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What are Selective Herbicides?

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  • Written By: C. Martin
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  • Last Modified Date: 25 August 2016
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Selective herbicides are weed killers that attack specific plants or plant types, while leaving desirable plants unharmed. Non-selective herbicides usually kill all plant life that they come into contact with, while a selective herbicide can be used on areas of mixed growth, where desired plants such as cereal crops are growing in the same area as the weeds that the herbicide is designed to kill. In terms of usage, selective herbicides are often used on gardens, lawns, and crops. A non-selective herbicide is more likely to be used to clear areas of waste ground, or sites such as railways from all vegetation.

The selectivity of a herbicide often depends on using the correct dosage and application. Many types of selective herbicides are selective, because they can kill the targeted weeds at a lower concentration than the desired plant species or crop plant. If such a selective herbicide is applied at a stronger concentration than is recommended, then both weeds and crop plants may be killed, or at least damaged.

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Some of the most common herbicides in this category are ones that kill broad-leafed plants while leaving grass species unharmed. These kinds of selective herbicides are extremely important in crop cultivation, and are also used to maintain lawns and turf. A widely used example of this type of herbicide is 2,4-dichlorophenoxyacetic acid (2,4-D). It has been in common use since the 1940s, and is still important in agriculture today. While 2,4-D has good selectivity, crop plants can still be damaged by it if too much of the chemical is applied.

Grass selective herbicides are a type of selective herbicide that kills grasses, but allows the continued growth of broad-leafed plants. Fluazifop is one example. These types of herbicides are important for the cultivation of broad-leafed crops such as peas and soybeans, and for controlling grass growth in orchards and vineyards.

Selective herbicides have a number of different methods of killing plants, and the mechanism of toxicity is directly related to how selective the poison is. Herbicides that contain a substance called ACCase inhibitors, for example, kill grass species, because they prevent the formation of fats in these plants. The inhibitors specifically target the enzymes used by grass species for fat formation, and hence cell growth. Broad-leafed plants use different enzymes for fat formation, and are therefore unharmed by these herbicides. Other types of selective herbicides mimic certain plant hormones, and are therefore selective in only affecting plant types that are sensitive to those kinds of hormone.

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anon314302
Post 6

Round-Up is non selective and some weeds and plants have developed natural defense and resistance to it. Round-Up/Glyphosate up binds heavily to soil particles and even particles in hard water, so moving through the soil is pretty near impossible.

JimmyT
Post 5

Is there any such thing as an organic selective herbicide? I have some different grasses that keep growing up along my fencerow, but I really don't want to use one of the commercial weed killers on them. I'm worried about it possibly draining into the creek on my property, plus I don't really like handling chemicals.

I have read about a few supposedly organic herbicides, but never got the feeling they were selective. Like I said, I would like to find a selective grass herbicide, but not something that will kill trees or flowers. Does anyone have any suggestions?

Emilski
Post 4

@kentuckycat - Don't forget about crops that are "Roundup Ready" like corn and soybeans. In these cases, I don't know if Roundup would be considered selective or if the plants would be considered excluded.

Roundup ready plants have genes implanted in them that make them resistant to weed killer. I don't remember if Roundup is usually used as a pre-emergent herbicide or post-emergent or both, but farmers can spray it over a whole field without having to worry about the crop plants. It saves a lot of time and money, but has been criticized for its possible environmental effects.

kentuckycat
Post 3

@Izzy78 - At least in my experience, Roundup is a pretty easy herbicide to use. If you sprayed it over your yard, though, it would kill everything, so it's not really selective.

One of Roundup's benefits is that you can spray it on the leaves of a plant, and it will kill it, but it is supposed to deteriorate quickly and not be absorbed into the soil, so that the effects shouldn't spread to any of the plants around it. This is true most of the time, but I've found it can sometimes spread.

Izzy78
Post 1

How long have these types of herbicides been around? It doesn't seem like it could have been too long, since scientists would have had to have known enough about the biology of different plants to know how to make something that would attack different enzymes.

I am only familiar with Roundup herbicide. Does it fall into the category of a selective herbicide, or does it kill everything?

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