Effect of glyphosate application time on yield parameters of South African glyphosate-resistant maize cultivars

Glyphosate is the most used herbicide in South Africa. Due to observations by some South African maize producers that the application of glyphosate to glyphosate-resistant (GR) maize cultivars resulted in reduced yield, we conducted an in-depth study under local conditions. Through field trials, over two seasons (2017/2018 and 2018/2019), we investigated whether the application time of glyphosate would impact maize yields negatively. Various yield parameters were measured subsequent to glyphosate application to the local GR maize cultivars DKC74-74BR, DKC78-79BR, KKS4581, KKS8408, BG5785BR, PAN6R-710BR, P1814R and P2880WBR. Four glyphosate products were included (Roundup PowerMax®; Slash Plus 540 SL; TouchdownForte® and MambaTM DMA 480 SL), resulting in 32 cultivar x glyphosate product combinations. Each product was applied at V4, V4+V6, V6 and V8 growth stages together with an untreated control. Yield parameters measured (ears per plant, rows per ear, kernels per row, thousand kernel mass and yield) were expressed as a percentage of the control. The trials were planted as randomised complete block designs with three replicates. Limited response was observed with all the parameters investigated, with a significant negative yield response, greater than the untreated control, observed in only 3.1% of the cultivar x glyphosate product combinations evaluated. No clear trends or discernible and consistent impacts on yield and yield parameters could be established based on the application time of glyphosate (within label recommendations) across seasons. The findings contribute significantly to the knowledge base and current understanding of the international community and local producers alike regarding the effective use of glyphosate and generic variations thereof in crops of diverse genetic backgrounds.Significance: Limited response in the yield parameters evaluated were obtained in response to the application time (V4, V4+V6, V6 and V8) of the four glyphosate products on eight GR maize cultivars tested (p=0.1). Inconsistent patterns or trends were detected in cases where significance was obtained, implying that it would not be possible to draw accurate conclusions or formulate recommendations. Application time of glyphosate did not result in a significant reduction in yield compared to the untreated control, in the majority of the cultivar x glyphosate product combinations investigated, confirming that glyphosate application conducted within label specifications would not reduce yield, irrespective of the glyphosate product or genetic background of maize.&nbsp