Early-season spatial plant-to-plant uniformity can affect soybean yields

Abstract Soybean (Glycine max L. Merril) seed costs have steadily increased over the past decades and triggered a growing interest in reducing seeding rates, while improving profitability via seed savings and better yields. However, little is known on the implications of early-season spatial plant-to-plant uniformity on soybean yield under the current reduced seeding rates and modern varieties. Therefore, the objectives of this study were to i) explore traditionally used and new metrics to characterize early-season spatial plant-to-plant uniformity, ii) select and identify the best metrics relating the spatial plant-to-plant uniformity and soybean yield, and iii) evaluate the effect of stand uniformity under varying yielding environments and seeding rates. Soybean trials evaluating different planters (Max Emerge® and Exact Emerge®) and seeding rates (160, 215, 270 and 321 thousand seeds ha-1) were conducted in rainfed and irrigated conditions during 2019 and 2020 in the US and Argentina. Early-season plant-to-plant spacing was recorded in all trials and multiple stand uniformity metrics were calculated. Spacing standard deviation (sd) and the sd of the achieved versus target evenness index (ATEI = observed spacing/theoretical plant spacing) showed the greatest correlation with soybean yield (R2 = 0.26 and 0.32, respectively). However, only ATEI sd exhibited a more consistent relationship with soybean yield across environments. Non-uniform stands, described by ATEI sd, negatively impacted soybean yields under low and medium yield environments (mean of 2.7 and 3.0 Mg ha-1, respectively), while high yielding environments (mean of 4.3 Mg ha-1) were not sensitive to stand uniformity. The effect of unevenness spacing was not affected by the tested seeding rates. Our study provides new insights on the ability of soybeans to compensate yield at low yield environments and suggests uniformity of spacing between plants as a target to maximize yields regardless of the seeding rate.