Planting soybeans early in the midsouthern USA raises concerns about both early flowering in indeterminate varieties and attaining sufficient height in determinate varieties for effective canopy formation and harvesting.
Early planting of soybeans will result in shorter plants regardlessof maturity group (MG), variety, or stem type. Expected changes in soybean plant stature after R1 (beginning bloom) can affect choice of planting date, MG, row spacing, seeding rate, and management options.
Soybean varieties planted in late March and early April will take 10 to 14 days to emerge. Indeterminate MG IV varieties planted in this timeframe begin blooming about 40 to 45 days after planting. Thus, flowering of indeterminate varieties may begin within a month after emergence. Determinate MG V varieties begin blooming about 60 days after planting.
Soybean varieties planted from mid-April to early May will take 7 to10 days to emerge. MG IV varieties planted during this time begin blooming about 35 to 40 days after planting. Again, flowering of indeterminate varieties may begin a month after emergence. MG V varieties begin blooming about 52 to 55 days after planting.
Research conducted in the early 1970s in Kentucky compared MG IV varieties with similar maturity but different stem growth habits (indeterminate and determinate) for height and node production. The results indicated that a determinate variety reached more than 80% of its final height and produced more than 90% of its final node number by R1. In contrast, an indeterminate variety reached less than 50% of its final height and produced less than 60% of its final node number by R1. These results have been used to establish an expectation that all determinate varieties increase height very little after R1.
A recent study conducted in the central midsouthern USA using rows spaced 20 inches apart resulted in two main findings.
1) In early April plantings of indeterminate MG IV varieties, average height (6 inches) at R1 was 21% of the average height (29 inches) at stem termination. Plants added an average 10.5 nodes between R1 and stem termination (5.5 to 16). In early May plantings of MG IV varieties, average R1 height (9 inches) was 24% of the 37-inch average height at stem termination. Plants again added an average 10.5 nodes (7.5 to 18) between R1 and stem termination.
2) In early April plantings of MG V determinate varieties, plants averaged 65% as tall at R1 (15.5 inches) as at stem termination (24 inches). Plants added an average 3 nodes between R1 and stem termination. In early May plantings of MG V varieties, R1 average height (19.5 inches) was 64% of the 30.5-inch average height at stem termination. An average 4 additional nodes were added between R1 and stem termination. Canopy closure of MG V varieties grown in 20-inch-wide rows had not been achieved at R1 regardless of planting date.
The above findings lead to two conclusions. 1) Indeterminate varieties grown in the midsouthern US will be short at R1, but should average at least a fourfold increase in height from beginning bloom to stem termination and more than double at-flowering number of nodes. 2) Determinate varieties grown in the midsouthern US should increase height by at least 50% from beginning bloom to stem termination and increase number of nodes by at least 3. These growth increases in determinate varieties result in canopy closure by stem termination time regardless of planting date.