With the introduction of Pursuit, Muster, and Assert herbicides in the last few years the necessity for rotational restrictions because of herbicide residue became commonplace on many farms. There are a number of farmers who want a rotation that can remain somewhat market sensitive. However, changing some seeding intentions while running the drill could prove disastrous if you forget your herbicide residue restrictions. Direct seeders must be especially aware of these restrictions because they are always looking for more products to add to their arsenal against weeds.
Saskatchewan Agriculture and Food publishes a very good publication on herbicides every year. It used to be the Weed Control Guide. This last year they entitled it The Crop Protection Guide. It contains just about everything on the herbicide labels and more to help you make wise pesticide decisions. But, by the time farmers recuperate from seeding the rush to get spraying is upon them. At 4:00 a.m., the only thing they want to read is how many jugs to dump in the tank before the wind comes up. I know, I've been there. As direct seeders you are the farmers on the cutting edge. Get this guide before seeding and be up to date on any products you might use! Be especially aware of the recropping guidelines under the precautions section of each herbicide.
Herbicide disappearance is a complicated process, probably not entirely understood by anyone and more or less different for each herbicide group. Generally though the disappearance of a large portion of the herbicides we apply can be grouped into two modes. One of these is microbial degradation. Conditions such as soil moisture and temperature that influence microbial activity also affect herbicide persistence in the soil. Fig. 2 shows how temperature affects 2,4-D breakdown. A warm fall could really increase the time for herbicide breakdown A dry year will mean more herbicide carryover into the following year. (See Fig. 1) Light textured soils have less moisture holding capacity and therefore a greater potential to carry over herbicides than heavier soils. Fig. 3 shows carryover of Avadex and trifluralins over 8 years. Notice the differences, especially after a drought in 1980.
Most of the other methods of herbicide degradation can be grouped together under the heading: chemical modes of disappearance. Many herbicides become more or less tightly bound to soil particles. Some will never be released while others are slowly released to microbial decomposition. In these processes, soil pH and organic matter play an important role in the rate of disappearance of some herbicides. Degradation by sunlight and volatilization (escaping to the atmosphere as a gas) also play a part in the disappearance of some groups of herbicides.
For specific recommendations, make sure you consult your label or the crop protection guide. Probably the most important time to keep residue restrictions in mind are when making crop planting decisions. Herbicide records with specific locations are very important. When taking over new land be sure to ask about herbicide residue. Here are some products with residue precautions on their labels. Restrictions apply to 2,4-D and Rustler when used as a burn-off. Ally, Muster, and Amber breakdown is affected by soil pH. Cyanamid's Assert and Pursuit have soil zone restrictions. DowElanco's Tordon 202-C has long term restrictions for legume crops. Lontrel and Prevail also have legume recropping restrictions in the year following application. Trifluralin and Avadex have restrictions for canary seed and oats respectively. There are a number of number of weed sprays used on alfalfa that have recropping limitations. Also using some products within too close a time frame can create additive effects.
When spraying new products and products that might have
residues, it would be well worth your time to leave a well-marked
check strip to use next year. You may not even be aware that
residue conditions are causing crop damage, without a check strip
for a comparison. If you think you have a residue problem, be
sure to get the company representative, and your Sask. Ag and
Food soils and crops agrologist to look at the problem. You might
even consider analyzing some plant tissue, if the cost can be
justified.
