Crop Report – August 4, 2021

Posted on by OMAFRA Field Crop Team in Crop Reports
Figure 3. Viable seed produced by purple-top turnip that survived burndown prior to corn planting in 2021.

Cover Crops for Weed Management

It’s the time of year when Ontario farmers are turning their attention to seeding cover crops following cereal harvest. Each grower has their own reasons for growing a cover crop – whether it’s to improve organic matter, hold onto nutrients from manure, provide a nitrogen credit or protect against erosion. In this report, we will explore how cover crops can help manage weeds. We will discuss which species are most effective, share a couple of Ontario examples and provide tips on how to avoid cover crops becoming weeds themselves.

How cover crops suppress weeds

Cover crops can suppress weeds in three main ways: through competition for resources (e.g. water and nutrients), modification of the soil environment (e.g. mulch preventing light penetration) and direct interaction with weeds via allelopathy. The degree of weed suppression depends on how quickly the cover crop establishes, how competitively it grows and whether it releases allelopathic compounds. Lasting surface residue is also beneficial. Cover crops are much more effective suppressing annual weeds versus perennials, which have well-established root systems and energy reserves.

A cover crop such as radish, which establishes and canopies very quickly, is an excellent weed fighter in the summer and fall. However, it leaves very little surface residue in spring, causing minimal residual weed suppression. Species such as winter rye or triticale don’t generate much biomass in fall but can grow aggressively and canopy fully if seeded early. In spring, they green up quickly and compete with early emerging annual weeds. These species also release allelopathic chemicals and can provide surface residue following termination.

For growers looking for a simple approach, straight oats (Figure 1) or a mixture of oats and radish can be very effective in suppressing weeds during the late summer and early fall. Those who are comfortable with overwintering species will benefit from added suppression of early-season annuals. The decision depends on your cropping system and your other goals for the cover crop.

Figure 1. An oat cover crop in the fall of 2020 in Waterloo region.
Figure 1. An oat cover crop in the fall of 2020 in Waterloo region.

 

 

 

 

 

 

 

Ontario research: impacts on glyphosate-resistant Canada fleabane

Recent research from the University of Guelph, Ridgetown Campus, led by Taïga Cholette, evaluated seventeen different cover crop single and multi-species combinations for their effects on glyphosate-resistant (GR) Canada fleabane suppression in corn. Cover crop treatments that included oilseed radish provided the fastest ground cover. All treatments provided greater than 75% suppression of Canada fleabane in May compared to a no cover crop control. However, the greatest and longest-lasting suppression was seen in treatments that included overwintering species such as cereal rye and annual ryegrass.

None of the cover crops evaluated provided full control of GR Canada fleabane and certainly should not replace primary forms of weed control. Also, relatively high seeding rates were used; at rates more typical to Ontario farms, expected weed suppression would be less. Nonetheless, this research highlights the value cover crops can bring in managing herbicide-resistant weeds.

On-farm case study

This season provided a strong example of the weed-suppressing power of cover crops to a Haldimand County grower. He drilled cereal rye last fall (40 lbs/acre) into his entire field except for a small section separated by a ditch. This spring, he planted IP soybeans into the rye before spraying it off. The field received a tank-mix of Roundup®, Canopy™ Pro and Boundary®.

By early July, the field was very clean (Figure 2A). The small section on the other side of the ditch, however, had a high density of fleabane plants poking above the soybean canopy (Figure 2B). Similar examples have been provided in recent years by Ontario growers who grow rye either as a crop or cover crop.

Figure 2. Field without any Canada fleabane present (A) and small section of field that did not have ceral rye cover crop, with high density of Canada fleabane (B). July 9, 2021.
Figure 2. Field without any Canada fleabane present (A) and small section of field that did not have ceral rye cover crop, with high density of Canada fleabane (B). July 9, 2021.
Figure 2. Field without any Canada fleabane present (A) and small section of field that did not have ceral rye cover crop, with high density of Canada fleabane (B). July 9, 2021.
Figure 2. Field without any Canada fleabane present (A) and small section of field that did not have ceral rye cover crop, with high density of Canada fleabane (B). July 9, 2021.

 

 

 

 

 

 

 

 

Cover crop cautions

While cover crops can provide weed management benefits, they can also pose risks. Certain species can be challenging to control in the spring and can themselves become weeds. Annual ryegrass, despite its weed suppression abilities, is very difficult to terminate and has escaped control in numerous instances in Ontario. Also, some species can unexpectedly overwinter and may not be controlled by a glyphosate-only burndown, depending on their growth stage. Figure 3 shows such an example, where purple-top turnip survived winter, was not killed in mid-May with a high rate of glyphosate and produced viable seed. Early control is key to achieving effective termination in such cases.

Figure 3. Viable seed produced by purple-top turnip that survived burndown prior to corn planting in 2021.
Figure 3. Viable seed produced by purple-top turnip that survived burndown prior to corn planting in 2021.

 

 

 

 

 

 

 

 

The bottom line

Cover crops are a tool that can augment the weed management program on your farm. If you’re new to cover crops, start small and simply with species that winterkill. If you are looking for additional weed suppression, consider species like cereal rye that provide spring growth. Integrating cover crops requires planning and additional management but can provide strong benefits, especially in assisting to control herbicide-resistant weeds.

Resources

References

Cholette TB, Soltani N, Hooker DC, Robinson DE, Sikkema PH (2018) Suppression of Glyphosate-resistant Canada Fleabane (Conyza canadensis) in Corn with Cover Crops Seeded after Wheat Harvest the Previous Year. Weed Technology. 32:244–250. doi: 10.1017/wet.2018.19

 

Weather Data

Location   Weekly July 26 – August 1 Accumulated
Year Highest Temp (°C) Lowest Temp (°C) Rain (mm) Rain (mm) April 1st GDD 0C April 1st GDD 5C April 1st CHU May 1st
Harrow 2021 32 12 2 376 2082 1480 2132
2020 33 13 47 354 2638 1904 2925
2019 31 12 14 414 2550 1813 2730
Ridgetown 2021 31 9 10 363 1955 1362 2000
2020 32 11 44 383 2507 1788 2752
2019 30 10 108 546 2413 1678 2569
London 2021 30 9 20 303 1942 1355 1974
2020 32 12 36 378 2445 1728 2660
2019 29 8 39 536 2309 1600 2482
Brantford 2021 29 7 17 282 1912 1322 1931
2020 32 12 1 213 2470 1759 2641
2019 32 8   274 2386 1664 2543
Welland 2021 28 9 19 292 1910 1317 1943
2020 30 13 7 292 2506 1788 2761
2019 30 10 32 403 2452 1714 2650
Elora 2021 28 6 17 258 1765 1185 1786
2020 30 9 18 299 2269 1577 2472
2019 29 7 13 417 1903 1255 2008
Mount Forest 2021 29 8 33 319 1766 1188 1795
2020 29 12 40 428 2246 1567 2494
2019 29 6   129 2153 1448 2319
Peterborough 2021 29 7 19 290 1761 1170 1767
2020 31 7 7 221 2138 1478 2283
2019 30 5 13 352 2133 1425 2209
Kingston 2021 28 10 9 211 1808 1217 1828
2020 30 8 59 319 2441 1718 2702
2019 28 12 14 364 2368 1634 2546
Kemptville 2021 29 9 10 239 1893 1296 1875
2020 33 8 21 343 2396 1683 2588
2019 32 8   203 2263 1551 2378
Earlton 2021 24 8 25 419 1638 1066 1602
2020 30 5 22 324 1926 1327 2133
2019 30 7 25 363 1791 1168 1919
Sudbury 2021 25 10 18 302 1682 1110 1676
2020 29 9 12 417 2006 1382 2232
2019 29 6 42 401 1841 1214 1984
Thunder Bay 2021 30 5 10 239 1538 980 1527
2020 28 4 11 229 1800 1206 1970
2019 29 4 54 322 1734 1103 1837
Fort Frances 2021 28 6 16 186 1630 1066 1663
2020 29 4 17 308 1965 1334 2141
2019 27 4 42 413 1859 1199 1962
Report compiled by OMAFRA using Environment Canada data. Data quality is verified but accuracy is not guaranteed. Report supplied for general information purposes only. An expanded report is available at www.fieldcropnews.com.