Ultimately, the difference between a weed and a crop plant is simply one of definition – the crop produces food or fiber that is useful to the harvester and the weed does not. Competition between weeds and crops is as old as recorded history and potentially catastrophic.
The Bible quotes a parable told by Jesus in Matthew, chapter 13 – “A sower went out to sow. And as he sowed … other seeds fell upon thorns, and the thorns grew up and choked them.”
There are rare instances when two plant species do not choke each other, when they will complement each other as they grow in the same space. For instance, Native American farmers have known for centuries that several species of grasses, like maize or corn, grow better when they are seeded with species of legumes, like beans. Coincidently, the resulting harvest of corn and beans provides a nutritionally balanced diet. Africans and Asians have grown grasses like sorghum or millet in combination with legumes like cowpeas or hyacinth bean. The symbiosis works because legumes fix nitrogen from the air and deposit it underground where the grasses can use it. The grasses bind the soil together and stimulate helpful microorganisms for the legumes.
But for the most part, plants will compete fiercely with each other for light and air above ground and for nutrients and water below ground. Even plants of the same species will compete if planted too closely together.
Scientific studies in the 60s showed that when researchers compared the total dry weight of a weed-free field with the total dry weight of a field of both weeds and crops, the weights were almost always the same. In other words, every weed reduced the weight of the crop by the weight of the weed.
Other studies looked at the corn yield reduction that various weeds exacted on the crop at various densities. Most of the reductions in yield ranged between 5 percent and 50 percent. But one particularly nasty weed – giant ragweed – at a density of 13.8 plants per square meter produced a whopping 90 percent reduction in yields.
In cotton, three weed species – barnyardgrass, black nightshade and johnsongrass – have been shown to be capable of completely eliminating the crop if they are allowed to grow unchecked for the full season.
The list of weed species that U.S. farmers and scientists are intimately familiar with is long. Most species are colorfully named – redroot pigweed, yellow nutsedge, coffee senna, silverleaf nightshade, barnyardgrass, milkweed, giant foxtail, green foxtail, hemp dogbane, sowthistle, itchgrass, quackgrass, buffalobur, common cocklebur, devil’s-claw, hogpotato, morningglory, fierce thronapple, pigweeds, sicklepod, lambsquarter, cutleaf groundcherry, beggarweed, shattercane, leafy spurge and velvetleaf.
In the 50s and 60s, the economic impact of weeds on crop yields was estimated in the hundreds of millions of dollars. Today, even after 50 years of technological progress, the USDA estimates losses in the billions of dollars.
Before World War II, mechanical cultivation – running through the rows of crops with weed hoes trying to chop out the weeds without hitting the crops – was usually the best a farmer could do. In some crops, like sugar beets, migrant farm workers had to be hired to physically weed each row. So, it’s no surprise that when relatively effective herbicides, like 2, 4-D, were introduced, farmers jumped at the chance to control weeds chemically.
Alex Martin (left) – now a weed science professor – says his earliest awareness that there might be problems in farming was about the challenge of weeds. “We [his dad and family] had terrific weed problems in our corn and soybeans,” he says. “I can recall the first time we used a herbicide [and] how miraculous it appeared to be to solve a major problem we had before. And so, I don’t know if that influenced me in choosing weed science as a career professionally, but it made a very strong impression on me.”
Dan Stork (right) says he probably decided to sell herbicides, in part, after pulling weeds by hand as a kid. “We hand-walked all our beans, up and down every row,” he says, “and pulled every weed that the cultivator and the pesticides didn’t get. We were the third line of defense… That was one reason I was looking at other employment opportunities after that,” he laughs. “On a hot 100 degree day, if you walk half a mile one way and a half mile the other way, and before you even got a drink of water, it wasn’t an awful lot of fun!”