Herbicide Choices (Post or Pre-Emergent)
By Hugh Martin and Greg Stewart, OMAFRA

Twenty-five years ago pre-emergence (after planting, but before the crop emerges) was a common time to spray corn and soybeans. Lately, it's fallen out of favour on many farms because most herbicides need moisture to activate them. If it doesn't rain within 10 days of spraying, weeds emerge and get too big for the herbicide to kill them when rain finally comes. In addition, pre-emergent sprays are sometimes at risk that heavy rains can move more soluble herbicides to deeper depths. This can dilute the herbicide and reduce control, or cause crop injury.

Attempts to build more reliability into pre-emergent herbicide programs has included incorporation by tillage. In post-planting operations, a shallow incorporation after application with a rotary hoe or light harrow moves the herbicide into the top 1-2 cm and causes some mechanical injury to very small weeds that have roots less than 3-5 cm long. Many of these same products can be incorporated before planting (always check the label); this has been common practice over the years. Incorporation before planting lessens the dependency on rainfall, but adds complications to time-sensitive field operations in that pre-planting window. Incorporation moves the herbicide to deeper depths, but due to the dilution of the herbicide, may require slightly higher rates per acre. This can put the herbicide deep enough to be effective against weeds such as nutsedge, but may be too deep to be effective against some shallow rooting weeds such as black nightshade.

In recent years, newer herbicide choices have permitted farmers to increase the use of post-emergent sprays and move away from some of the aforementioned risks faced with soil applied herbicides. Farmers can usually finish their planting and then start to spray, which adds convenience and allows some opportunity to see which weed species are present, to optimize herbicide choices. The problem is that if there's a week of rainy weather, or if it's windy every day, there's usually just a narrow window of time when the growth stage of the weeds and the crop are appropriate for optimum control. If you spray too late you run the risk of crop injury and lack of weed control, with limited options for rescue treatments.

Time management is the key to successful post-emergent herbicides. In three years out of 10 (about 30 per cent of the time) there will be three days per week in early June when the soil is unsuitable on clay loam soils and one year in ten (10 per cent) on sandy soils. You can also expect a couple more days per week when the winds will be a limiting factor.

If you have 1,000 acres to spray, how many hours will that take you? And can you get this all sprayed within the optimum crop stage? Similarly, if you are depending on a custom applicator to apply your post-emergence herbicides and that same applicator is forced to cover huge acres to make it an economically viable operation, can he get to your field on time?

Pre-emergence may not fit into everyone's program. But it's an option for growers who are having problems scheduling all of their spraying for post emergence, or for those wanting more weed control early to make sure they don't miss the critical periods for weed control.

For 2000, Ontario growers have two new pre-emergence herbicide options, Converge and Axiom.

Converge is a new product from Aventis. It is being sold as a co-pack of Converge 75WDG and Converge 480. Converge 480 is an atrazine product. Converge 75WDG contains isoxaflutole (IFT), which was sold last year in the U.S. as Balance. Converge has shown excellent results for season-long weed control during the past two seasons even where we have had dry springs. But it requires some rain for suitable activation. The Converge pre-emergence treatment has been shown to be as effective as other leading PPI and post-emerge treatments.

Converge can cause some crop injury when used on sandy soils. There, rains can move the product down into the root zone and the new leaf of the corn plant may turn white. Most injury goes away in a few days and is not serious, but in some cases plant loss has resulted. The product is not recommended on sandy soils or fields with sandy soil and low organic matter areas in them.

Axiom is a new product from Bayer. Axiom contains two ingredients, flufenacet and metribuzin, and is sold as a 68 per cent dry flowable product. Flufenacet is a herbicide from the same family of herbicides as Dual and Frontier. The metribuzin is added at a rate that is about 25 per cent of the normal rate of metribuzin.

The strength of Axiom is the control of annual grasses. The label also lists control of pigweed and suppression of lamb's-quarters (not triazine resistant) and common ragweed. Broadleaf control is somewhat limited, and a tank mix with atrazine, Banvel or Marksman would be recommended in corn or with metribuzin or linuron in soybeans.

Axiom rates are dependent on soil type and soil organic matter content. Do not use Axiom on sandy soils or on soybean varieties that are susceptible to metribuzin.

The trend towards using post-emergent applications of herbicides in corn has been significant over the past five years, and has resulted in moving some field operations out of the planting time period and lessened our dependence on rainfall for herbicide activity. However, timing is critical within post-emergence applications and producers may find conflicts with nitrogen application, custom operator availability, or other on-farm operations. Specific weed control challenges and time management issues will all need to be factored in when selecting from a growing list of herbicides and application timing options.

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