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Weed Control Management & Tactics for Organic Farms

“An integrated weed management strategy of cultural and mechanical control (and for some biological), is essential on organic farms,” says Katherine Stanley, a Research Associate in the Natural Systems Agriculture Lab at the University of Manitoba.

“A good crop rotation, a competitive green manure year, mechanical weed control, competitive crops and higher seeding rates are also all important,” adds Katherine.

“It’s about managing weeds in a way to suppress or reduce their numbers,” says Ruth Knight, an Independent Consulting Agronomist.   “There are many complexities around weed management, and fields go thru evolutions of weeds,” she adds.

“It’s really going to make a difference if your farm is livestock based, and using manures or not,” says Ruth.  “With hay in the rotation that can really help with perennial weeds such as thistles.”

Pile of manure, great fertilizer, countryside shot

“Incorporating manure in the rotation after a winter cereal and before a cover crop is a great way to apply manure when there is less risk of compaction and complements the fertility cycle. The manure or compost improves the growth and impact of the cover crop and gives less opportunity for weed seeds to grab onto the nutrients,” notes Ruth

“Organic farmers use multiple techniques or “many little hammers” to help keep their weeds under control,” Katherine adds.  Some of those techniques are equipment focused.

“Flex tine harrows are a great tool for organic farmers. Easily available, and you can use them pre-seeding, pre-emergent and post-emergent. Different crops have different optimal timings of use of the harrows, and it’s important to know what this is so you don’t cause too much damage to your crop,” notes Katherine.

“Harrows best control weeds when they are very small or at the white thread stage which means  there is a relatively small window of time for flex tine harrows to be used effectively and requires scouting early in the season,” adds Katherine.

“Traditional inter-row cultivators are also important when growing corn and soybeans.   However, if only using inter-row cultivation, weeds within the crop row can still be problematic,” says Katherine.

“Adding camera- guidance to an inter-row cultivator allows for narrow, as well as wide row spacings.  You can operate at speed with good accuracy reducing contact with the crop. On wide row spacings you can add finger weeders which help to control weeds within the crop row. Inter-row cultivators are able to control larger weeds than the rotary hoe or harrow, so your window of use is longer,” notes Katherine.

“I also think the CombCut has some interesting potential,” says Katherine.  “Unless you have high weed populations you may not see a yield increase in that year after use, but it helps to improve harvest and reduce weed seeds in future years. More farmers are using it, and I think it’s a helpful piece of equipment to keep in mind.”

“Especially this spring [2019], it was very challenging to decide on the best time to go in the field for mechanical weed control,” says Ruth.  “Farmers got impatient – understandably, but because they went in on wet fields, that caused compaction which negatively impacted water infiltration and nutrient cycling in the soil.  It’s a balancing act.”

“Crop vigour and health also are key,” says Ruth.  “If your crop grows better, then your canopy closes faster and blocks out the weeds.  The soil wants to be covered so avoid gaps.  Cover crops, row spacing are important.”

“Ensuring good seeding rates for cover crops is important.  You want a high biomass,” emphasizes Katherine.  “If you have a legume in your mix that is slow growing to start, pair it with rye or oat or something that grows faster and covers the ground.”

Field Peas

“4010 forage peas are one of my favourites, but we are seeing farmers move away from this as more peas are being included in their regular crop rotation. Many farmers have been using clovers as well. Yellow blossom sweet clover as an example,” says Katherine.

“Even with cover crops, you still need to watch for weed infestations, and if that happens be sure to terminate it before the weeds go to seed,” advises Katherine.

Ruth adds, “if weeds become too dense in the cover crop for example, they can be managed with mowing before the weed seeds mature.  This reduces weed seed shed and the fertility taken up by the cover crops will be relayed to the next crop and will enhance the soil carbon sponge.”

“If weed seeds have already been shed, many will degrade, rot or be eaten if left on the surface. It can be more advantageous to leave the weed seed on the soil surface rather than working them immediately with tillage which would effectively be seeding the weed seeds into the soil,” notes Ruth.

“Weeds are also pieces of the puzzle and an indicator of what’s happening in the soil,” notes Ruth.  “In higher fertility fields you’ll see lamb quarters and pig weed.  Ragweed is more reflective of low fertility soil.”

”As with many aspects of managing organic crops, planning ahead for fertility, weed pressure and pest management is the significant difference between managing a system versus reacting to issues,” summarizes Ruth.

Katherine and Ruth will both be part of the “Organic Cropping Forum” taking place on Thursday, January 23rd, 2020 at the Guelph Organic Conference.  Additional organic cash crop workshops take place on the Friday, Saturday and Sunday of the conference.

A free trade show takes place Saturday, January 25th and Sunday January 26th.  A variety of exhibitors including grain traders, input suppliers, certification companies and equipment suppliers will be showcased.

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