Laura Schmidt, Production Specialist – West, MPSG – Spring 2022 Pulse Beat
POWER IN CROP DIVERSITY
In the broader prairie context, Manitoba farmers actually have pretty diverse crop rotations. When we first think of crop diversity, we probably think of a good mixture of broadleaf and grass crops, incorporating a legume for the nitrogen-fixation benefit. There’s also an advantage to diversity in growing season length, and that came through in 2021. Soybeans shined in areas where they were able to capture and utilize August rains thanks to their longer growing season. In years with even a touch more moisture, peas tend to shine as one of the first crops to come off, freeing up time in fall.
MASC updated their yield response to crop rotation information last year (Table 1). Planting into pulse or soybean stubble has provided a yield benefit for several crops, most commonly for cereals. Of the three legumes in the table, planting into navy bean stubble commonly returned yield responses of 110% or more. Flax purports an astonishing 126% yield response to being sown into pea stubble.
It’s not the best practice, but soybeans following soybeans still report 95% yield response (Table 1). Considering high nitrate fields last fall and that soybean fields are those testing lowest in residual N — soybeans on soybeans may be tempting for some farmers. If your fields have not had noticeable root rot disease pressure or significant weed concerns, this may be an option — but I wouldn’t make it common practice. If this is the route you plan to take, consider that soybeans are also exporting quite a bit of phosphorus off-field in the seed. Plan your P rotation accordingly to maintain or build those levels over time.
FERTILITY – N & P
Fertilizer prices have been overwhelming this year. Pulse and soybean crops are obviously quite attractive right now, thanks to biological N-fixation. Soybeans, peas and faba beans can produce their own nitrogen if they are accompanied by their compatible rhizobia species (Bradyrhizobium japonicum/elkanii for soybeans, Rhizobium leguminosarum for peas and faba beans). But for N-fixation to take off, it is optimal that they are planted into low residual N soils (<50 lbs N/ac). That might be the challenge this year. Research observations suggest that soybeans going into fields with a history of soybeans are better able to still produce nodules under high-N environments. This may be true for peas, too. If you are planning to plant into higher N soils (50–100 lbs N/ac), consider a double inoculation strategy and include a granular form of inoculant to maximize nodulation potential and to ensure they are competitive at fixing N. Above 100 lbs N/ac, it will likely be more economical to pivot those acres to a crop more suited to use that residual N. Read more about planning for legume crops with high N soils here. Dry beans are one crop that could take advantage of high residual nitrate levels in soils testing 90–100 lbs N/ac.
On the phosphorus side, soybeans and pulses are great P scavengers. Soybeans don’t often respond to P in small-plot research, regardless of rate or placement — and this lack of response occurred at even very low soil test levels. For peas, research has indicated a benefit to starter P, but large yield responses are not common. Dry beans, on the other hand, have responded well (more than 300 lbs/ac yield increase) to in-furrow-applied 10-34-0-0 at two to three GPA rates in research conducted by NDSU at Carrington, ND. Broadcast or mid-row band-applied P did not affect yield in those experiments and higher rates of 10-34-0-0 reduced dry bean plant populations.
However, if you’re going to cut P to soybeans this year, remember that they also export quite a bit of P off-field in the seed (0.84 lbs/ bu, so a 40 bu/ac crop would remove 34 lbs P/ac). Peas are also P hogs (0.69 lbs/bu, meaning a 50 bu/ac pea crop would also remove 34 lbs/ac). If ratcheting back on P, be sure to balance inputs and removal throughout the rest of your rotation to adjust for these high-P use crops.
If we’re heading into another dry spring, reduce the amount of seed-placed starter fertilizer to avoid toxicity to seedlings. For peas, the maximum safe seed-placed P is 20 lbs P2O5/ac with 15% seedbed utilization (SBU) under good soil moisture conditions. On 10-inch row spacings, a hoe drill with a 2-inch spread would have a 20% SBU and a disc drill with 1-inch spread would have 10% SBU (divide opener width by row spacing to calculate SBU). A safer bet is to band it away from the seed — below or to the side.
SEEDING
It’s a good year to pay attention to germination test results for your peas. Early reports are showing plenty of seed lots with low germination rates. With more seed handling, these rates can be driven down even further. To plan for success with your peas, check out Anastasia Kubinec’s article where she covers factors to consider for plant establishment.
When it comes to timing seeding this spring, know that early seeding of peas and faba beans avoids flower blasting during the hottest part of the summer and maximizes the competitive ability of both crops against weeds. With soybeans, you have flexibility with the seeding window (first to third weeks of May). So there is an opportunity to work them into wherever they fit best during a busy May. This is a general statement, so for your respective region within the province, be sure to watch for the risk of a late spring frost. For dry beans, on the other hand, it’s best wait until after May long to plant and to target warm soils for rapid emergence.
MPSG’s On-Farm Network (OFN) has been evaluating different soybean seeding rates for ten years and has conducted 100 seeding rate trials across Manitoba. Compiled results suggest that seeding rates in the neighbourhood of 150,000 to 180,000 seeds per acre should be adequate to achieve full yield potential. This aligns well with the small-plot research that tells us to target 140,000 to 160,000 live plants/ac.
HERBICIDES
The first thing to pay attention to in 2022 is your risk of herbicide carryover. Most of our products are broken down by microbial activity, which requires adequate soil moisture and warm temperatures. Residual herbicides can remain in the soil longer if less-than- normal rainfall occurs, increasing the risk of herbicide injury to sensitive crops. As a result, we can use the precipitation map from mid-June to mid-September to inform our risk of carryover for different regions (Figure 1).
There are some caveats here. This map was created using Manitoba Agriculture and Environment Canada weather station data and rainfall can be pretty localized, so it is a generalization for an area. In June and July, we also had fewer rains that dropped larger amounts of rain, so soils dried out between rainfall events. This may not have provided enough continuous moisture for microbes to do their job. Sandy soils with low organic matter are at the greatest risk. Refer to the Guide to Field Crop Protection and the label for recropping information on specific products. In-season, if you have herbicides with residual carryover, you will see injury pop up after a rain, which releases the herbicides from soil particles and into solution to be taken up by the roots.
Supply chain constraints on glyphosate availability are a concern for both in-crop applications in soybeans and pre-season burndowns in advance of other crops. Explore what other options are available to you and develop a plan B and C with your suppliers. If you’re looking at herbicides you haven’t dealt with before, double-check the crop rotation restrictions.
For peas and dry beans, there are not a lot of in-crop options. And those that are there largely rely on Groups 1, 2 and 6. With more and more Group 1- and 2-resistant weeds and Group 6/bentazon having only contact activity, a pre-emergent herbicide with residual soil activity is critical.
Cultural management tools like seeding rate and row spacing are also important considerations. Shade out weeds by choosing narrow row widths and aiming for adequate plant stands (soybeans: 140–160,000 plants/acre; peas: 320–360,000 plants/acre or 7–9 plants/ ft2).
DISEASE DRAW-DOWN?
Will we need a fungicide in 2022? In MPSG’s OFN trials, soybeans rarely respond to a fungicide application. We just don’t have many foliar diseases that cause yield loss and it has been rare to see much for white mould or other stem diseases. After the last few dry years, disease loads have generally been drawn down. It is unlikely we will see severe diseases unless we get excess moisture in 2022.
That being said, there are fields with tighter soybean rotational history. In those fields in 2021, Phytophthora root rot was common despite dry conditions. Root diseases are a one-two punch in dry years since the smaller infected root mass can’t seek out and access moisture like a healthy plant. Soybean cyst nematode is a similar situation — infected roots can’t sustain a plant unless there’s plenty of ambient soil moisture.
A similar story for peas — Aphanomyces root rot will be the disease to watch for, and yield can take quite the hit. If you’re seeing root rots in your peas, get them tested. Whole plants (roots and shoots) can be sent to the Crop Diagnostic Lab for testing, or you can submit soil samples to labs that test for results outside of the growing season.
Peas, on the other hand, are also more likely to respond to a foliar fungicide to control Mycosphaerella blight. In MPSG OFN trials from 2017 to 2021, peas responded to fungicide roughly one-third of the time. The key here is the timing of disease infection and if symptoms are developing up the crop canopy when it’s cool and humid to infect the mid to upper portions of plants. Use the Fungicide Decision Worksheet for Managing Mycosphaerella Blight in Peas to make informed spray decisions.
The best bet to know what is going on in your fields is to get out and scout. Be curious. Dig around.
HOPPERS AND WEEVILS
Lucky for us, peas are not a preferred food source for grasshoppers. Infestations of 10/m2 (1/ft2) are not expected to cause economic losses in peas. However, they do enjoy the fact that soybeans and dry beans stay green longer into the season than other crops. As other crops are harvested or as hay and ditches are cut, scout your beans. We use defoliation thresholds to inform management decisions. For soybeans at V stages: >30% defoliation, at R1–R5: 15% and at R6–R8: 25%. For dry beans, defoliation thresholds are 35% during V stages and 15% after R1. Randomly select two plants at five areas of the field to estimate. Since they move into the crop at field edges, edge sprays are often effective.
Pea leaf weevil is on the rise in Manitoba. Learn more about these weevils in John Gavloski’s article. Seed treatments are currently the most effective management option, but it is unlikely that treatment will be economical given our current pea leaf weevil population levels in the province. The best way for us to get more information on this management practice and its impact on weevils would be to conduct seed treatment trials in peas through MPSG’s On-Farm Network. If this is something you’re interested in, please reach out to our OFN Agronomist, Leanne (ph: 204-751-0439 or email: leanne@manitobapulse.ca).
We are also planning to kick off a pea leaf weevil survey in May and early June to determine weevil abundance to further inform management decisions.
Happy scouting! ■