Marestail taller than 6 inches is effectively uncontrollable at spring burndown, and that single threshold framed weed-control failures across Pennsylvania row crops this season, according to Penn State weed scientist Dwight Lingenfelter, who compiled grower reports on the season's most stubborn species.

Lingenfelter's assessment is blunt on the management principle: "If you stay vigilant and consistently use a combination of management tactics, the negative impacts of stubborn weeds on crop yield can be lessened."

Why did marestail control break down?

Poor herbicide choices, off-target application timing, improper adjuvant use, inadequate spray volume and unfavorable weather drove most failures, especially in soybeans. In-crop glufosinate still delivers effective control in most situations. But in Roundup Ready or Plenish soybean platforms there are no post options, because most marestail populations resist both glyphosate and ALS (Group 2) herbicides.

That leaves a complete burndown — Sharpen plus 2,4-D — and residuals at planting (Valor, Authority, metribuzin) as the critical pass. Lingenfelter notes Plenish E3 varieties should be more plentiful next season, widening post-emergence choices.

What happened with annual ryegrass termination?

Terminating annual or Italian ryegrass cover crops was challenging again this spring. Growers reported incomplete glyphosate control, particularly under cool, wet, cloudy conditions. Causes include cover crop height at spraying, poor environmental windows, inadequate rates and adjuvants, and tank-mix antagonism. Resistant seed in the bag is also possible.

A Penn State study found Liberty/glufosinate applied post in corn delivered more than 90% control of ryegrass about 12 inches tall.

Why are cocklebur and ragweed back?

Cocklebur and common ragweed resurged, partly because corn growers use less atrazine and partly because hardy soil-microbe populations now degrade the herbicide faster than in past decades. Some ragweed populations also resist triazine, glyphosate or ALS chemistry.

Many planting-time herbicides run out of effectiveness after four to five weeks, while these weeds keep germinating through midseason. Glyphosate, Liberty, dicamba and Group 2 products control cocklebur; Liberty, Reflex and Cobra handle ragweed. Giant ragweed — often resistant to several herbicide families — is emerging as the next major contender, Lingenfelter warns.

What does Penn State research recommend?

For burcucumber, Palmer amaranth, waterhemp and marestail, two-pass programs with overlapping residuals are the proven route. Specific guidance:

• Add Peak or mesotrione post in corn for more consistent burcucumber control.
• In soybeans, combinations of Valor, Authority, metribuzin, Zidua, Dual Magnum, Warrant/Enversa, Reflex and Liberty suppress waterhemp and Palmer; Enlist One, Engenia and Stryax fit their respective platforms.
• Thick cereal rye or triticale cover crops reduce small-seeded weeds (Palmer, waterhemp, marestail) but do not stop large-seeded burcucumber, cocklebur and giant ragweed.

Pokeweed responded well in Penn State trials to glyphosate or dicamba sprayed mid-June to mid-July, with good coverage essential. Annual grasses — foxtails, crabgrass, fall panicum — broke through in corn because Group 15 products (s-metolachlor, acetochlor, pyroxasulfone) applied at planting dissipate within weeks, yet grasses still germinate after drought-breaking rain. A post application is necessary.

How are hayfields and small grains affected?

Horsenettle, hemp dogbane, milkweed, Canada thistle and smooth bedstraw continued to plague pastures and hayfields. Routine in-season mowing plus a well-timed fall systemic herbicide can slowly reduce populations over several growing seasons.

Large crabgrass, yellow foxtail and Japanese stiltgrass persisted in grass forages. The recommended program: pendimethalin (Prowl H2O or Satellite HydroCap) applied mid-to-late April, then again five to six weeks later. Facet/QuinStar gives some post control of foxtail and crabgrass under 1.5 inches tall; Plateau at 4 to 6 fluid ounces per acre controls Japanese stiltgrass but can injure timothy and tall fescue. Penn State is running field studies to find additional options.

In small grains, common chickweed — with a head start going into winter — overran many fields, and ALS-resistant populations render Harmony Extra ineffective; adding metribuzin helps with chickweed and other winter annuals. Osprey and PowerFlex HL control annual ryegrass and roughstalk bluegrass best when started in fall, since spring treatments only suppress overwintered plants.

For spring oats, no labeled herbicide in the region controls foxtails and other summer annual grasses. Penn State studies show Prowl H2O, Dual II Magnum and diuron hold promise, but registrants still need to permit their use in the region — a label decision that could open the first real grass-control window for oat growers.