September, 1999

Volume 15, No. 1

MARYLAND    WEED REPORTER

With corn harvest just underway and the planting of small grains not too far distant, I thought I would put out a MARYLAND WEED REPORTER just to give some thoughts on the potential of carryover, new products for use in small grains and a few odds and ends to wrap things up for the season.

Carryover

The three families of herbicides we need to be concerned about are the triazines, the imidazolinones and the sulfonylureas.

Triazines

The triazines are made up of the following products:Triazine Herbicides

While dry weather may exacerbate the carryover potential of these herbicides, the real culprit behind their carryover activity is soil pH. Under low soil pH conditions, these herbicides will actually bind to soil particles. The result may be a lack of weed control. Once you lime the soil, though, the lime will actually compete with the herbicides for exchange sites on the soil particles. In general, the lime will win, resulting in the availability of the triazine herbicides for plant uptake. The problem lies in Fall liming. If the soil pH is low and triazine herbicides were used that year, Fall liming may release the herbicides. If planting a fall crop, injury may occur because of plant uptake of the triazine herbicides. Thus, one needs to plan ahead when using triazine herbicides and liming the soil.

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Imidazolinones

The imidazolinones are made up of the following products:Imidazolinone Herbicides

The imidazolinones are known for their carryover potential under two conditions, dry weather and low soil pH. Both of these conditions will exacerbate the carryover of these soil applied herbicides, with Scepter and Scepter package-mixes more prone for carryover. Particularly in the case of corn, the label states, "...field corn may be planted in the spring of the year following Scepter 70 DG application unless extreme drought conditions develop (less that 15 inches of rainfall or irrigation is received from 2 weeks prior to the date of the last application through November 15 of the same year). In the case of small grains, the rotational restriction for wheat is 3 months, but for barley it is 11 months for Scepter.

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Sulfonylureas

The sulfonylureas are made up of a tremendous amount of single and package-mix products. Most are listed below.Sulfonylurea Herbicides

A number of environmental conditions play a role in the breakdown of the sulfonylureas. Two key issues are microbial breakdown and a chemical process called hydrolysis. Under normal soil pH conditions, these two processes occur and the sulfonylurea herbicides are easily broken down. However, under high soil pH conditions, microbial breakdown slows down and hydrolysis may not occur, resulting in carryover of these herbicides. In turn, the herbicides will be available for plant uptake. With the amount of lime amended sludge being broadcast across the state, we have seen a rise in soil pH in many counties. If sulfonylurea herbicides were used, they may carryover to the following crop, resulting in injury or even death of the crop.

To give an example, the rotational restriction for Canopy goes from 10 months to 18 months for alfalfa when going from a soil pH of 7.0 or less to a soil pH greater than 7.0 to 7.5. For many vegetables, given the same scenario, the rotational restriction goes from 18 months to 30 months. In the case of Canopy XL, they don't even recommend that you apply it to soil with a soil pH greater than 6.8.

In corn, you can get a double whammy with some of the package-mixes. For instance, in the case of Basis Gold, the rotational restriction for fall planted small grains is 10 months. The label also states, "If Basis Gold is applied after July1, do not rotate with crops other than corn or sorghum the next year or injury may result." As listed above, Basis Gold has both a triazine herbicide and two sulfonylurea herbicides. Depending upon liming practices and soil pH, herbicide availability could exist for subsequent crops.

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Fall Applications

The Fall of the year is an excellent time to apply systemic herbicides to perennial weeds. Oftentimes, perennial weeds have produced seed and are now manufacturing sugars/carbohydrates for storage in their below-ground plant parts in order to survive the Winter. By applying a systemic herbicide, it will also move with the plant flow to the below ground parts. In turn, these herbicides will kill the underground plant parts.

Banvel/Clarity, 2,4-D and Roundup Ultra are the three most widely used products that are Fall applied. I generally recommend 1.5 to 2.0 quarts of either product for good Fall perennial weed control. You could blend them together, such as 1.0 quart/acre of Banvel/Clarity plus 1.0 quart/acre of Roundup Ultra. Touchdown 5 is now also labeled for post corn harvest weed control. Keep in mind that if you will be planting anything in the Fall, do not use Banvel/Clarity or 2,4-D. These products have residual activity and may kill anything that is planted in the Fall at those high rates.

Harmony Extra is also labeled for post corn harvest. It's utility lies in the control of dock and wild garlic in the Fall of the year.

All of these products have rotational restrictions. The following table addresses some of the rotational restrictions.

Rotational Restrictions for Fall Applied Herbicides

Banvel/Clarity - Fall seeded small grains are restricted to 20 days after application per pint of dicamba used.

Harmony Extra - Wheat barley and oats may be replanted anytime after the application of Harmony Extra. Most other crops require a 45 day waiting period from time of application to planting.

2,4-D - Fall seeded small grains are restricted to the following year for 2,4-D applications.

Roundup Ultra - Roundup Ultra can be applied before, during or after planting of small grains. In general, I would suggest waiting 7 to 14 days after application before planting in order to make certain good translocation of the product has occurred before disturbance of the plant through tillage or by running a planter over it.

Touchdown - As with Roundup Ultra, Touchdown 5 is labeled for application before, during or after planting of wheat but before crop emergence. As discussed above, I would suggest waiting 7 to 14 days after application before planting in order to make certain good translocation of the product has occurred before disturbance of the plant through tillage or by running a planter over it.

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Small Grain Weed Control

Harmony Extra probably has the lion's share of the small grain market in terms of broadleaf weed control. Unfortunately, it does not have activity on the number of weedy grass species now invading our small grain fields. Some of these grasses are discussed below.

Annual ryegrass - Hoelon is the only product labeled in our area for control of annual ryegrass. In wheat it is labeled preemergence and early postemergence. In many fields that I have walked over the past several years, it would appear that Hoelon is not working. Part of the problem was timing and tank-mixing. Hoelon can control annual ryegrass, but you need to apply it early, when the plants are only 2 to 4 inches in height. Also, Hoelon is not recommended to be tank-mixed with most broadleaf herbicides, including Harmony Extra. Antagonism can occur, resulting in loss of grass control.

We do have some pockets of Hoelon resistant annual ryegrass. Currently we are looking at alternatives to Hoelon. In our research trials last year, we examined several products including preemergence applications of Dual II Magnum and Axiom. Little to no injury was observed and good ryegrass control was obtained. We will continue this work this Winter.

Annual bluegrass, bromegrass/cheat, and bulbous oatgrass are also becoming a nuisance in many small grain fields. Harmony Extra nor Hoelon will control these species. We have been investigating a new product called Maverick. It has activity on all three species when applied early postemergence. We will be exploring this product one more year with hopes of getting it registered in our area next year. Unfortunately, it is a sulfonylurea, with a very long residual. The current label will not allow soybeans to be planted for at least 12 months after application. We have tried to plant STS soybeans, and they have tolerance. At this point in time, we will be working with the manufacturer on a label for our area in the near future.

Odds and Ends

What chemical companies will be left in the future is certainly uncertain. Buyouts and mergers continue to happen. Most analysts predict that there will only be 6 or so basic manufacturers down the road. Time will tell!

American Home Products still owns American Cyanamid. American Cyanamid has been active in the development of IR corn lines and the use of their chemistry with these crops. Since AmCy is also developing other kinds of IR crop plants, they have decided to change the name from IR to a term known as, "Clearfield Production System." This "System" will allow the use of herbicide-resistant crops coupled with the use of imidazolinone chemistry. At last report, we will see imidazolinone-resistant wheat available this Fall. Thus, the term IR will be gone and replaced on all IR crops with the term, "Clearfield Production System."

Who's making, buying and selling Roundup? Give up? The following table lists the company that will be having Roundup available, whom they are buying it from, and the reason for buying it.Who is Making, Buying and Selling Roundup?

*Premixes - to be used in company premixes such as a Roundup + atrazine prepackage mix. KD - stands for knock-down in no-till situations. RR crops - stands for use in Roundup Ready crops.

Will be putting out another newsletter before the close of the year regarding changes in EB237.

Until next time,

Ronald L. Ritter

Extension Weed Control Specialist

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