Should I Spray For Weeds Before Seeding

Spray weeds or get seeding? Perennial grasses are best hit with an early glyphosate application. Grasses are very sensitive to glyphosate, and produce leaf area more quickly and earlier in the Preparing For Overseeding : Seed first or spray weeds? Early weed control is an important step in profitable canola production. Saskatchewan research on preseed weed control in wheat showed that early weed control was more

Spray weeds or get seeding?

Perennial grasses are best hit with an early glyphosate application. Grasses are very sensitive to glyphosate, and produce leaf area more quickly and earlier in the season than broadleaf perennials. Grass weeds in this canola field (above and below) in 2011 should have been hit pre-seed.

Seeding is a priority for many growers this week, but fields with a large population of weeds, especially advancing winter annuals, should get a pre-seed burnoff. For annuals and winter annuals, glyphosate needs only 24 hours to get to the growing point and set the control process in motion. (It may be quicker than that for some specialty glyphosates in good growing conditions.) After a day, the crop can be seeded. For perennial weeds, the recommended delay ranges from 3 to 5 days depending on weather conditions. If sunny and warm, translocation will take place fairly quickly so 3 days should be enough. If weather is cloudy and/or cool when applied, 5 days is recommended before seeding. When deciding how long to wait to seed, remember that the conditions on the day the glyphosate is applied are the most important to efficacy, making them relatively more important than conditions in the days that follow.

In weedy fields, the economic return will likely be higher for a pre-seed burnoff than if the grower seeded over these weeds and then took a chance on getting them sprayed before crop emergence or in-crop.

In addition, weeds present when the crop emerges will have a substantially greater negative impact on the yield of the crop if the pre-emergent application is missed altogether.

University of Saskatchewan research shows that where weed competition is significant the weed control provided by an early burnoff application can have a greater influence on crop yield than seeding date. The study looked at seeding dates in early May vs. late May with burnoff treatments applied the day previous to seeding. The third treatment was a burnoff conducted with the early seeding date but not seeded until the late seeding date. The late burnoff treatment yielded significantly lower than either of the early spraying treatments, regardless of when they were eventually seeded. The study was done on wheat, but lead researcher Ken Sapsford says results would be similar for canola. One of the benefits of early weed control is that it stops weeds from taking up moisture and nutrients. Those nutrients will be returned to the soil eventually after the weeds decompose, but not in time to be useful to this year’s crop.

Post-seeding/pre-emergence spray window. The cleaner you have the field as canola emerges from the ground, the better. One option if growers miss the pre-seed burnoff, is to apply glyphosate post seeding and pre-emergence. But since shallow seeded canola can emerge within 5 days under warm and generally moist soil conditions, and since growers need to leave weeds for a couple days to start growing again after the seeding operation, the window for post-seed/pre-emergence spraying is very narrow. With all the wind these days, that opportunity may never come. (The time lag between seeding and spraying is necessary to allow weeds buried by the seeding operation to re-emerge and to allow weeds uprooted to overcome the stress placed on them.)

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In crop sprays and glyphosate resistance. A 2010 predictive model by Hugh Beckie, research scientist with AAFC in Saskatoon, ranked kochia first among weeds at risk of developing glyphosate resistance on the Prairies. The prediction was accurate. The next most likely weeds to develop resistance, in order, are wild oats, green foxtail and cleavers. While these weeds are unlikely to develop glyphosate resistance when glyphosate is applied in crop only one year in four, tight rotations of Roundup Ready canola or seasonal pre-seed applications timed to target these weeds increase that risk substantially. Read the article on page 11 of the March 2011 Canola Digest.

Preparing For Overseeding

Thatch is an accumulation of surface roots, dead plant parts and other debris on the soil at the base of the grass plants. While a thin layer of thatch does not normally present a problem, it must be removed before overseeding to expose bare soil for the new seed.
If the thatch layer is thick, consider renting a dethatching machine or a “power rake” to loosen the thatch and simultaneously scarify the soil without pulling out existing grass plants. With a thin layer of thatch, a very close mowing of the lawn and a brisk raking with a garden rake will do the job. Either way, rake up the loosened thatch and use it as mulch around the yard or store it for later use in a compost bin. See Dethatching Rakes in the Tool Shed.

Deal with Existing Weeds

Many annual weeds die out as fall approaches so they are not a problem when overseeding at this time of year. However, if your turf shows a significant number of broadleaf weeds, which are often perennial and will return next year, remove or kill them before overseeding. Judicious use of the herbicide 2,4-D will efficiently rid the turf of dandelion, plantain, ground ivy and their ilk.

Because the vigorous, dense turf that results from overseeding will discourage future weeds of this kind, this is likely to be the only time you will need to use this herbicide. Spray it as directed on the product label at least ten days to two weeks before you plan to sow grass seed. Broadleaf weeds will start to turn brown in three or four days and be completely dead by the end of the 10 day wait. The herbicide will have begun to break down after ten days, so new grass seedlings will not be harmed. What remains is bare soil to be overseeded. See Lawn Weed Control Products in the Tool Shed.

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Using Broadleaf Herbicide

Read the label carefully and follow the instructions.
Spray when weeds are actively growing (by late summer weeds have abundant, mature foliage).
Skip one mowing before spraying so weed foliage offers maximum surface for the herbicide application.
Plan to spray 10 to 14 days before the date scheduled for overseeding.
Do not spray when rainfall is expected within six hours.
Do not spray if winds exceed 5 mph to avoid drift onto other plants.
Do not walk on, or allow pets and children to walk on sprayed area for at least 12 hours; a 24 hour wait is better.

Aerating the lawn is always beneficial and–if time and energy permit–while preparing for overseeding is a good time to do it. It is not essential to the success of overseeding, though.
Punching holes in the turf soil by means of a mechanical or hand aerator introduces oxygen into the top layer where plant roots grow. Existing grass benefits enormously. It also loosens the soil to make it easier for seeds to contact soil and sprout and new grass seedlings to become established.

Mow Lawn Close and Remove Debris

The most important step in overseeding is preparing the seed bed with an existing turf already in place. Since the grass seed needs to contact the soil and the new grass seedlings need lots of light to grow, it is necessary to mow the existing grass very short.
Set the mower as low as it will go–so its cuts at about ½ inch–to remove as much foliage as possible without scraping or harming the crowns of the existing plants. If you have a bag attachment collect the clippings in the mower bag. Then rake up the area or go over it with a blower-vac to be sure that no debris covers the bare soil between the existing grass plants. See Grass Rakes in the Tool Shed.

Spread Fertilizer (Optional)

It is a good idea, but not critical, to spread some slow-acting granular fertilizer when you are overseeding. This will be your fall (or spring) application of slow release nitrogen fertilizer for the lawn. There is no need to spread lime on the lawn when overseeding. Since it takes six months for lime to begin breaking down and affecting the pH of the soil, it will not influence the environment of the newly germinated seed. See organic lawn fertilizers. in the Tool Shed.

Seed first or spray weeds?

Early weed control is an important step in profitable canola production. Saskatchewan research on preseed weed control in wheat showed that early weed control was more important to yield than early seeding.

Pre-seed weed control will manage weeds that emerge ahead of seeding, reducing crop competition for light, moisture and nutrients.

Growers waiting to seed may find that fields too wet for the seeding unit may support the sprayer — although deep ruts are not great for the seedbed. High flotation tires on the sprayer will reduce rutting.

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Cool, wet conditions that are holding up seeding can also reduce herbicide efficacy, but early weed control with lower efficacy is generally preferable to no control at all or late control with higher efficacy — as long as weeds are present and not frost damaged.

Clark Brenzil, provincial specialist, weed control, with the Saskatchewan Ministry of Agriculture, explains how cool, cloudy conditions affect herbicide efficacy:

In the case of herbicide applications following a nighttime frost or near-frost event, the herbicide activity on a cloudy cool day would be next to zero. Biological activity would have stopped during the night, and would not start up again until the plant warmed to at least 5°C — and even then it would be very slow. A few hours between 5°C and a daytime peak of 10°C would not be enough warmth to get plant metabolism going to a point where herbicide was all that effective, especially with the cloud. No biological activity = no herbicide activity. Ideally, you want a day or two of warm sunny days and night time lows of 4°C or higher before spraying. If applied more than 48 hour before the frost event, efficacy on living plants will be retained and the plant will continue to decline when it warms up again.

If faced with a decision to spray some fields and not others, be sure to spray Clearfield fields before seeding. Otherwise you have to wait until the two-leaf stage of the crop to spray.

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