Considerations for Switching Soybean Maturity Groups for Delayed Plantings

Considerations for Switching Soybean Maturity Groups for Delayed Plantings

Shawn P. Conley, State Soybean and Wheat Extension Specialist, John Gaska, Outreach Specialist, University of Wisconsin, Madison

Delayed corn planting coupled with frequent rainfall events and poor planting conditions have postponed soybean planting across many parts of WI. Since we have just reached the switch date for corn hybrids two common questions have arisen regarding soybean plantings. These are:

1.      When during the planting season should a producer switch to an earlier maturing soybean variety?

In southern Wisconsin, full season soybean varieties (>1.8RM) out yielded earlier maturing varieties (<1.8RM) by 15 bu per acre at early May planting dates compared to only 2 to 5 bu per acre at late May planting dates. In northern Wisconsin, late maturing varieties (1.7 to 1.9RM) also out yielded early maturing varieties (<1.7RM), however the difference was not as great. Switching to an earlier maturing variety when planting after the first week of June will reduce the chance of damage from an early fall frost (Fig. 1.). Unfortunately growers will realize a yield penalty if they choose to move to an earlier maturity groups and lowered seeding rates (Table 1.). It is also important to note that if you do choose to switch to an earlier maturity group soybean, do not use a variety that is more than 0.5 RM earlier than you normally would plant.

2.      When is the latest soybeans can be planted in Wisconsin and still expect a grain yield?

Research from the 1990’s in southern Wisconsin indicates that in two out of three years, grain can be harvested from soybeans planted as late as June 26, although the yields are usually minimal and not generally economically feasible. The frequency of harvesting grain from soybeans planted this late can be increased by using early maturing varieties   (<1.8RM)   in   southern Wisconsin, and <1.0RM in northern production areas. A planting date of June 20 in southern Wisconsin and June 15 in northern Wisconsin, using early maturing varieties, was considered to be the latest practical date. However today’s grain prices coupled with opportunities for late-season discounted treated soybean seed may entice growers to push the planting date window in 2011.

Figure 1. Median frost date for WI.

Table 1. Expected relative soybean yield at four replanting dates compared to predicted yields for a range of plant populations resulting from an optimum planting date of May 1-20 for full season maturity or short season maturity varieties.

Replanting date
Early plant population
Initial planting
(May 1-20)
June 1
June 10
June 20
July 1
ppa x 1,000
——————– % of maximum yield ———————–
200
100
86
89
90
75
75
68
67
61
60
180
98
85
88
87
75
72
66
64
63
60
160
97
84
87
84
73
70
64
61
63
58
140
95
83
85
81
72
67
62
57
62
56
120
93
81
82
78
70
65
59
53
60
52
100
91
80
80
76
67
63
57
49
56
47
80
88
79
77
73
64
61
54
44
51
40
60
86
78
73
70
61
60
51
39
44
33
40
83
77
69
67
57
59
47
34
35
25
* Figures in bold italics are for shorter season maturities.
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