Features Seeding/Planting Weeds
Improving productivity and pest control: are longer crop rotations better?

Well-planned crop rotations can provide many agronomic and economic benefits to wheat and other crops in rotation. In a recent project, researchers wanted to assess whether longer crop rotation intervals could lead to increased crop productivity and stabilize economic returns while reducing input requirements compared to more typical summer annual crop rotations.

“We initiated a five-year crop rotation study in 2013 focusing primarily on wheat and canola rotations of different intervals between various host crops,” explains Kelly Turkington, plant pathologist with Agriculture and Agri-Food Canada (AAFC) in Lacombe, Alta.

The project was conducted at three AAFC locations including Lacombe, Alta., Melfort, Sask., and Normandin, Que., and funded by the Western Grains Research Foundation. “We wanted to determine if longer crop rotation intervals with a range of crops including wheat, barley, canola, field peas and flax could lead to increased crop productivity and stabilize economic returns, compared to more typical rotations of canola-cereal-canola-cereal, or continuous cereal or canola. As well as comparing 10 different host crop rotation combinations, we also compared a fungicide application versus no fungicide treatment for each crop in rotation. We also assessed other factors such as influence on pest abundance, soil microbes and soil quality.”

The project

The rotation trials compared one versus two years between wheat crops, as well as different pre-crop treatments including flax, canola, barley and field peas. In Normandin, flax was not included in rotation as it is not a crop commonly grown or adapted to that region. In the final year of the study in 2017, wheat was grown in all rotations. Various factors were assessed and data collected for plant development, weed infestations, canopy disease severity (canola and wheat only), crop yield and kernel characteristics, and soil microbial characteristics. Canopy diseases assessed for wheat included Fusarium head blight (FHB) and leaf diseases, while blackleg and sclerotinia disease severity was assessed for canola.

“Throughout the project, disease levels tended to be low to moderate in most locations, with 2017 disease levels somewhat lower,” Turkington says. “Rotational effects on disease were limited, although there were indications that wheat following barley may be at an elevated risk of increased Fusarium-damaged kernels [FDKs], which would result in downgrading the subsequent wheat crop. The alternating wheat-barley rotation tended to have the lowest yield, and perhaps this rotation had an impact on soil-borne disease issues, such as root rot and take-all, but these were not measured in the current experiment.

“Wheat and barley have a similar spectrum of seed and seedling disease issues and the risk of these issues was likely higher in wheat following barley versus broad-leafed crops. Overall, fungicide application tended to have the most consistent effect on grain yield and thousand kernel weight, especially when the risk of leaf disease and Fusarium development is moderate to high and resulted in direct protection of upper canopy leaf and head tissues.”

Benefit to canola before wheat

There definitely was a benefit when alternative crops of canola, field peas or flax, and particularly canola was grown before wheat in terms of dealing with issues of FDKs and the potential impact on root health in the subsequent wheat crop. Trying to extend the rotation interval between wheat crops or susceptible crops by at least two years is recommended particularly for leaf spots and FHB. A single year between wheat crops for example is not long enough for decomposition of any wheat residues that might carry Fusarium or other pathogens. As well, dockage was an issue at Lacombe and largely reflected volunteer barley development in succeeding wheat crops. This can also be an issue in malting barley crops that follow wheat, where wheat volunteers can increase dockage.

A single year between wheat crops for example is not long enough for decomposition of any wheat residues that might carry Fusarium or other pathogens.

In the canola plots, low to moderate levels of sclerotinia and blackleg were observed, with very low severity rating for both diseases in all years. Fungicide applications included Headline at the two- to six-leaf stage, and Proline at the early bloom to full bloom in canola, the recommended timing for sclerotinia. Turkington notes even with both of those fungicide applications, they did not see any significant increase in canola yield compared to the untreated plots. Canopy diseases were also not prevalent for flax or field peas and therefore no significant impact of a fungicide application in either crop. However, in field peas in some years leading up to 2017, there were issues with aphanomyces root rot disease that had a big impact on yield in those fields. Looking at this trial and others in fields with aphanomyces disease, rotations of at least five-years between field pea crops is recommended to reduce the potential for disease losses.

Managing weed competition

“We also had weed competition challenges in both field peas and flax, in particular with Group 2-resistant cleavers that had a significant impact on yields in those plots, as well as a significant negative impact on subsequent wheat crops,” Turkington explains. “Flax is not very competitive with weeds, and field peas to a certain extent, combined with the fact that both crops had large populations of Group 2 resistant cleavers meant we did not see the benefit we expected by having a non-host crop like flax or field peas in the year prior. Certainly, field pea and flax are excellent rotation crops from a disease perspective, but there may be other issues like weed competitiveness that need to be considered. Rotations with canola prior to wheat helped to limit weed development likely via competitive crop stands and more effective herbicide options.”

“Certainly, field pea and flax are excellent rotation crops from a disease perspective, but there may be other issues like weed competitiveness that need to be considered.”

In the final year of the study, soil microbial assessments indicated that fungicide application reduced microbial biomass C (MBC) at Normandin, and increased MBC at Lacombe. Crop rotation effects were also variable: at Lacombe, pea-based rotations had higher microbial biomass contents and sulphur cycling activities than the other rotations, and canola-wheat-barley-canola and wheat-flax-wheat-flax rotations had the lowest; and rotation effects at Normandin were different from those at Lacombe even though the low sulphur cycling activities in the canola-wheat-barley-canola rotation was also low.

The yield question

“Overall, we didn’t see huge impacts on yield due to rotation except where issues of weed competition, in particular Group 2-resistant cleavers in flax and field peas, and less effective herbicide options resulted in significant weed populations and weed interference,” Turkington adds. “Although rotation can be an effective strategy for improving crop productivity in wheat, avoiding non-competitive crops as well as monitoring shifts in herbicide sensitivity will be critical in terms of avoiding a negative impact due to weed competition on wheat productivity.

“When thinking about crop rotations, be cognizant of the weed population and the history of the use of particular active ingredients in a field. If weed resistance issues are suspected, monitor populations and have suspected resistant populations tested. Also look at modifying the rotation and agronomics to produce more competitive crops, such as narrow row spacing with flax and higher seeding rates, and look at regularly changing pest control active ingredients.”

“It is as important for a farmer to be able to grow a crop successfully as is the ability to sell the crop.”

Turkington notes that when planning crop rotations, growers need to look over the long term and at crops they know they can be successful at growing, as well as being successful in relation to access to a reasonable market.

“It is as important for a farmer to be able to grow a crop successfully as is the ability to sell the crop. This may impact some excellent cropping options, but a variable and inconsistent market may not make them as favourable an option in some rotations. Overall, an integrated approach to crop and pest management including long-term crop rotation intervals of at least two years between wheat crops and other susceptible crops, is important for promoting higher yields and productivity.”