Phenological development in soybean can be delayed when it is grown under long photoperiods (>14 hours). Maturity group (MG) classifications, often not established in Manitoba (MB), are usually based on time (days) or crop heat units (CHU) to maturity, and do not always accurately predict critical crop growth stages (such as the beginning of flowering or maturity) in MB. In MB, soybean rated MG 000 to MG I can be grown depending on the amount of CHU received. Each MG classification is further sub-divided into decimal units. The objective of this study is to confirm differences in soybean phenological development in MB from an established soybean production region, Ontario (ON), and use this information to develop an accurate model to predict critical growth stages for farmers and agronomists in MB and ON. The model was based temperature, photoperiod (latitude), and soybean MG. Furthermore, a controlled environment experiment was conducted to isolate the potential delaying effect of photoperiod.

The field experiments coupled with the growth stage model will aid farmers and agronomists in proper MG selection for their respective environment and provides a timeline of crop development which can be used to estimate when R1 might occur, which can end the window for application of an in-crop herbicide. Results from the growth chamber experiment are valuable for soybean breeders and plant physiologists. If a breeder requires reduced time spent from VE to R1, photoperiodism must be well understood in order for a cultivar to be selected. Increased understanding of how soybean behaves in northern, short growing environments will enhance breeding program efficiency and ultimately result in cultivars capable of successful production in these environments. Furthermore, results from this study have showed that all cultivars tested were photosensitive. It is common for early maturing cultivars to be described as ‘photoperiod insensitive’; however, this entirely depends on the photoperiod or latitude in which it was grown in.

Field experiments should be repeated in additional environments in Manitoba to improve the phenology models developed in this study. Soybean production in Canada continues to expand north and west, into environments with shorter growing environments and longer photoperiods experienced in MB. Additional site-years in new locations will address the same gap in knowledge this study did and provide more data that can be integrated into the growth stage models, which should increase their accuracy and range in geography in which the models can be used across the Prairies.