Efficacy of glyphosate for fallow weed management

Field and greenhouse experiments were conducted at Gezira research station during season 2004/05-2005/06 to study the herbicidal efficacy of glyphosate on fallow weeds as affected by time of application, rate, use of adjuvants, tank mixing with 2,4-D and relative humidity. Glyphosate, regardless of rate, was less effective when applied late in August or in mid October in comparison with applications made in September. The highest activity and best control of weeds were achieved from treatments made in mid September. Glyphosate at 0.18kg a.e./fed displayed moderate (52%) effects on grasses with poor persistency (27%) when applied late¬August, satisfactory activity when applied late-September and excellent performance (93-96%) on grasses when applied mid-September. The herbicide efficacy of glyphosate was dependent on rate. At 0.18kg a.e./fed control of grasses and broad-leaved weeds ranged between 80-100 and 57¬60% respectively. Incre asing the herbicide rate to 0.36 or 0.54kg a.e./fed effected 82-100% and 75-88% control of grasses and broad-leaved weeds, respectively. Generally the herbicide was more effective on grasses than broad-leaved weeds. Herbicidal activity also varied with the weeds speCies m that Ociinum basilicum was excellently controlled (88-100%) by a dose of 0.18kg a.e./fed whereas Rhynchosia memnonia, Corchorus Jascicularis and Ipomoea cordoJana required 0.72 to 0.90kg a.e./fed for excellent (80%) control. Soil samples taken from glyphosate treated plots revealed that application of the herbicide reduced weed seed bank by more than 40% regardless of the herbicide rate. Greenhouse studies showed that glyphosate applied at a sublethal dose (0.36kg a.e.lfed) before seed setting, did not kill Xanthium strumarium, but it reduced the weed potential of seed production by 78%. Adjuvants such as ammonium sulphate (AMS) or oils have poor effects on weed when added alone. However, they increased efficacy of glyphosate considerably irrespective of rate. In this regard glyphosate alone at 0.27kg a.e.lfed displayed 55 - 95% and 52 - 56% control of grasses and broad-leaved weeds, respectively in comparison with a respective control of 78 - 100% and 77 - 85% on addition of AMS and 70 to 100% and 69 to 89% on addition of oil to the same herbicide dose. Adjuvants also broaden the spectrum of weeds controlled by the herbicide. 1. cordo/ana is tolerant to glyphosate at 0.36kg a.e.lfed while the same rate resulted in 71 and 100% control of the same species on addition of AMS or oil. The same trend of effects of adjuvants on glyphosate was observed under greenhouse conditions. The herbicide alone at 0.36kg a.e.lfed effected 17 to 50% control of X strumarium, but when it was mixed' with AMS or oil each at 1 %, a 70 - 100% control of the weed was achieved. Glyphosate alone at 0.18-0.36kg a.e.lfed was more effective on grasses than broad-leaved weeds. 2,4-D alone was effective o n broad-leaved weeds but had no effects on grasses. Mixing both herbicides broaden the spectrum of control of weeds. Under field conditions glyphosate alone at 0.36kg a.e.lfed resulted in 95 and 70% control of grasses and broad-leaved weeds, respectively. 2,4-D alone at 0.16kg a.e.lfed resulted in poor control of grasses and 81 % control of broad-leaved weeds. However, when the herbicides were mixed at these rates, 97% control of grasses and broad-leaved weeds was achieved. Under greenhouse conditions, glyphosate alone at 0.36kg a.e./fed gave 50% control of X strumarium, but when it was mixed with 2,4-D at 0.16kg a.e.lfed excellent (100%) control of the weed was achieved. Performance of glyphosate varied with weed growth stage. Spraying of X strumarium, in pot experiment, at 4, 6 and 8 weeks after emergence revealed that younger plants were more susceptible and required lower dose for effective control. Herbicidal efficacy of glyphosate varied with relative humidity. In a p o t experiment a 100% control of X strumarium required 8 h of exposure to the herbicide under low humidity while only 4 h were required to reach the same level of control under high humidity