Marestail taller than 6 inches, annual ryegrass surviving glyphosate burndown and a resurgence of cocklebur have driven weed calls to Penn State Extension this season, weed specialist Dwight Lingenfelter reports.

"Like most farmers, I am sure you confronted many stubborn weeds this year," Lingenfelter writes. His advice: "If you stay vigilant and consistently use a combination of management tactics, the negative impacts of stubborn weeds on crop yield can be lessened."

What is happening with marestail?

Marestail (horseweed) remains the leading burndown failure. Once plants exceed 6 inches, control drops sharply. In soybeans, mistakes include poor herbicide choice, late application timing, weak adjuvants, low spray volume and adverse weather.

In-crop glufosinate still works in most situations. But in Roundup Ready or Plenish platforms, no post-emergence option exists because most populations resist both glyphosate and ALS (Group 2) herbicides.

• Full burndown: Sharpen + 2,4-D at planting
• Residual partners: Valor, Authority, metribuzin
• Outlook: Plenish E3 varieties should be more plentiful next season, expanding post choices

Why is annual ryegrass still escaping burndown?

Cool, wet, cloudy springs produced incomplete ryegrass control with glyphosate, Lingenfelter reports. Causes include cover crop height, poor environmental conditions, low rates, weak adjuvants and tank-mix antagonism.

Resistant seed in the bag is another risk. In a Penn State trial, Liberty/glufosinate applied post in corn delivered more than 90% control of 12-inch ryegrass.

Are old-school weeds really returning?

Yes. Cocklebur and common ragweed reappeared this season. Lingenfelter attributes part of the shift to reduced atrazine use in corn and enhanced microbial degradation that has stripped performance from atrazine over years. Some ragweed populations resist triazine, glyphosate or ALS herbicides.

Many at-planting residuals lose activity after four to five weeks, while these species keep germinating into midseason. Glyphosate, Liberty, dicamba and most ALS (Group 2) herbicides still control cocklebur. Reflex and Cobra add ragweed control.

What is the next major weed to watch?

Giant ragweed, a close relative of common ragweed, is starting to rear its ugly head, Lingenfelter writes. Most populations resist several herbicide families, making them hard to manage. Burcucumber, Palmer amaranth, waterhemp and marestail also continued to cause problems.

The recommended approach is a two-pass programme with overlapping residuals.

• Burcucumber in corn: add Peak or mesotrione post
• Waterhemp and Palmer in soybeans: combinations of Valor, Authority, metribuzin, Zidua, Dual Magnum, Warrant/Enversa, Reflex and Liberty; Enlist One or Engenia + Stryax in their respective platforms
• Thick cover crops of cereal rye or triticale suppress small-seeded Palmer, waterhemp and marestail, but do not stop larger-seeded burcucumber, cocklebur or giant ragweed

Pokeweed also persisted. Lingenfelter's research found glyphosate or dicamba applied mid-June to mid-July performed well, provided spray coverage is solid.

Annual grasses — foxtails, crabgrass and fall panicum — caused problems in corn. Group 15 herbicides (s-metolachlor, acetochlor, pyroxasulfone) applied at planting dissipate after several weeks, yet the grasses can still emerge after late-season rainfall. A post application is necessary.

What is breaking in hayfields and pastures?

In grass forages, horsenettle, hemp dogbane, milkweed, Canada thistle and smooth bedstraw continue to plague farmers. Routine in-season mowing plus a well-timed fall systemic application can shrink populations over a few years.

For large crabgrass, yellow foxtail and Japanese stiltgrass, Lingenfelter recommends pendimethalin (Prowl H2O or Satellite HydroCap) in mid- to late April, repeated five to six weeks later. Plateau at 4 to 6 fluid ounces an acre controls stiltgrass but can injure timothy and tall fescue. Penn State field trials are underway to find additional options.

What about small grains?

Common chickweed dominated some fields after a strong autumn start. ALS-resistant populations mean Harmony Extra no longer works in many cases; metribuzin added to fall programmes helps. Annual ryegrass and roughstalk bluegrass are again tough — Osprey and Powerflex HL deliver the cleanest results, and often both fall and spring applications are needed.

In spring oats, no herbicides are labelled locally for foxtail control. Prowl H2O, Dual II Magnum and diuron have shown promise in Penn State studies, but manufacturers have not yet authorised regional use.

Lingenfelter asks growers to report giant ragweed escapes so Penn State can track its spread ahead of next season, calling the weed "our next major contender" for major row-crop losses.