LAUSR

Soybean [Glycine max (L.) Merr.] planting date and maturity group are important agronomic decisions. This study quantified how maturity group selection and later than optimal planting dates affected grain yield and crop development across Iowa, US. Field experiments were conducted in seven locations between 2014 and 2016. Cultivar maturities ranged from 2.2 to 3.7 MG and planting dates targeted for 20-day intervals from early May to early July. Soybean grain yield ranged from 0.27 to 7.54 Mg ha-1. Cultivar maturity had little to no effect on grain yield at 4 of 7 sites while planting date was significant at all sites (p<0.001) and the planting date and cultivar maturity interaction was not significant. As planting date was delayed, the VER3 and R3-R7 periods were each shortened by up to 15-20 days. The shorter growing period resulted in less radiation and growing degree day accumulation. A exponential-plateau relationship between relative yield and GDD was evident for the VE-R3 phase, with a plateau at 700oC days. A linear relationship between yield and GDD was evident from R3-R7, suggesting greater yield with more accumulated GDD. The opposite relationships were found for photoperiod which had a linear relationship for the VE-R3 and curvilinear for the R3-R7 phases. These results showed that yield potential would be maximized by planting before 20 May. We concluded that planting earlier in the spring was a better management practice than maturity selection to maximize yield and the R3-R7 period duration was critical in determining potential yield.