Site-specific optimisation of soybean seeding rates: evidence from local on-farm trials

Abstract

Limited research exists on optimal soybean (Glycine max L.) seeding rates and plant populations in South Africa. This study utilised on-farm precision experimentation data to evaluate the impact of three management approaches on productivity and profitability. Variable seeding rate trials were conducted over 12 site-years between 2019/2020 and 2023/2024, in three productions regions. Indeterminate soybeans from late maturity to medium maturity groups were planted at seeding rates ranging between 150 000 and 450 000 seeds ha⁻¹. Optimising soybean seeding rates using both flat-rate and variable-rate approaches resulted in improved performance compared to producers’ standard status quo seeding rates. While the optimised variable-rate approach generally achieved the highest yields, its performance was similar to that of the optimised flat-rate approach, indicating that most benefits arose from seeding rate optimisation itself, with variable-rate application providing only marginal additional gains. The magnitude of the benefits varied by region, highlighting the importance of local context in seeding rate decisions. The optimal flat-rate seeding recommendation averaged 293 900 seeds ha⁻¹ with an inter-annual variability of 10.8% and an average yield of 3.68 t ha⁻¹, being site- and year-dependent. Variable rate seeding led to an average increase of 21.5% in seeding rate for eight site-years and a decrease of 11.5% in four site-years, compared to the status quo approach. For the variable rate approach, yield improvements ranged from 30 to 990 kg ha⁻¹, with one site-year experiencing a slight yield decline of 50 kg ha⁻¹. The utilisation of on-farm precision experimentation to optimise seeding rates can enhance on-farm productivity and profitability when integrated with strategic cultivar selection, optimal planting dates and effective soil management tailored to seasonal conditions.