We may be at the beginning of a sea change with respect to weed control in soybeans and cotton.

Judging from what I am hearing from folks who sell cotton and soybean seed, it looks like weed control is starting to move from a glyphosate (Roundup) based system to a glufosinate (Ignite) based system.

In 2008, just a trace amount of cotton acres in Tennessee were planted to Ignite-tolerant cotton. In 2009, that increased to over 40 percent of the cotton acres being planted to Ignite-tolerant cotton.

From what I have heard this spring, it would appear that over 60 percent of the cotton acres in Tennessee will be planted to an Ignite-tolerant cotton variety.

In soybeans there was virtually no interest in LibertyLink varieties in 2009. Though the seed was in very limited supply, a fair amount went unplanted. In 2010, with quite a bit more LibertyLink soybean seed available compared to 2009, LibertyLink soybeans were virtually sold out before Christmas in the Mid-South.

In my opinion, there are two big drivers for this change in technology. First, growers are finding that these glufosinate-tolerant varieties are yielding favorably with other varieties they have been planting. The other big driver is that growers are looking for the next best option to try and raise cotton and soybeans in fields infested with glyphosate-resistant weeds.

Glyphosate-resistant Palmer amaranth is not always the major concern as I talked with growers who will plant glufosinate-tolerant varieties in fields that have glyphosate-resistant horseweed and giant ragweed or, in some cases, combinations of all three glyphosate-resistant weeds.

A large proportion of the Ignite-tolerant cotton varieties are WideStrike varieties. As most growers now realize, cotton varieties with WideStrike insect protection are linked to Ignite tolerance. The level of Ignite tolerance is not as good as in a true LibertyLink variety and one can expect 15 to 20 percent leaf burn after application. Even though we have seen this leaf burn in our research, we have never seen it result in significant yield loss or delay in maturity.

However, though we have four years of testing, we cannot test all environments, and in areas where some stress is already affecting the cotton, an application of Ignite on WideStrike cotton may be enough added stress to delay maturity a little.

Please keep this in mind as neither Bayer, the manufacturer of Ignite, nor Dow, who sells PhytoGen’s WideStrike cotton, will cover you if there is a problem after Ignite is sprayed on WideStrike cotton. Nevertheless, growers I’ve talked to who used Ignite on WideStrike cotton in 2009 thought the 20 percent leaf burn a fair trade off when the other option was significant yield loss and quality problems due to glyphosate-resistant Palmer amaranth or giant ragweed infestations.

The most reliable way to make an Ignite-based system work in either soybeans or cotton is to use a pre-emergence material. Ignite will control large giant ragweed and horseweed but can be very inconsistent on Palmer amaranth or many grassy weeds much over 6 inches in height. As a result, a pre-emergence is critical to give you some time to spray all your fields before weeds get too large.

As Ken Smith mentioned a couple weeks ago, repeated Ignite applications on large Palmer amaranth will greatly shorten the time until we develop Ignite-resistant Palmer amaranth.

lsteckel@utk.edu