A quirk in the development of corn with traits makes it possible to obtain certain hybrids with corn borer and/or rootworm resistance that also carry Liberty resistance. These hybrids carry what's called the Liberty-Link system. Herculex hybrids fall into this classification.
The reason they carry resistance to the broad spectrum herbicide, Liberty, now marketed by Bayer Corporation, is that the Liberty-resistance gene was used as a marker gene in developing the hybrids with the insect-protection traits, notes Dave Nanda, consultant for Corn Illustrated, an exclusive Farm Progress Companies project, and a long-time plant breeder. He is also currently president of Bird Hybrids LLC, Tiffin, Ohio.
The two genes, one conferring Liberty resistance and the other conferring insect protection, are very tightly linked, Nanda says. In fact, they're linked so tightly that when the insect-protection gene passes to plants during breeding, the Liberty gene goes along as well. So a convenient way to know if the insect-resistance gene is in a plant during breeding is to spray with Liberty herbicide. Plants that contain the liberty resistance gene, and thus also the insect protection gene, survive, Those that don't have the Liberty gene don't have the insect-protection gene either. Since they don't survive Liberty applications, they're removed from development by default. That's why Liberty resistance has become known as a marker, or indicator, of insect-protection traits.
Hybrids carrying insect protection and Roundup (glyphosate) resistance were developed using other methods, Nanda notes. Glyphosate resistance is not a marker in their development. So it's another trait which must be included in the breeding process.
The upshot of all this is that if glyphosate is a trait n a hybrid, there's an extra charge for it, typically amounting to about $10 per acre. However, some hybrids carrying Liberty resistance because of how the hybrid was developed for insect protection are sold without an extra price tag attached for the herbicide resistance.
"If you're going with a pre- herbicide early or don't have many weed problems and may not spray all of your acreage, one option is going with hybrids that have Liberty resistance," Nanda says. "So if you don't need to come back and spray, and thus don'[t need the herbicide-resistance trait, you haven't paid for it."
Not everyone agrees that the logical conclusion is to move toward Liberty hybrids. While the price picture for '08 between glyphosate and liberty herbicides is still coming into focus, in the past glyphosate has been cheaper to apply than Liberty. That may or may not hold true this year.
In any event, they're not the same herbicide, notes Tom Bauman, Purdue University weed control specialist. Both can deliver good results, but Liberty tends to be less forgiving, adds Glenn Rice, another Purdue weed control expert.
Sources around the Midwest seem to agree that Liberty can be used successfully, but it's a contact herbicide, not a systemic herbicide. So it is not translocated into the roots. What you see in terms of kill is what you get. Browning of tissue does tend to happen faster than if glyphosate is applied.
Glyphosate is more effective against bigger weeds, the weed control experts say. For Liberty to be effective, it needs to be sprayed when weeds are smaller, especially if grass is involved. It tends to not be as effective on some tougher grasses as glyphosate, especially once they get more than a few inches tall.
Anyone trying to decide between the two systems ought to sit down and consider the pluses and minuses of each product, the specialists say. Since Liberty is a contact herbicide, it must be sprayed at higher volumes, say 15 to 20 gallons per acre, with flat fan nozzles. Coverage is the key for success with that product, the specialists conclude.