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Crop weeds: stop weed seed set

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Pasture spray-topping

One of the tactics for reducing annual grasses and retaining desirable species in pastures is spray-topping. This tactic involves applying a non-selective herbicide at a critical time to target weed seed set, followed by heavy grazing.

The composition of a medium term pasture changes over time. The pasture may be legume dominant in year one, but without intervention pastures become dominated by annual weeds (generally grasses). Typical grass species include annual ryegrass, silver grass, barley grass and brome grass.

These grass weeds cause:

  • a build up of weed seeds in the seedbank
  • reduced availability of nitrogen from pasture legume input as weeds use nitrogen reserves
  • a build up of cereal root disease
  • increased risk of eye injury and meat contamination in sheep and increased vegetable faults in wool (resulting from the grass weed seeds).


One of the tactics for reducing annual grasses and retaining desirable species in pastures is spray-topping. This tactic involves applying a non-selective herbicide at a critical time to target weed seed set, followed by heavy grazing. Both paraquat and glyphosate can be used for spray-topping. Spray-topping pasture is possible because annual grasses become much more sensitive to non-selective knockdown herbicides during flowering. So low rates of herbicide can be used to sterilise the grass seeds, with limited effect on desirable pasture legume species.

Controlling Weeds Post-Harvest in Winter Wheat

With the recent precipitation many winter wheat fields have rather large broadleaf and grassy weeds growing at this time. A number of these fields received harvest aid treatments. If weeds are cut off with the combine at harvest, you will need to let them regrow before herbicide application.

Timely control of weeds following winter wheat harvest can limit soil moisture loss to weeds and prevent the deposit of more weed seeds in the soil, two factors that can benefit the next crop’s yield. In addition, timely control of volunteer wheat is essential in reducing the spread of wheat streak mosaic disease.

Figure 1. The effect of winter wheat crop residue levels on weeds in corn following winter wheat treated with herbicides post harvest. In general, 1 bushel of wheat produces 100 pounds of crop residue or 60 bushels of winter wheat equals 6,000 lb of crop residue. (Links to larger version.)

The effectiveness of post harvest weed control is influenced by production practices used with the previous wheat crop, such as

  • winter wheat variety selection,
  • fertilizer practices,
  • row spacing,
  • planting date,
  • seeding rate, and
  • weed control in the growing wheat.

Other factors include:

  • weed size,
  • cutting off weed tops with the combine,
  • crop rotation,
  • temperature when spraying,
  • rain the day of spraying,
  • weed seed distribution, and
  • streaks caused by sprayers, terraces, dust, straw, and chaff.

The amount of residue from this winter wheat crop affects how the next crop will compete with weeds.


Weeds under stress are difficult to control, however, this may be less of a problem this year as many areas have excellent soil water. It’s a general rule that you can wait up to 30 days after harvest to spray wheat grown as part of a three-year rotation. If the wheat was planted without an 11- to 14-month fallow period, spray it within 15 days of harvest. Examine each field separately and adjust your treatment schedule accordingly. This year some fields will need to be sprayed before 15-30 days. The key is to prevent weeds from using soil water and producing weed seeds.

As with all weed control, it’s essential that you closely watch for weed developments and spray at the proper time to achieve maximum control. Most labels state that weeds must be treated before they are 6 inches tall. If weeds are under severe drought stress, wait for rain and spray about a week later.

Effective Cultural Practices To Aid Weed Control

Many options besides increasing herbicide rates are available for weed control after wheat harvest. Vigorous winter wheat stands will compete better with weeds. To achieve maximum control:

  • Prepare a good firm seedbed.
  • Control weeds in a timely manner.
  • Fertilize if needed.
  • Seed properly.
  • Plant at the optimum time.
  • Select a competitive winter hardy winter wheat variety.

Split Treatments

Split treatments, which have a good history of effectiveness, should be especially beneficial this year. In Kansas, there was a 20-bushel increase in corn yields the next year for treatments applied in July vs. mid-August. When using a split treatment, apply the glyphosate products with companion herbicides (adding surfactant, if needed, plus ammonium sulfate) as the first application in July or early August. Some glyphosate products include sufficient surfactant while many products require more. Be sure to check the product label.

For all glyphosate brands, add ammonium sulfate (spray grade) at 17 lb per 100 gallons of spray solution. (The ammonium sulfate is the first item put into the spray tank after the water.) Ammonium sulfate is especially helpful when stress conditions are present. Liquid ammonium sulfate, with or without a drift retardant, also is available. It’s difficult to recognize weed stress so it’s wise to always add ammonium sulfate.

Improve control by increasing the rate of glyphosate. Allow at least six hours — and longer with some weeds — for the glyphosate product to become rainfast. Barnyardgrass may require 24 hours without rain for maximum control. With glyphosates, use a spray volume of 5 to 10 gallons per acre and don’t apply when temperatures reach or exceed 95°F.

Producers should control weeds in wheat stubble fields by applying the full label rate of glyphosate with 2,4-D or dicamba to the glyphosate unless susceptible crops or other sensitive vegetation are in the area. Other herbicides that may be added to glyphosate include Spartan, Charge, Authority, MTZ, Valor, and Sharpen. Tank mixes with glyphosate will help control weeds that are difficult to control with glyphosate alone and will help reduce the chances of developing glyphosate-tolerant weed populations. Always check labels and rotation restrictions.

If temperatures are above 80°F, use the amine formulation of 2,4-D and dicamba. For additional information, treatments, and rates go to the Ecofarming Section of the 2014 Guide for Weed Management, EC130.

The second part of the split treatment should be applied in September. It should contain at least 0.5 lb per acre of atrazine and possibly Gramoxone Inteon (add surfactant), depending on the amount and size of volunteer winter wheat, downy brome, jointed goatgrass, or other weeds present.

Weed Challenges

Several options are available for using nonselective herbicides with difficult to control weeds. With Gramoxone Inteon, use a minimum of 2 pints of X 77, or equivalent surfactant, per 100 gallons of solution. Use 2 quarts of X 77 per 100 gallons of spray solution if using less than 20 gallons of carrier. The active ingredient varies among products so check labels and adjust rates accordingly.

The atrazine rate varies with soil and rainfall patterns. In southwest Nebraska, use 2 quarts of atrazine per acre unless the soil or the following crop limits the rate to a lower amount. In the Panhandle the maximum allowed in one season is often 0.5 quart per acre.

  • Volunteer Wheat and Grasses. The advantage of split treatments is that they provide excellent control of volunteer winter wheat and other winter annual grasses. Using one quart or less of atrazine before September 10 allows winter wheat to be planted 12 months later in most areas and soils. If sufficient soil water is available the following spring, corn could be planted or if moisture is limited, the field could be fallowed and winter wheat could be planted in the fall.
  • Downy Brome. If downy brome is a problem and a winter wheat fallow rotation is being used, tillage is usually recommended immediately after harvest to plant the seeds and ensure maximum weed germination during the fallow period. Do not till if only a limited amount of crop residue is present after harvest since tillage will make the soil susceptible to wind and water erosion. Herbicides are available to control downy brome in the growing winter wheat and are best applied early. If jointed goatgrass and/or feral rye is a problem, use a rotation where wheat is not planted for at least three years under good moisture conditions and even longer under dry conditions.
  • Jointed Goatgrass and Feral Rye. Herbicide-tolerant winter wheat varieties are available for fields with jointed goatgrass or feral rye problems. Beyond™ herbicide is then applied in the growing wheat. (Grower training is required before this herbicide can be purchased.) Check the label for additional information.

Robert Klein
Western Nebraska Crops Specialist, West Central REC

Learn How To Beat Weeds on Your Hunting Land

Learning to handle your weeds can be extremely beneficial to your land management. Not all weeds are bad. Learn how to manage your weeds for a successful deer season.

  • By David Hart
  • May 31, 2018


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You spent countless hours on your tractor. You sprayed off the existing plant growth, disked the soil into a nice, loose, fluffy texture and spread and buried your seeds. By the time you added all of the expenses together, you figured you had nearly a thousand dollars in your food plots.

Now look at them. What started out as blankets of lush, green clover, beans and other plot plants are now little more than weed-choked fields. Everything from annual and perennial grasses like fescue and Bermuda grass to a variety of broadleaf weeds are swallowing all that money and effort. Those weeds aren’t just taking up valuable space, they are robbing your plot plants of moisture, nutrients and sunlight.

Don’t fret. There is hope even for the most weed-ridden food plots.

Get a Jump on Weeds

You can have more success controlling unwanted plants if you take a few thoughtful steps before you plant the first seed. Proper site preparation is one of the most important steps in controlling future weeds, says Whitetail Institute Vice-President Steve Scott.

“Spray the area with a non-selective herbicide like Roundup at least two weeks before you plant,” he says. “If you can, start well before that and undergo a spray, disk, spray, disk routine to kill any new weeds that sprout after you’ve disked. The more you prepare in advance, the more weeds you can kill before you plant your food plot seeds.”

Starting a new plot well in advance also gives the roots of dead weeds time to loosen. That makes disking easier and produces a nicer seed bed. More importantly, multiple disking and spraying cycles will knock back a huge amount of weed growth before you plant by killing each new round of growth. Spray the area, let it die and then disk it. Wait until a rain or two generates new weeds and then spray them. Disk again, wait a few weeks and spray the next round of weeds. After a couple of cycles, you will have eliminated a large portion of the existing weed seeds.

Before you start preparing a plot site you should have given careful consideration to the site itself. Is it suitable for the seeds you want to plant? Some plants — clover, for instance — don’t do very well in soil that doesn’t hold moisture.

“If you use the wrong plant in the wrong location, you automatically give the weeds a head start,” Scott says. “They are opportunistic and will grow when other plants won’t. For that same reason, it’s also a great idea to conduct a soil test and properly amend the ground with the right pH and nutrient levels. Again, weeds will take over if the plot plants are struggling.”

You can have more success controlling unwanted plants if you take a few thoughtful steps before you plant the first seed. Proper site preparation is one of the most important steps in controlling future weeds.

Better Plots Through Chemicals

Weeds are inevitable, no matter how far in advance you prep the plot site. Some weed seeds can lay dormant in the soil for decades, sprouting only when they are subject to the ideal soil temperature and moisture level. Disking can bring those seeds up to the surface.

Even if you don’t disk, there will always be at least some weed seeds in your plots waiting to pounce. They can arrive on the wisp of a breeze, birds can carry them in or they might hitch a ride on your tractor or equipment. That’s why you’ll need to be prepared to jump on the weeds before they swallow your plots. Herbicides are a critical tool in the weed war. It’s virtually impossible to win without them.

Weed seeds will sprout about the same time your plot seeds sprout. Don’t worry — at least not yet. Although they will compete for nutrients and moisture, young weeds are not a major threat.

“You want to avoid spraying when plants are real young,” Scott says. “The chemicals could hurt the good plants.”

However, once your plot plants grow to about 3 or 4 inches, it’s time to treat them with a herbicide. Before you mix the first tank, make certain you read the label. It will not only tell you how much to use for specific plant varieties and plant ages, but it will also offer guidance on safe handling procedures and other precautionary items. Scott insists that whatever you do, don’t use the “glug” method.

“Two glugs per 50 gallons, that sort of thing,” he says. “It’s extremely important to use the proper amount. Too much and you could harm your plants. Too little and you won’t get the desired result. Using the right amount will also save you money. Herbicides aren’t cheap, so the less you waste, the better off you’ll be.”

Know thy Enemy

Enemies can be effective, though, as long as you use the right ones. Some herbicides are non-selective; they kill every plant they come in contact with. Others are selective and kill specific plants or plant families. Spraying random herbicides on your plots might actually cause more harm than good.

That’s why you have to know the enemy and the best methods for controlling them. For instance, do you know the difference between fall panic grass and nutsedge? Which herbicides kill one but not the other?

Equally important, do you know which plants in your plots are members of the grass family? Spray a grass-specific herbicide like Whitetail Institute’s Arrest Max on your wheat, corn or sorghum and you just killed it. All those are members of the grass family. So are rye and oats.

Some herbicides are designed to kill broadleafs, but a few kill only certain types of broadleafs. Ammonium salt of imazethapyr, the active ingredient in Pursuit herbicide, kills broadleafs but it is safe to use in clover and alfalfa. Other broadleaf killers are fatal to all types of broadleaf plants, but are safe to use on grasses.

“Read the label,” Scott says. “Always read the label before you mix and before you spray. It will tell you which plants it can kill and how much to use to control a variety of weed species.”

Mechanical Maintenance

Herbicides certainly do the job asked of them if they are used properly, but not all weeds need a shot of chemicals. Some can be controlled with your rotary mower. That’s a good thing. Gas for your tractor is a lot cheaper than herbicides for your sprayer. Mechanical weed control only works in perennial plots, though. Annuals that are mowed are, for all practical purposes, killed. Additionally, there’s little point in controlling weeds in fall-planted annuals like brassicas and turnips. Most weeds go dormant that time of year, so they won’t have much of an impact on your plot plants.

“Mowing annual weeds that grow above your perennial food plot plants will prevent them from producing a seed head and giving you more trouble in future growing seasons,” Scott says. “Just make sure you mow before they flower and produce a seed. It won’t control all the weeds, but it will reduce competition from annual weeds that grow above your plot plants.”

Set your mower height to where it cuts the very top of your plot plants. That not only controls the taller weeds, but it also stimulates plants like clover and alfalfa to grow new leaves, which is exactly what the deer prefer.

Mowing, however, won’t control shorter weeds and it has virtually no effect on perennial weeds like fescue and Bermuda grass. In fact, it might actually stimulate them to grow even more aggressively. That’s where herbicides come into the picture.

Start Over

Sometimes, even mowing and chemicals can’t beat the weeds. It’s a battle that can’t always be won. In the most extreme cases, it’s simply better to torch the entire plot and start from scratch.

“I know some people that want to kill everything and start over when five percent of the plot has weeds in it,” says Scott. “Other people don’t worry about how many weeds there are. They just want to know if the deer are still using it.”

No matter what your attitude is about weed growth, remember that all food plots, even perennials, are temporary. Killing everything and starting over is as simple as spraying the field, disking it and throwing down some new seed. Just don’t forget to stay on top of the weeds as soon as they show up. If you wait too long, you will have a hard time beating the weeds.