Many people have heard the saying “an ounce of prevention is worth a pound of cure." In agriculture, this saying is extremely appropriate in terms of weed control, especially with the increase in herbicide resistance noted in area weeds.
To better understand this, let’s take a look at the average number of years seeds are known to survive in the soil. Most grasses don’t survive all that long, with smooth brome seeds only surviving for two years, while barnyard grass is known to survive for 13 years. Most grass species are in between those numbers. For example, annual ryegrass can survive up to nine years, but perennial ryegrass is only about three years.
Broadleaf weed seeds survive much longer. Mustard seeds seem to last for centuries, with some germinating as long as 600 years after entering the soil. By comparison, curly dock and 10-plus years of survival seem very short in comparison.
Soil longevity of common broadleaf Nebraska weeds include 50-plus years for field bindweed, 40-plus years for common lambsquarters and 20-plus years for Canada thistle.
When one considers that weeds like common waterhemp can produce 400,000 to over 1 million seeds per female plant, it doesn’t take long for seeds to accumulate in the topsoil. Experiments at the University of Illinois found that there was better germination and emergence of these seeds in no-till systems. Soil seed longevity of Amaranthus species such as common waterhemp is less than four years, however.
The latter fact is very important, and is the “ounce of prevention” aspect. Two Amaranth species weeds local growers fight include common waterhemp and Palmer amaranth. If growers can keep weeds such as redroot pigweed, common waterhemp and Palmer amaranth from going to seed in their fields for several consecutive years, the seed bank for these species should be eliminated.
Many growers are also concerned about marestail, also known as horseweed. Even though the seed of this weed species is similar in survival to that of Amaranthus species (less than three years under field conditions), eliminating the weed from your fields doesn’t necessarily mean your fields will be free of marestail/horseweed. This is because horseweed can produce 200,000 seeds per plant, which can then disperse with the wind, similar to dandelions. Seeds are deposited everywhere, including fields that didn’t previously have them. Like dandelions, horseweed/marestail seeds need a disturbed soil surface for germination.
Some individuals have inquired about the effectiveness of cover crops for weed control the following year. Studies at the University of Missouri found that a fall-planted cereal rye cover crop reduced the population of glyphosate-resistant horseweed by approximately 70 percent when evaluated at planting.
For common waterhemp, cereal rye was also the best cover crop species evaluated for early season waterhemp control and was similar to the fall herbicide or full residual herbicide treatment. When evaluated for late-season common waterhemp control, cover crop treatments only provided some reduction with not much difference by cover crop species. The fall herbicide-only treatment (approximately 24 weeds per square meter) and the full residual herbicide treatment (approximately five weeds per square meter vs. 145 for control) provided superior control compared with cover crops for late-season waterhemp.