry Matter of Sugarcane Varieties Grown the Pongola Area

In order to determine the suitability of varieties for the Pongola area, two trials were established at the SASA Experiment Substation at Pongola, one harvested every spring and the other every autumn. Random samples of cane stalks were taken prior to harvesting and the ERS % C was determined. Using these data it was possible to study the variation in ERS % C with crop age and time of year. It was found that ERS % C increased markedly during the months of April and May and was tending to reach a peak value during September. NCo 310 was superior, in terms of ERS % C, to NCo 376 and NCo 334,in all cases, except in the plant crop of the autumn harvest. The ERS % cane of N52/219 compared favourably with that of NCo 310 in all cases, except in the second ratoon of the spring harvest. The yields, in terms of tc/ha/100 mm effective water, for NCo 376 and NCo 334 were consistently greater than those of NCo 310 and N52/219 for both the spring and autumn harvests. This resulted in similar yields, in terms of ters/ha/100 mm effective water, for all varieties in all cases. Only in the second ratoon of the spring harvest and in the first ratoon of the autumn harvest was the performance of NCo 376 statistically significantly greater than that of NCo 310. Third leaf sample analysis indicated that the varieties differed markedly in their foliar N % dm content. Introduction During the 1973-1974 season at Pongola, the variety NCo 310 occupied 54 per cent of the total area of land planted to sugarcane, while NCo 376 occupied 42 per cent, NCo 334 1 per cent and other varieties 3 per cent (Havenga4). Havenga states that the switch from NCo 310 to NCo 376 is still taking place too slowly considering the enormous increase in the incidence of smut disease in NCo 310. In August 1971 and April 1972 two trials were established at the SASA Experiment Substation at Pongola to determine the suitability of eight varieties to a spring harvest cycle, viz. October to October, and an autumn harvest cycle, viz. May to May. The varieties planted were NCo 376, NCo 334, NCo 310, N52/219, N55/805, N51/168, CB 36/14 and N7. Only the yields of the first four varieties have been considered for this paper as these are considered the important varieties for the Pongola area. Observations at Pongola have shown that NCo 310, N55/805 and N7 are susceptible to smut disease, while CB 36/14 and N51/168 have not performed well. The variety N52/219 has recently been released and is resistant to smut disease. Rostrong harvested 40 week-old-cane during the months of January, March, May, June, August and October at Pongola and determined the sucrose per cent cane at each harvest. He found that the sucrose was low in January, gradually increasing through to March with a marked increase occurring between March and May, and finally reaching a peak value at the end of August. Stalk samples were taken in both of the variety trials to confirm this reported trend and to observe whether there were differences between varieties in their estimated recoverable sugar per cent cane (ERS 76 C) with respect to the time of year. Gosne112 had found the latter to be the case in Rhodesia. Gosnell and Long3 noted the influence of variety on the nitrogen content of the third leaf. They also noted that increasing crop age caused very large reductions in foliar N, while season also had marked effects on N, which declined markedly between August and April. In an experiment at Shaka's Kraal on a Waldene (Dwyka) fine sandy loam, Moberly7 found that the uptake of N was more efficient in crops grown during a spring harvest cycle, compared with an autumn harvest cycle. Third leaf samples were therefore taken in both trials to see whether the same trends in foliar N could be detected for sugarcane grown in the Pongola area. A split-plot design was used for both trials, where two levels of nitrogen fertilizer were applied to the eight varieties. So far the response to the higher level of N has been inconsistent and therefore the mean yields, ERS % C and the third leaf N contents, for both levels of N, have been used in this paper. Procedure The experiments were planted on a soil of the Makatini series and were fully irrigated. Both trials received 60 mm of effective irrigation water applied on a minimum 25 day cycle. The drying-off period was 10 to 11 weeks in all cases. The dates of planting and harvesting, and the crop age at harvest are presented in Table 1. TABLE 1 The dates of planting and harvesting, and the crop age at harvest for the spring and autumn harvests Autumn harvest Spring harvest