Herbicide

Some liquids kill bad plants called weeds.

We use these on big farms. They help our food grow better.

Some killers only hit certain plants. These leave the good crops safe. Other killers hit every plant they touch.

People used to move soil to stop weeds. They also used water to flood them. Long ago, people used harsh chemicals. These were not very good.

In a big war, scientists found a new way. They made a liquid called 2,4-D. It kills wide leaves but not grass. This helped much more food grow.

Now, we use these tools in many places. We use them in woods and fields. They help us grow what we need.

Herbicides are substances used to control plants we do not want. These unwanted plants are called weeds.

Some herbicides are selective. This means they kill specific weeds but leave the crops safe. Other kinds are non-selective. These kill almost any plant they touch.

In the past, people used ways like flooding fields to stop weeds. Later, they used harsh chemicals like arsenic. These were often unsafe or did not work well. During the Second World War, scientists made a big discovery. They created 2,4-D. This was the first successful selective herbicide. It kills broad-leaved plants but does not harm grasses like wheat or corn. This helped farmers grow much more food.

Herbicides work in different ways. Some are applied to the soil before seeds are even planted. Others are sprayed on leaves after the plants grow. Some chemicals work by stopping a plant from making parts it needs. For example, ALS inhibitors stop a plant from making amino acids. These are small parts that plants need to grow. Without them, the plant cannot make DNA. This eventually causes the plant to die.

Herbicides are special substances used to control plants people do not want. These unwanted plants are often called weeds.

Some herbicides are selective. This means they can target specific weed species while leaving the desired crops safe. Other types are non-selective, which means they kill plants without making a distinction.

These tools help farmers grow much more food. In fact, using herbicides helped increase crop yields by three to six times between 1900 and 2000.

There are many ways these substances work in a field. Some are applied to the soil before any seeds are even planted. This is called preplant application. Other types are applied to the leaves after the weeds have already grown.

Some chemicals are absorbed through the roots in the soil. Others enter through the foliage, which are the leaves of the plant. The weather also plays a big part in how they work. High humidity can help a spray stay on a leaf longer. However, heavy rain might wash a spray away before it can work.

Before modern herbicides, people used different ways to stop weeds. They might change the salt or acidity of the soil. They also used mechanical ways like flooding a field or digging the soil. In the late 19th century, people used chemicals like arsenic or kerosene. These were often expensive, flammable, or even toxic.

Most of these early methods were not very effective at controlling weeds.

Major breakthroughs happened during the Second World War. Scientists in the United Kingdom and the United States were researching how to use herbicides in war. W. G. Templeman at Imperial Chemical Industries first synthesized a compound called 2,4-D. This was the first successful selective herbicide. It was released commercially in 1946. It works well because it kills broad-leaved plants, known as dicots. It does not harm most grasses, which are called monocots. This allowed for much better weed control in crops like wheat, corn, and rice.

Today, herbicides work by interfering with the inner workings of a plant. They often mimic natural plant hormones to trick the plant. Some belong to a group called ALS inhibitors. These chemicals stop a plant from making certain amino acids. Without these amino acids, the plant cannot make DNA. This eventually causes the plant to die. Other groups, like ACCase inhibitors, stop a plant from making cell membranes. By targeting these specific steps, scientists can control which plants grow and which ones do not.

Herbicides are chemical substances used to control undesired plants, commonly called weeds.

These tools are vital for modern agriculture and forestry. They allow farmers to protect crops from competition for resources. By managing weeds, herbicides help increase crop yields significantly. Between 1900 and 2000, the use of herbicides, fertilizers, and improved cultivars increased major crop yields by three to six times per acre.

In the United States in 2012, approximately 91% of all herbicide usage by weight was applied in agriculture.

Herbicides are classified by how they act on plants. Selective herbicides target specific weed species while leaving the desired crop unharmed. For example, 2,4-D, mecoprop, and dicamba control many broadleaf weeds but do not harm turf grasses. Non-selective herbicides, sometimes called defoliants, kill plants indiscriminately. These are often used to clear industrial sites, railways, or waste grounds. Common non-selective examples include paraquat, glufosinate, and glyphosate.

Timing and application methods also define how these chemicals work. Preplant herbicides are applied to the soil before planting. Some are mechanically incorporated into the soil to prevent them from breaking down through light or evaporation. Preemergence herbicides are applied before weed seedlings emerge from the soil. These chemicals kill weeds as they grow through the treated zone by affecting cell division. Postemergence herbicides are applied after the weeds have already emerged. These can be absorbed through the leaves, known as foliar absorption, or through the roots.

Environmental factors heavily influence how well a herbicide performs. Climatic conditions like humidity, light, and temperature change how a plant absorbs the chemical. High humidity can help foliage-applied herbicides enter a leaf by keeping the spray droplet wet longer. However, intense light may break down some herbicides or cause the leaf cuticle to thicken. This thickening can make it harder for the chemical to be absorbed. Precipitation can also play a dual role. Rain might wash away a spray from leaves, but it can increase the absorption of soil-applied herbicides by the roots.

Modern herbicides work by interfering with the biochemical machinery of the plant. They often mimic natural plant hormones or enzyme substrates to disrupt metabolism. One group is known as ACCase inhibitors. These target acetyl coenzyme A carboxylase, which is part of the first step in lipid synthesis. By stopping this process, they prevent the production of cell membranes in the meristems of grasses. Another group is the ALS inhibitors. These target acetolactate synthase, an enzyme used to synthesize branched-chain amino acids like valine, leucine, and isoleucine. Without these amino acids, the plant cannot synthesize DNA, which eventually leads to death.

History shows that herbicide use has evolved from simple methods to complex chemistry. Before widespread chemical use, people used cultural controls like changing soil pH or salinity. They also used mechanical methods like tillage or flooding. In the late 19th and early 20th centuries, people used inorganic chemicals like arsenic, kerosene, or sulfuric acid. However, these were often toxic, flammable, or corrosive, and they were not very effective. The real revolution began during the Second World War through research in the United Kingdom and the United States.

Major breakthroughs occurred when W. G. Templeman synthesized 2,4-D at Imperial Chemical Industries. This was the first successful selective herbicide. It was commercially released in 1946 and allowed for better control in cereal crops. It works because it kills dicots, which are broadleaf plants, but not most monocots, which are grasses. Later, the triazine family, including atrazine, was introduced in the 1950s. Atrazine is notable because it can cause groundwater contamination if it is not broken down quickly in the soil. Finally, glyphosate was recognized for its herbicidal activity in the 1960s and marketed as Roundup in 1971. It is now used extensively alongside glyphosate-resistant crop plants.