Dr. Ramona Mohr, Agriculture and Agri-Food Canada – Brandon – Spring 2021 Pulse Beat
Effective early-season management practices are key to growing successful soybean crops in short-season growing regions like Manitoba.
Despite ongoing improvements in soybean genetics that have led to the introduction of varieties well-adapted to the prairie climate, and supported widespread production of soybeans in Manitoba, soybean is inherently a cold-sensitive, long-season crop. Damage from cold temperatures at seeding along with spring or fall frosts remains a risk in some locations and years. Cold and wet spring conditions may significantly delay crop emergence, contribute to seedling disease and reduce crop vigour and/or stand, whereas spring frosts may cause varying levels of crop damage or loss depending on the timing, severity and duration. Similarly, late-season frosts can result in yield reductions or total crop loss depending on the stage of crop development and frost severity.
These risks can be partly managed by growing soybean varieties that are well-adapted to local conditions, planting into warm soil to reduce the risk of chilling injury and hasten emergence, and by considering the 24-hour forecast following seeding to avoid cold and wet conditions that may contribute to chilling injury. Calendar date is another important consideration for soybeans in short-season areas. Seeding soybeans too early increases the possibility of cold temperature damage and spring frosts, whereas late seeding may reduce overall yield potential and increase the chance of fall frost damage. Managing the order of seeding on-farm so that more frost-tolerant crops like cereals are seeded prior to soybeans may be helpful in reducing the risk of cold temperature damage to soybeans in spring. It may also contribute to increased yield potential in crops like cereals.
There is also some interest in the potential for using different residue management strategies to create a more suitable seedbed for soybean establishment. Theoretically, using residue management practices to modify the early growing season micro-climate or give the crop a competitive advantage under stressful conditions, might help create growing conditions more conducive to soybean establishment, growth and yield. The question is how effective this is under Manitoba conditions, and what are the relative costs versus benefits.
To begin to address this question, a series of small-plot soybean trials were conducted in western Manitoba from 2015 through 2017. In all cases, soybeans were planted near or during the recommended planting window for soybeans in Manitoba, into soils measuring 15°C or higher. The effects on soybean of preceding crops, including wheat (tilled, stubble with straw removed or returned), oat (stubble with straw removed or returned) and canola (stubble with straw returned) were studied over 12 site-years. In about half of the site-years, tillage and straw management practices influenced seedbed conditions at soybean planting. In those cases, tillage warmed the seedbed at planting by 1 to 5°C compared to untilled treatments, while straw removal increased the seedbed temperature by ≤ 1 to 3°C compared to stubble with straw returned. Tillage and straw removal also reduced seedbed moisture in about half of the site-years.
Although tillage and straw removal resulted in a warmer and/or drier seedbed in a number of cases, these practices hastened soybean emergence in select cases only, by up to one to two days, and had no effect on final soybean stands. More importantly, residue management practices had no effect on soybean yield in the great majority of site-years (10 of 12 site-years) and limited effects on soybean seed weight, protein and oil. In two of the 12 site-years where yield differences were observed, higher yields were not consistently associated with higher spring soil temperatures.
In part, the early-season differences in seedbed temperature and moisture measured in the various residue management treatments may not have been large enough to affect soybean yield, or the soybean crop may have been able to compensate for differences earlier in the season. Although the results of this study suggest that residue management is likely to have limited effects on soybean seeded into warm soils during the recommended planting window, it is possible that residue management may have a greater effect under more marginal growing conditions.
To look more closely at this question, a follow-up study is currently underway in western Manitoba and eastern Saskatchewan to assess the effect of a wider range of residue management practices (fall-tilled; fall-burned; short stubble with straw returned; tall stubble with straw returned; short stubble with straw removed; tall stubble with straw removed) on earlier and later planted soybean (approximately May 10 and two weeks later, as weather permits). In this study, preceding crops are wheat at Brandon and Carberry, and canary seed at Indian Head.
Based on preliminary results from 2018 and 2019, residue management practices had a similar effect on soybean yield regardless of whether soybean was planted earlier or later. There were differences between 2018 and 2019 in the effects of planting date, however. In 2018, seeding date had no effect on soybean yield at any site, whereas in 2019 earlier seeding increased yield by an average 7.7 bu/ac at Brandon and 3.5 bu/ac at Indian Head compared to later seeding (Figure 1A). At those sites, soybeans had been seeded on May 9 and 29 at Brandon and on May 14 and 30 at Indian Head. In part, earlier seeding in 2019 likely helped the soybean crop avoid cold conditions early in the fall that delayed crop maturity and harvest. In both 2018 and 2019, residue management affected soybean yield only at Indian Head, with tall stubble showing some yield benefit over other residue management practices (Figure 1B). Analysis of 2020 field samples is currently underway, with field trials set to wrap up at Brandon following the 2021 growing season.