Non-selective herbicides such as Roundup and Liberty are dynamite. When applied to tolerant crops, they polish off nearly every weed in the spectrum.
But perfect they aren't - at least not on broadleaf weeds. There are places where even these Herculean herbicides need help.
"Roundup is very strong on grasses and needs no assistance there," says Southern Illinois University weed scientist George Kapusta. "It may need help on yellow nutsedge (whichis neither a grass nor a broadleaf) and to a lesser extent on such broadleaf weeds as velvetleaf and ivyleaf morningglory, and to an even lesser extent on common ragweed."
There are two ways to overcome those soft spots, says Kapusta.
If a field has a history of those weeds and is planted to Roundup Ready soybeans, the grower can use a split application of Roundup.
"Make the first application, at 1 1/2 to 2 pints per acre, when weeds are four to five inches tall," Kapusta advises. "Then make the second application, at 1 to 1 1/2 pints per acre, a week to 10 days later. That should give a very high level of control on grasses, broadleaves and nutsedge."
Another approach is to use a soil-applied product ahead of Roundup. It should be a herbicide that controls yellow nutsedge, velvetleaf, ivyleaf morningglory and common ragweed - if you have them in your area.
"Authority is a logical choice since it's potent on yellow nutsedge and morningglory," says Kapusta. "However, it has not been available as a single product. It is teamed with Classic as either Canopy XL or Authority Broadleaf."
FirstRate, recently introduced, controls morningglory and ragweed, but not the others. Canopy is another alternative, but it can't be used on soils with a pH above 6.8, says Kapusta.
Consequently, Kapusta notes, the soil-applied herbicide choices are limited.
Possible post-applied tankmix partners for Roundup are Blazer, Cobra, Reflex, Flexstar and FirstRate, says Kapusta. They're all good on morningglories and common ragweed. Basagran can control nutsedge and velvetleaf.
"However, all those products add substantially to cost," Kapusta points out. "And Blazer, Cobra, Reflex and Flexstar can cause antagonism when used with Roundup under some conditions and on certain weeds."
If postemergence broadleaf herbicides are used with Roundup, apply them when weeds are 2-4" tall or about 21 days after planting, says Kapusta. Then apply Roundup alone a week later. Do it in that order because broadleaf weeds are tougher to control and their treatment takes precedence.
"What all this gets back to is the fact that a split application of Roundup is the best approach," says Kapusta.
The weed scientist says Liberty is comparable in performance to Roundup on annual grasses and broadleaf weeds and has similar weaknesses. Neither provides any residual control. And Liberty does not get perennial weeds.