Charles Geddes, Research Scientist – Weed Ecology and Cropping Systems , AAFC – Lethbridge – Pulse Beat 96, Fall/Winter 2022
BREEDING EFFORTS ARE serving up soybean varieties that are better adapted for growth in northern climates. However, weeds in these locations tend to emerge quickly and compete early – two things that are still unfamiliar to today’s soybean crops. Weed communities that dominate prairie cropping systems thrive in cool environments, giving them a competitive edge over warm season crops. This can lead to herbicide-only programs in soybeans that result in large selection pressure for herbicide-resistant weeds.Herbicide-resistant weeds are a growing issue in Manitoba and the new frontiers of soybean production. Recent surveys across the Canadian prairies have shown that the majority of kochia populations tested are now glyphosate resistant, while dicamba- resistant kochia populations are also found in the soybean-growing areas of western Canada. In addition, the overlap of canola and soybean growing areas in western Canada creates a unique issue for managing volunteer canola in soybeans, since both crops have similar herbicide resistance traits. There is a need to bolster the competitive ability of soybeans in western Canada crops to help mitigate and manage the impact that herbicide- resistant weeds have in soybeans currently, but also down the road.
A five-year research project began in 2019 to understand how agronomic practices could be used to improve the competitive ability of soybeans grown in western Canada. This project, funded by Manitoba Pulse & Soybean Growers, Western Grains Research Foundation, and the Governments of Manitoba and Canada through the Canadian Agricultural Partnership, set out to provide soybean farmers with recommendations on how to support soybeans in the “war on weeds”.
Preliminary insights from the first three years of research show how agronomic decisions can aid weed management in prairie soybean production.
VARIETY SELECTION
Genotype by environment interactions suggest the optimal soybean genetics in one region will differ from those in another. However, across six site- years of research in Portage la Prairie, MB (2019 and 2020), Saskatoon, SK (2019 and 2020), and Lethbridge, AB (2019 and 2020), the highest-yielding soybean varieties under weed-free conditions within each location also tended to have the highest yield under weed competition. This is good news for soybean farmers because it suggests that breeding efforts aimed at improving soybean yield have not compromised the traits that lead to a competitive crop. Another observation was that growing a bushy soybean variety resulted in greater yields under both weedy and weed-free conditions compared with a slender variety at one of three research sites.
PLANTING DATE
A study comparing different soybean varieties planted early (May 13-22), mid (May 23-June 1) or late (June 2-11) showed that the impact of planting date on weed competition varies among locations, and depends on the timing of weed pressure. For example, about 18% greater soybean yield loss was observed due to weeds emerging after an early planting date compared with mid or late planting dates in one of two years in Lethbridge, AB. In Saskatoon, SK, however, about 20-30% greater yield losses were observed in both years when planting later, while differences were absent in Portage la Prairie, MB. In another study, soybean yield losses were greater when soybeans was planted early compared with mid in two of three environments. Weed density and biomass were also greater in early-planted soybeans compared with mid in all three environments. These results suggest that the impact of soybean planting date on weed competition varies depending on the timing of weed emergence within the field, which is often related to the accumulation of growing degree days and the timing of precipitation events.
Figure 1. Soybean yield loss due to weeds when planted at target densities equivalent to 0.5X (80,000 plants/ac), 1.0X (160,000 plants/ac), 1.5X (240,000 plants/ac), and 2.0X (320,000 plants/ac) the recommended density. Different letters within each environment indicate significant differences (p = 0.05).
ROW SPACING
One study showed that growing soybeans in narrow (9 inch) compared with wide (27 inch) rows resulted in greater yield both in the presence and absence of weeds in two of two years at Lethbridge, AB, but not in Carman, MB (7.5 vs. 30 inches).
SEEDING RATE
Data from Carman, MB (2021) and Lethbridge, AB (2020 and 2021) showed that increasing soybean target densities from current recommendations (160,000 plants/ac) could make the soybean crop more tolerant of weed competition (Figure 1). For example, soybean yield losses were lowest when targeting a very high density of 320,000 plants/ac in Carman, MB, while in Lethbridge, AB, yield losses were lowest when planted at 240,000 plants/ac. Current recommendations for soybean target densities (160,000 plants/ac) resulted in similar yield loss to that of half (80,000 plants/ac) of the recommended target density. These results suggest that higher soybean densities could help to maintain soybean yield when competing with problematic weeds.
COVER CROPPING
Planting soybeans in narrow (7.5-9 inch) rows into a fall rye cover crop terminated with the pre-plant burndown herbicide halved mid-season weed biomass, compared with wide (24-30”) row soybeans without a cover crop. This result was observed across four sites in Carman, MB (2021), Indian Head, SK (2021), and Lethbridge, AB (2020 and 2021). These results suggest that both fall rye cover cropping and narrow soybean rows could interact to help improve the competitive ability of soybeans grown in the Canadian prairies. However, further research is required to determine the potential impact of the cover crop on soybean yield in the presence and absence of weed competition.
Overall, the first three years of this prairie-wide research project have started to uncover which agronomic tools could aid weed management programs in soybean production. However, the efficacy of many of these cultural weed management tools varied depending on the location. It is therefore critical to use these non- chemical practices in combination to help improve the potential weed management benefits of a competitive soybean crop.
INTEGRATED WEED MANAGEMENT
Integrated weed management principles suggest that the use of multiple diverse weed management practices can add up, leading to reduced selection pressure for resistance to a single weed control tool (such as herbicides). This means that the weed management strategies discussed above should be implemented in combination to achieve the greatest benefit. It is noted that many of these options require additional investment in weed control programs, which can be difficult to pencil out if economics of the farming operation are considered only on a year-to-year basis. Rather, proactive investment in the form of integrated weed management takes a longer-term focus on the health of farming systems, suggesting that a small investment in integrated weed management today could help prevent the necessity for large investments down the road.
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