| CROP and SOIL NEWS/NOTES OSU Extension Service |
May, 1999 Vol. 13, No. 5 |
Quackgrass is a creeping, pernicious, aggressive perennial grass that is a serious weed in most of the northern temperate regions of the world. For much of its history, it has been named Agropyron repens, but in recent years, more and more taxonomists (as well as the Weed Science Society of America) know it as Elytrigia repens. In many areas, it is known by the common name, couchgrass (coochgrass).
It probably has been in the U.S. since the early to mid 1600s. It is strongly competitive with numerous crops, often tying up over 50% of the available N, P, and K. Its quality as a livestock feed is satisfactory, and it has been used as pasture or hay, perhaps because "if you can’t beat it, join it." It is not as productive as our better forage grasses, so it is not preferred.
Quackgrass produces a relatively low number of seeds compared to many of our broadleaves. Reports range from 15 to 400 seeds per plant stem. Seeds certainly can spread the weed, but most of the spread in a locality probably is from the rhizome system. The rhizome system is generally shallow, most rhizomes less than 6" deep. One study found that the 14 rhizomes from one plant created a patch 10’ in diameter with 206 shoots produced from them in one season.
Seeds germinate in the early spring. Rhizomes begin to form when the shoot has about three leaves. Buds and roots are formed at the rhizome nodes, but because of apical dominance, most of the buds remain dormant unless the rhizome is broken by tillage.
Buried seeds may remain viable for up to 4 years, but most are less persistent. Seeds can pass through the digestive tracts of horses, sheep, and cattle, but not of swine.
The story on allelopathic properties (releasing chemicals that harm other plants) of
quackgrass is somewhat uncertain. No question that ground-up rhizomes spread on the soil
surface will seriously inhibit germination and growth of many plants such as alfalfa. But
a number of investigators believe the allelopathic chemicals are not produced by living
quackgrass plants but from dead plant parts. A few years ago, a grower sprayed a stand of
quackgrass with Roundup, worked it up, and planted barley. The barley looked terrible, but
only in the area where quackgrass was sprayed with Roundup. The obvious conclusion was
that Roundup residue in the soil was hurting the barley. But we were able to find some
research indicating that quackgrass can be infected with the take-all disease and that
killing the plants increases infection of other plants. So it was not the Roundup, it was
take-all from the dying quackgrass that was the problem. Could have been some allelopathic
effects, as well.
How to control quackgrass? Rhizomes are susceptible to both frost and drought. The practice of shallow tillage with something like a springtooth to bring rhizomes to the surface to dry out during our dry summers has helped convert a solid stand to a much less dense infestation. Of course, the flip side to that is the possibility of spreading rhizomes, so a small patch can become a field-wide infestation. One reference discusses using repeated tillage for carbohydrate starvation, but in the early 1950s, Virgil Freed expressed the opinion that this did not work well with quackgrass. He said that not much root reserve was used sending up a few leaves. The new leaves very quickly began replenishing the underground parts, so the process was not nearly as effective as in Canada thistle or field bindweed.
For years, we have had herbicides that work pretty well in cropland. Examples are Eptam in beans, corn, et al., Kerb in legumes, and Sinbar and Assure II in mint. In non-crop areas, it is hard to beat Roundup. When Roundup was new, we learned an important fact. We located a beautiful thick stand of quackgrass to experiment with. It was late in the season and the quackgrass was tall, so we mowed it down, waited until the regrowth was 2 to 3" tall, and applied the Roundup. It was beautiful. All the sprayed plants were eliminated. But the next spring, we could not tell that anything had been sprayed. Same old story; we should have known better. We sprayed too soon when sugars were moving upward to form new foliage, so no Roundup went downward. Killed the tops nicely, but not the roots and rhizomes. The label says to apply only after the quackgrass is 6-8" tall, and I can certainly agree with that.
Quackgrass heads are sometimes mistaken for those of ryegrass, but they are easy to distinguish. The flat side of the quackgrass spikelets face the rachis stem, whereas the edge of the ryegrass spikelets are appressed to the rachis.
We have discussed before some pros and cons of organic farming. A recent article in the Investor’s Business Daily in Los Angeles (it was passed along to me, I don’t subscribe) discusses an argument that had not occurred to me. Because organic farmers do not use manufactured fertilizer, they often depend a great deal on animal manure, some composted and some not. A primary reason why people buy organic produce is to not only reduce the chances of exposure to pesticide residues, but also the belief that food produced from ‘natural’ soil constituents is healthier than food from manufactured fertilizers. Now, some food scientists and analysts are saying that organic food is actually riskier than conventional food because of the way it is fertilized.
For years, a wide variety of medical and food science groups have said that the risk from pesticide residues in our food is minuscule and that risk from contamination with microorganisms is much greater. The director of the Hudson Institute says, "Organic is now obviously the deadly choice in food." He says the animal manure may infect the food with deadly bacteria that are carried in feces. The head of the Center for Food & Nutrition at Georgetown University says that eating organic food carries "quite a risk" if farmers use improperly composted manure. An epidemiologist from the Center for Disease Control has written that composting standards are not stringent enough to kill bacteria.
In 1996, data from the Hudson Institute and the Center for Disease Control indicate that organic and natural food made up about 1% of the nation’s food supply, but that 24% of the confirmed cases of E. Coli 0157:H7, the bad one, came from organic and natural foods.
Naturally, the organic people strongly disagree and point out that this is based on speculation, not data, and does not represent reality.
There are other reasons, still unproven, why organic food could be considered riskier, such as production of natural defense chemicals in plants in response to attack by disease or insects, which protective pesticides can help prevent. But I had not heard the fertilizer argument before. I don’t know who is right, and I believe that much more research is needed. Wouldn’t it be ironic if people paid higher prices for "safer" food that actually carries a higher risk?
Herbicide adsorption (not absorption) is the physical or chemical attraction of herbicide molecules to soil particles. This is hugely important because this process affects essentially everything that can happen to the herbicide in the soil. Adsorbed herbicides are unavailable for plant uptake, so this can act as a "savings account", releasing herbicide slowly, thus providing enough chemical to control the weeds but not too much to injure the crop (assuming the rate is correct). It can either reduce the rate of breakdown (paraquat) or increase it (atrazine). Adsorbed herbicides are less available for evaporation or leaching.
So what are some factors affecting adsorption? Of course, the chemistry of the herbicide is important. Some are tightly bound (Gramoxone paraquat, Roundup glyphosate, Treflan trifluralin), some medium (diuron, atrazine), and some loosely (borates, Ramrod propachlor). We can’t affect that, just recognize it and adjust usage accordingly.
Important soil factors include texture, type of clay, organic matter content, soil moisture, and pH. Texture influences extent of adsorption by affecting the surface area available. The more surface area, the more adsorption. Clay particles are much smaller than sand particles, so a clay soil has much greater surface area than does a sandy soil per unit of volume. Start with a big block of wood and cut it into smaller pieces. Same total volume, but the total amount of surface now is greater. Higher rates must be used in clay soils for adequate activity. Of course, leaching is greater in sand because a) more of the herbicide is in soil solution and not tied up, and b) water moves downward more freely in sandy soils. As you know, selectivity of many herbicides in many crops depends on them not leaching down to the roots, so this selectivity, such as simazine in ornamentals or diuron in alfalfa and orchards, becomes more questionable in sandy soils.
Montmorillonite clays have greater exchange capacity and can adsorb larger amounts of herbicide than can an illite or kaolinite.
Organic matter (O.M.) is perhaps the most important soil factor affecting adsorption. O.M. has a tremendous amount of surface area and also seems to have the chemical nature to tie up the more oil-soluble herbicides. Some soils that are very high in O.M., such as muck soils, barnyards, or areas around sawmills, have such a tremendous adsorptive capacity that soil-applied herbicides seldom are effective. On the other extreme, some of our desert soils have an O.M. content of less than 1% and great care must be exercised to prevent crop damage because the herbicides are readily available and are very effective.
We now know that water can compete with the herbicides for adsorptive sites on the soil colloids. This is illustrated in the figure below. In the case of volatile materials applied to the soil surface, loss is much greater from a moist soil than from a dry soil. The volatile herbicide can be adsorbed more quickly and more completely on the dry soil and is not lost so readily into the atmosphere, whereas on the wet soil the water prevents it from being adsorbed and herbicide loss is great. On the other hand, with herbicides that tend to be adsorbed too much for optimum activity anyway and that are not volatile, we may consider it desirable that less of it is adsorbed. For example, when we apply diuron for ryegrass control in wheat, we find that the material is less effective when applied to a dry soil. Results are much better if application is delayed until after the soil has been moistened by fall rainfall. The principles are the same in these two cases; we simply want adequate adsorption with slow release in one case and less adsorption in the other case. And whether the herbicide or the water gets there first seems to be important.
The soil pH seems not to affect many herbicides, but for some, it can be important. The Cl-triazines, like atrazine, are adsorbed somewhat more tightly in acid soils than basic ones. This seems to have been a factor in the observation a few years ago that atrazine was not very effective on the surface of non-tilled soil that had been receiving acidic type fertilizer. It likely was just bound up too tightly. There also are reports of crop damage in high pH soils, probably because the atrazine was more available for plant uptake.
Likewise, some of the sulfonylurea herbicides, such as Glean chlorsulfuron, are greatly affected by pH. The water solubility of Glean is 587 ppm at pH 5 and 31,800 ppm at pH 7. The higher water solubility will encourage it to partition into the soil solution more readily instead of being tied up, so it leaches much more at high pH. On the other hand, the imidazolinones , such as Arsenal imazapyr or Pursuit imazethapyr, are adsorbed less at lower pHs.
An excellent reference on some quantitative properties of pesticides is the OSU
Extension Pesticides Properties Database by P.A. Vogue, E.A. Kerle, and J.J. Jenkins.
Among other things, it includes pesticide movement rating, sorption coefficient, water
solubility, and soil half-life. Check with your county extension office or write to
Agricultural Communications, OSU, Corvallis.
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