The Effects of Post-treatment Moisture Stress and Varying Amounts of Applied Nitrogen on The. Ripening Responses of Sugarcane to Glyphosate

The effects of longor short drying-offperiodsand varying amounts of applied nitrogen fertilizer on the ripening response to glyphosate and Ethrel were tested on variety NCo376. The responseto glyphosate was markedlyreduced when post-treatment moisture stress occurred but the responseto Ethrelwasnot affected as severely. Resultsconfirm previous findings that responses to glyphosate were not affected by the amount of applied nitrogen but responses to Ethrel were slightly greater with larger amounts of applied nitrogen. No damaging residual effects on the following ratoon cropswereevident whensoilmoisture stressdeveloped after glyphosate had been applied. Introduction The potential for the useof ripenersisgreateston irrigated cane grown in the northern semi-arid parts of the South African sugar industry. Increases in sucroseyieldshave averaged18%(3,2ts ha-I) in irrigatedcaneat Pongola(Clowes and Inman-Bamber;' Donaldson and Inman-Bamber)' and in the Philippines,a 26%increasein grams sucrose per stalk was recorded with the use of chemical ripeners (Tiancoand Gonzales).' The effect that agricultural practices may have on the potential yield increase from ripeners is important and these have been discussed by Clowes and Inman-Bamber,' It has been established that the physiological state of the cane at the time of spraying affects the response to the ripener. Clowes and Inman-Bamber' showed that the occurrenceof moisture stressaffected the ripeningresponse to glyphosate while Rostronshowed that responses to Ethrel werenot affected by stress unlessthe symptomswereclearly apparent. In the Eastern Transvaal, irrigation is usually suspended for about six to eight weeks before harvesting in the winter months and for about four to five weeks in the summer months. Because the average evapotranspiration rate (Et) during the summer months is 6 to 7 mm d-' it is likelythat the readily available soil moisture would probably be depleted within about 14 days of suspending irrigationin soils with less than 100mm available moisture, unless there is rain. The effect that drying-offof irrigated cane has on the responses to glyphosate and Ethrelare reported in this paper and the interaction between the levels of applied nitrogen (N) and the responses to glyphosate and Ethrel are also discussed. Materials and methods The experiments were sited at the Shakaskraal farm of the South AfricanSugarAssociationExperimentStation on the north coast of Natal. A split-plot randomized blockdesign with four replications was used. Whole plots which comprisedsixcanerows 1,378 m apart and 12,2 m longwere treated with varyingamounts ofN. The plots were then split lengthwise for the ripener treatments and at the time of harvest the cane from the net plots comprising four rows 5 m long was weighted and sampled. The variety used in the experiments was NC0376 which was grown on a Longlands form soil with a rooting depth varyingfrom 450 to 900mm. The mean availablemoisture capacity (AMC = field capacity to wilting point) in a saturated soil profile wascalculatedto be 75 mm. Rainfall was supplemented with irrigation by means of overhead sprinklers delivering 25 mm when the available soil moisture (ASM) wasbelow50 mm according to a soil moistureprofit and loss account. Because of technical problems, irrigation was inadequate and soil moisture was depleted on several occasions in Experiments I, 2 and 4. However, at the time of ripener application the condition of the cane wassuitable for chemical ripening. The various treatments were applied to the 5th, 6th, 7th and 9th ratoon crops (Experiments 1 to 4 respectively) and the 8th ratoon crop was cut back to change the cropping cycle so that the effects of Ethrel could be tested on cane harvested in autumn. Details of the experiments are shown in Table 1. Nitrogenand potassium wereapplied as split dressings in the form of urea (46%) and potassium chloride (KCQ,50%) respectively. Superphosphate was applied where necessary in Experiments 1,2 and 4 and third leafsamples weretaken from all plots at regular intervals to monitor the N levels. Irrigationby overheadsprinklerswassuspended fora period beforeharvesting. Half the number of plots (W, plots)were then irrigated by means of perforated pipes placed in the interrows so that a predetermined amount of water could be applied to these plots to maintain the soil moisture at the required level until the time of harvesting. In these W, plots irrigation was suspended about ten days before harvesting. The remaining plots (W2) were not irrigatedduring this period in order to simulate an extended drying-off period of 30 to 60 days. The estimated ASM in each of the two treatments from the time of spraying to the time of harvestingthe crop is shown in Figure I for each of the four experiments. Ripenerswereappliedas split plot treatments.Glyphosate was applied to the same plots in Experiments I, 2 and 3, and in Experiment 4 the plots which received Ethrel treatment were re-randomised. The ripeners were applied by a hand-operated knapsack sprayer with an overhead boom fitted with two TK 1,0 floodjet nozzles. The spray mixture was pressurised to 200 kPa by CO2 and was delivered at a rate of 70 I ha-'. The rates at which the ripeners were applied are shown in Table I.