Hindsight is a major part of what a crop adviser does and we do it best on rainy days. As I write this article, a “Bomb Cyclone” is passing by and so is herbicide resistant horseweed.

The horseweeds: For all of their 24 million years of evolution, this sunflower genus still prefers wind to disperse its tiny seeds. Horseweed seeds at 1 millimeter in length are smaller than the tip of a sewing needle. They are attached to a pappus that looks like dandelion fluff. The most common species globally are Conyza bonariensis, Conyza canadensis and Conyza sumatrensis.

Just before this storm hit, I watched several different combines blow clouds of Conyza canadensis seeds out with the soybean chaff. Once they are aloft they go for miles. The vast majority will germinate immediately upon touching the ground.

When was GR horseweed discovered? Glyphosate resistant (GR) horseweed “showed up” in Kent County, Delaware, in 2000. That means seed dispersal occurred in 1999; or prior. Tennessee also saw GR horseweed and the Weed Science Society of America and Southern, Northeastern and Northcentral Weed Science societies quickly got the word out. Researchers went looking for GR horseweeds across the United States. The map pinged with discoveries in 2001, 2002 and 2003. GR biotypes were found in the Mid-Atlantic states and in the South, but rarely out west.

The DFI trifecta of 1999: Does anyone remember Hurricane Dennis? That system brought strong northeasterly winds into Delaware on Aug. 24 through Sept. 9, 1999. The origin of Dennis can be traced to a tropical wave that moved off the coast of Africa on Aug. 17. Then, on Sept. 16, came Hurricane Floyd, traced back to a tropical wave that emerged from western Africa on Sept. 2. Hurricane Irene started out as a broad area of low pressure over the southwestern Caribbean on the 8th of October. Irene became an “Intense extratropical storm over the North Atlantic” (according to the Hurricane Central Freeservers Web site).

Did the first GR horseweed seeds ride these winds out of Africa in 1999?

I ask because I was a graduate student at Penn State surveying for triazine resistant common lambsquarters (Chenopodium album) and pigweed species (Amaranthus spp.) on Pennsylvania livestock-crop farms in 1999. Although numerous reports of TR common lambsquarters had been reported in Pennsylvania since 1984, no detailed attempt had been made to investigate the prevalence of the resistance populations in the state. My research focused on problematic weed species in Pennsylvania corn fields: pigweeds, common ragweed, velvetleaf and foxtails. Horseweed was not an issue yet. It wasn’t even on our radar. Neither palmer amaranth or water hemp emerged from soil cores on all 156 fields sampled in 19 counties. Could GR horseweed have been in those cores? It’s quite possible.

I worked as a CCA in Tioga and Potter counties in 2000-2002; and in eastern Pennsylvania/New Jersey/New York in 2003-2005. I clearly remember telling growers, “Don’t worry about it. We don’t have that southern weed in our fields yet. Or palmer, or waterhemp.” I came back to Pennsylvania in 2005 as the weed specialist for the Pennsylvania Department of Agriculture’s noxious weed eradication programs. Palmer amaranth and waterhemp were rare to unknown in state cropping systems. GR horseweed was turning up everywhere. This July, I saw bolting horseweed explode in continuous no-till fields in northern Tioga County soybeans. Once GR horseweeds are greater than 6 inches, the only option is glufosinate. Most of the beans in this area are mid II’s and not LL traited. Suffice it to say, the horseweeds didn’t die. Why did they show up this year?

Infield resistance? For the last 17 years these continuous no tillage fields have probably had a spring burndown of glyphosate (plus/minus something else); followed by mid-crop glyphosate. With the push the last four years to plant cover crops, perhaps a fall glyphosate application.

Manure contamination? It’s a strong possibility. Fields here are taking imported chicken manure so southern concentrated animal feed operation can comply with nutrient management plans.

Seed contamination? Even with state noxious weed seed laws, GR horseweed is probably moving with certified seed.

Wind dispersal? The remnants of Hurricane Hermine and Tropical Storm Julia brought heavy rain into Tioga County last September.

However it got here, horseweed was at the peak of seed dispersal in northern Pennsylvania before the writing of this article. How you deal with it is up to you, but it’s worth noting: The seedling stage of annuals like horseweed and pigweeds is not tillage resistant.

Melissa Bravo is a freelance writer who farms in Tioga County. She is also a certified crop adviser, and livestock and land management consultant at Meadow Lake Farm Consulting Services. She can be reached at 814-574-4067.

Melissa Bravo is a freelance writer who farms in Tioga County. She is also a certified crop adviser, and livestock and land management consultant at Meadow Lake Farm Consulting Services. She can be reached at 814-574-4067.