Physiological Drivers of This Strategy
Adequate soybean stand establishment is required for agronomic and economic success. Many growers have adjusted seeding rates based on the theory that stand establishment (early season plant stand ÷ seeding rate) in areas of low productivity is reduced. Therefore, it is reasonable to assume that reduced stand establishment is the driving principal of why seeding rates should be increased in lower productivity areas of the field. However, this is not the case according to Gaspar et al. (2018), who found stand establishment was not affected by yield level, regardless of geographical location in the U.S. Thus, yield level and stand establishment are not connected, and stand establishment is not the driving factor behind the relatively higher seeding rates required in low productivity areas of a field.
Plant attrition is defined as the unexplained plant stand loss from emergence through harvest. Much like that of early season stand establishment, the amount of plant attrition throughout the growing season, and therefore harvest stand, did not differ between yield levels. However, Gaspar et al. (2018) did find that seeding rate affected the amount of plant attrition, in that higher seeding rates experienced greater amounts of plant attrition (data not shown).
Averaged over seeding rates, the effect of plant attrition on final yield is displayed in Figure 2. At the high yield level, plant attrition does not affect yield, while a negative relationship exists for the average and to a greater extent, low yield levels. Therefore, in low yield levels, not only are higher seeding rates required to reach the AOSR, but maintaining this increased plant stand throughout the growing season is critical to maximize yield (Figure 2). The use of seed treatments, appropriate tillage and planting practices, narrow rows, and adequate fertility are all components that can minimize in season plant attrition in areas of lower productivity.
Figure 2. Plant attrition relation to relative yield in three different yield levels.
A key point to remember is that soybean yield is linearly related to light interception, and this relationship is typically more critical in the Northern U.S. versus the Southern U.S. (assuming typical planting dates). Simply put, greater season-long light interception equals greater yield. In highly productive environments, current varieties can maintain yield with slightly reduced plant stands because the individual plant growth rate is not limited, and maximum light interception, and therefore yield, is still achievable. Furthermore, breeding efforts have increased the yield produced per plant, and specifically, this increase is attributed to the branches, not the main stem of the plant (Suhre et al., 2014). This complements lower plant stands by increasing the plant’s compensatory ability where plant stands are lower within highly productive areas (Carpenter and Board, 1997). However, in the inverse direction, breeding efforts have also made current soybean varieties more responsive to higher seeding rates. This complements the increased seeding rate required in areas of lower productivity, where plant growth rate and branching can be limited due to many potential factors, such as precipitation amount, soil water-holding capacity, nutrient supply, rooting depth, etc.
These factors, most commonly limiting in low productivity areas, can challenge the ability of soybean plants to maximize season-long light interception. Increased plant density is therefore required to maximize light interception and yield in these lower yield levels. In the same line, total season-long light interception is typically limited in northern latitudes, and this explains why the increase in seeding rate to reach the AOSR within the low yield level is relatively greater in northern versus southern latitudes (Gaspar et al., 2018). These aforementioned factors all contribute to the true physiological basis driving this soybean VRS strategy.