Pointing out a field planted with the same crop year-after-year is a good way to make crop pathologists wince. It means any crop-specific pathogens have multiple years to build infection load.
Rotation isn’t just about disease though, one pulse and soybean expert notes. It can be a big factor when the combine hits the field the following year.
Why it matters: Yield, soil health gains and reducing disease risk and pathogen loads are among the benefits experts say producers can see if they’re planning their rotation strategically.
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There are certain crop pairs that tend to give each other a yield advantage when one is planted after the other, according to data from the Manitoba Agricultural Services Corporation (MASC).
Looking at rotation information and yield from 2011 to 2020, the agency finds that, when planted on their own stubble, crop yields tend to fall below average. Red spring wheat, for example, produces just 85 per cent of its average yield when grown on its own stubble. Canola yields 93 per cent of average under similar conditions.
In contrast, corn sees a dramatic increase when grown after potatoes, yielding 118 per cent of its average.
For Dennis Lange, pulse specialist with Manitoba Agriculture, interest lies in the best partners for crops like soybeans, navy beans and peas.
MASC data puts soybeans on red spring wheat stubble at 101 per cent of average yield. After winter wheat, they perform even better, at 104 per cent. Interestingly, the top precursor crop for soybeans is more closely related. Soybeans planted after navy beans tended to yield 113 per cent of average.
For peas, wheat, canola and —interestingly —soybean stubble at seeding gave slightly above-average yields. Meanwhile, farmers who want to plant navy beans might want to look at their wheat or corn ground (resulting in 111 per cent of normal), old potato fields (126 per cent of normal) or winter wheat or barley (104 and 103 per cent, respectively).
The same data set highlighted how soybeans and pulses, often noted for their nitrogen fixation, impact crops following them.
Cereals (except for winter wheat), canola, flax, sunflowers, corn and peas all saw a boost the year after soybeans, with barley registering the highest boost at 110 per cent of average yield. Non-winter-wheat cereals and canola yielded higher after peas, while flax yield after peas jumped to a whopping 126 per cent of normal.
Navy beans had perhaps the greatest impact. While canola saw 101 per cent of normal yield, wheat fields, oats, barley, soybeans and corn all saw 110 per cent or higher of normal yield when planted into navy bean stubble.
“Probably the biggest thing is, when you’re planning your rotation, and you’re going from a pulse to a cereal crop or another crop, is to kind of look at what nutrients you have left in the soil,” Lange said.
He also urged caution on some crop partnerships due to potential contamination. Peas were listed among the crops with a yield boost after soybeans (106 per cent of normal), but soy is also listed as an allergen to some pea and dry bean buyers, and planting peas or dry beans after soybeans could be a recipe for marketing headaches.
There’s also crop chemistry to consider. When going from a cereal to a pulse, Lange urged producers to review any of what they’ve sprayed and weigh carryover risks. If it’s a field they’ve just acquired, they should get that previous year’s information.
The case of peas
Last year, Statistics Canada reported about 261,700 acres of field peas in Manitoba. It’s a number that has zig-zagged, from just under 218,000 in 2023 to 255,500 the year previous and just over 200,300 the year before that.
“We have longer rotations to start with. Typically, one in six years, one in seven years for peas on a piece of ground is very common,” Lange said.
Diseases of particular note for peas underpin those long rotations.
“If your rotation is too tight, things like aphanomyces root rot can really take a hold in a pea crop if you have a wet year — dramatically reducing your yield,” Lange said. “(The) long-term pea grower realizes that shortening that rotation can really be devastating with respect to disease issues.”
Many pea producers choose to plant a cereal like spring wheat the following year. It’s a good strategy, according to Lange.
“It kind of like maximizes your yield potential when you’re putting a cereal on (after peas),” he said.
Soybeans
The potential profit may tempt growers to plant soybeans in the same field two years in a row.
Lange cautions against that. Planting soybeans back-to-back is akin to speeding while driving, Lange said:
“Eventually, you’re going to get caught, and then you’ve got a price to pay for that.”
He also urged producers to soil test their fields.
“That soil test is really important for determining not only nutrient levels, but things like your calcium carbonate levels and soluble salt levels,” Lange added.
If salts and carbonates are relatively high, and producers plant a soybean variety that is susceptible to iron deficiency chlorosis, they could see a yield reduction if growing conditions aren’t ideal that year.
