The Efficacy of Roundup for Killing Sugarcane

The results of nine experiments to study factors influencing the efficacy of N(phosphonomethyl) glycine (as Roundup) in killing sugarcane are reported. These factors include varying rates, methods of application, disturbance of the cane after application, the presence of dew on the foliage during application and the susceptibility of sugarcane varieties. Other factors such as season, soil type and stage of cane growth were also considered. Ten litres per hectare consistently produced the best kill. Band sprays were superior to overall sprays when the coverage was adequate. Low volume applications were marginally superior to high volume applications. Disturbance of the cane one day after treatment only affected the kill very slightly under favourable conditions, while dew had no effect on cane kill. Varieties differed in their susceptibility but at 11,2 Iitres per hectare all varieties tested were adequately killed. The kill was unacceptable in all experiments where cane was sprayed in winter. Cane growing on light soils was more susceptible to treatment than cane growing on heavy soils. The stage of cane growth had a marked effect on the kill achieved. Introduction Roundup, a formulation of glyphosate containing 360 grammes per litre of acid equivalent of the isopropylamine salt of N-(phosphonomethyl) glycine, was registered in 1975 for killing sugarcane in South Africa. Being a broad spectrum herbicide with no soil residual activity it is ideal for eradicating the old sugarcane crop when using the minimum tillage system to re-establish a new plant crop quickly. Complete eradication of the old crop is essential to prevent the transmission of diseases, particularly Ratoon Stunting Disease (RSD), to the new crop. Extensive use of Roundup since 1975 has confirmed its ability to kill cane under the right conditions when applied at a rate of ten litres per hectare. Regrowth after such an application can be controlled mechanically or by hand. An increase in the cost of the chemical to about R19,OO per litre however, has created the need to investigate the possibility of using lower rates by means of different application techniques or the use of additives to Roundup. Commercial applications of Roundup are generally made by means of tractor-mounted boom sprayers or leveroperated knapsack sprayers fitted with floodjets which are held directly over the cane rows. This means that adequate coverage of the interrows occurs at the same time thus killing any weeds that are present, but also resulting in a loss of Roundup on non-target areas. As a result of this some emphasis has been placed in the trials on row only or band sprays of Roundup. The effects of additives to Roundup, disturbance of the plant after application, stage of plant growth and rates of Roundup on the efficacy of the treatment on various weed and crop plants have been reported by many workers (Anon1, Baird and Begeman" Fernandez and Bayer", Parker5, Suwunnamek and Parker"), and it was considered desirable to test these findings on sugarcane. Experimental The commercially available fonmulation of glyphosate, Roundup, was used in all the experiments reported here. All experiments except one were conducted using a randomised block design with four to six replications. Plots consisted of four to six rows which were six to ten metres long, giving 24 to 60 metres of cane row per plot. Eight of the experiments were used to evaluate various methods of application, four were used to test the effects of additives to Roundup, and two included a number of sugarcane varieties. Two experiments included treatments to evaluate the effects of disturbing sugarcane foliage after the application of Roundup and one was used to test the effects of dew on the foliage at the time of spraying. Methods of application included the use of conventional high volume equipment such as the leveroperated knapsack sprayer fitted with various types of nozzle, and low volume equipment such as the controlled droplet applicator and the motorised knapsack sprayer. High volumes ranged from 150 to 350 litres per hectare and the nozzles used included floodgets such as the Spraying Systems TK5, Demarquest APM3, Delavan D5 and 1CI yellow polijet and even-spray fanjets such as the Spraying Systems 8003-E and 8004-E. Nozzles were always held directly over the cane row except when extra nozzles were used to spray the sides of the cane plants. Swath widths varied and full coverage was invariably achieved by using the TK5, APM3 and D5 jets while the swaths produced by the polijet and fanjets were narrower and provided only a band of spray over the row. In the case of the band sprays the rate of Roundup applied per sprayed hectare differed from the rate sprayed per total hectare. All rates in the tables are listed as the rate applied per total hectare. Low volumes ranged from 19 to 45 litres per hectare and applications were directed over the cane row. The soil series and clay contents of the soils on the experiment sites and the season in which each experiment was conducted are indicated in Table 1.