Pulse Beat

Decisions About Soybean Residue Management

Dr. Yvonne Lawley, Department of Plant Science, University of Manitoba

With the increase in harvested acres of soybean in Manitoba, decisions were made about how to manage soybean residue for over 2 million acres in Manitoba in 2017.

For many, these decisions were simple. For others, there are questions about what approach to take with this low-residue crop.

The black snow visible along highways across southern Manitoba during this past open winter reminded many of us how important these decisions are.

Results from a recent four-year on-farm study focused on soybean residue management may provide decision makers with new information about crop performance following contrasting approaches to manage soybean residue after harvest.

Let’s rewind to the fall of 2013 when the soybean residue management project was just getting started. Deep tillage with double-discs and cultivators was commonly used in conventional tillage systems across Manitoba to bury and incorporate soybean residue. Vertical tillage tools were still relatively new, but rapidly gaining in popularity.

Soybeans are considered to be a low-residue crop and stems are cut very close to the ground at harvest. However, some of the farmers that I have talked with described soybean as tough and impenetrable to water – or hydrophobic – so that water would not move into the soil or dry underneath the residue in the spring. Further to the problem, soybean residue was prone to catching and balling up in some seeding equipment.

Later in the study, during the fall of 2016, conversations I had with farmers about decision making for soybean residue management after harvest have had nothing to do with the residue itself, but rather the deep ruts left after trafficking wet soils during soybean harvest.

As a researcher, I have been learning that any project involving residue and tillage is tough. We decided to take an on-farm approach for this project so that we could compare the ability of a range of tillage tools to manage soybean residue.

Four tillage treatments were compared: 1) a deep-till cultivator or double disc – representing the standard practice in conventional till systems when the project started, 2) no tillage or direct seeding, 3) vertical tillage low disturbance (discs set on 0o angle so that residue is somewhat incorporated but mostly left of soil surface), and 4) vertical tillage high disturbance (discs set on a 6o angle so that residue is incorporated with little residue left on soil surface) – which has become a more common practice during the time frame of the project.

We wanted to evaluate the impact of these residue management treatments on spring seedbed conditions (temperature, moisture) and on the stand and yield of subsequent crops. In the last year of the experiment, with the formation of MPSG’s On-Farm Network, we simplified our approach to compare a farmer’s standard tillage method for soybean residue compared to direct seeding into soybean residue.

On-farm experiments were an essential choice for this project due to the large size of tillage equipment and the operating speeds required to optimize equipment performance. Field-length plots were used and each treatment was randomized and replicated three or four times in the field, as space would allow.

The farmer’s own cultivation equipment was used for the standard tillage treatments and the vertical tillage equipment was provided by neighbours or local equipment dealers if it was not available on the farm.

Experiments were established in the fall after soybean harvest, from 2013–2016, in five fields on farms near Boissevain, Winkler, Homewood, Linden and New Bothwell. The soil types for these five fields ranged from loam to clay. You will find reports summarizing the site history, management, and results of each experiment on the On-Farm Network page of the MPSG website.

Test crops were planted the following spring in each experiment. The test crop selected depended on the farmers’ rotation and ranged from wheat to corn and soybeans. Test crops were managed based on standard practices for each farm. All plots within each experiment received identical management across all residue management treatments in the test-crop year.

 

Figure 1. Ground cover by soybean residue after harvest with and without fall tillage.

HOW MUCH SOYBEAN RESIDUE WAS THERE?

First, ground cover by soybean residue was quantified in the fall following tillage treatments (Figure 1). Fall ground cover by soybean residue was reduced in the residue management treatments involving tillage compared to direct seeding (Table 1). As expected, greater reductions in soybean residue ground cover occurred in the disc and vertical till high disturbance treatments.

Across the five experiments in the study, soybean residue in the direct seeded treatment – where there was no residue management after harvest – ranged from an average of 40 to 88% in the fall. In two of the studies, ground cover was quantified in both fall and in the following spring (Table 1).

Ground cover by soybean residue was lowered by 31% and 57% from fall to spring, in these two studies. This gives us a better estimate of how much soybean residue is broken down between harvest and spring planting in Manitoba, even when soybean residue is left on the soil surface.

SEEDING CONDITIONS NEXT SPRING

The impact of soybean residue management on soil conditions at seeding the next year was of great interest in this project. During the emergence period of the test crops, soil moisture and temperature at a 5 cm (2 inch) depth was compared. This depth represents soil conditions around an average seeding depth.

No significant differences in soil temperature or moisture were observed between soybean residue management treatments during test-crop emergence for any of the experiments. When walking in the field in the spring, it was often hard to visually distinguish between soybean residue management treatments (Figure 2). Given the low amounts of soybean residue in the spring, the similarity between residue management treatments was not very surprising.

Figure 2. Soybean residue management treatments (1) Vertical till – high disturbance, (2) Vertical till – low disturbance, (3) Conventional till – deep tillage cultivator, (4) No tillage – direct seeded.

TEST-CROP PLANT STANDS AND YIELDS

Despite the different tillage tools used, there were few differences in test-crop performance between the soybean residue management treatments compared in this project.

No statistical differences in test-crop stand were measured in four out of five experiments for both early and final stand counts.

There were also no statistical differences in test-crop yields between soybean residue management treatments for four of the five experiments.

These are a few simple sentences summarizing a lot of on-farm research effort!

What happened at the sites where there were differences?

Spring conditions during emergence of the 2016 corn test crop at Homewood were challenging. Corn was planted on April 30, 2016 and two weeks of very dry conditions followed, resulting in uneven corn emergence within all treatments in the experiment. Final corn-plant stands in the no-till treatment were 3,000 plants/ac lower than the conventional and vertical tillage conservative treatments.

Soybean test-crop yield was three bushels higher for the fall tillage treatment relative to the no-till treatment at Linden in 2017. There were no differences in plant stand or soil temperature and moisture during emergence to explain this difference.

Although unexpected, soil moisture at 30 cm was slightly higher in the conventional tillage treatment in July during flowering and early pod fill. It is hard to explain why soil moisture would be higher in the standard tillage treatment, but it would account for the higher soybean yield.

WHAT WAS LEARNED?

There were remarkably few differences between soybean residue management treatments in this four-year study using a number of different measures. Once the test crop was planted, it was often hard to distinguish treatments within the field.

Decisions about residue management are always farm, field and equipment specific, but the results of this on-farm study should provide food for thought about our current thinking and practices for soybean residue management in conventional tillage systems in Manitoba.

Some of the concerns about direct seeding into soybean residue that were not addressed in this project, such as ruts after harvest and the seeding equipment or openers for planting into soybean residue would be good topics for future research projects.

The financial and time costs for residue management as well as the risks of losing soil to wind erosion after soybeans are good reasons to test your own ideas about how to best manage soybean residue on your farm.