by Gaylon Morgan and and Josh McGinty.
There are more herbicide tolerant (HT) traits on the market than ever before and with more to come in the future. These HT traits provide new opportunities for weed management, but growers are going to have to remain diligent on the various HT traits and in which varieties these traits are included. The previous article (https://agrilife.org/texasrowcrops/2017/02/22/best-management-practices-for-auxin-tolerant-cotton-technologies-current-12017/) discussed the differences and similarities in the various dicamba products (XtendiMax®, Engenia™, and FeXapan™) and Enlist Duo®. One factor that was discussed in this article is the cotton variety tolerance to glufosinate (Liberty® by Bayer, Cheetah™ by NuFarm, or Kong by Solera) in GlyTol® LibertyLink® cotton varieties, XtendFlex® cotton varieties, and Widestrike® cotton varieties.
From trials conducted in the Southeastern CottonBelt and grower observations in Texas, there can be varying levels of crop injury from glufosinate, when applied to the different HT trait varieties. Glufosinate is the most effective for weed control when the weeds are actively growing (lush) and humidity is high. Under these same conditions, the potential for herbicide injury to cotton is also increased. This has been observed in South and East Texas, but no research has been conducted to validate the HT trait differences. However, in the Southeastern US, Dr. Stanley Culpepper and colleagues have conducted such research. Below is a summary of some of the research.
– 28 trials were conducted in the Southeastern states. Of the 28 trials, only 3 trials exceeded 15% crop injury from glufosinate. These 3 trials occurred under great cotton growing conditions and with very high humidity, and were coined “extreme environment for injury”.
– Under the extreme environment for injury, the table below shows differences in tolerances to Liberty applications.
Percent Visual Cotton Injury at 3 Days After Treatment in TyTy, GA in 2015. Data provided by Dr. Stanley Culpepper, University of Georgia.
|Stoneville 6448GLB2||DeltaPine 1553B2XF||Phytogen 499WRF|
* Liberty = 32 oz/a; Roundup Powermax = 32 oz/a; Warrant = 48 oz/a
** Herbicides were applied at 15 gpa and no adjuvants were added
The level of herbicide tolerance to glufosinate can vary between the different herbicide tolerant traits and varieties with the LibertyLink trait. Why the differences in tolerance in varieties with the LibertyLink traits? LibertyLink is a Bayer Crop Science patented trait. It is our understanding that the LibertyLink trait is the same for GlyTol LibertyLink varieties and the XtendFlex varieties. The XtendFlex varieties use a different trait introgression process which may occasionally result in less tolerance to Liberty; however, the XtendFlex varieties must still exhibit a very high tolerance to Liberty in order to be approved as LibertyLink by Bayer Crop Science. Cotton varieties with the Widestrike insect protection trait (WRF) have two copies of the pat gene, which provides partial tolerance to field rates of glufosinate, though less than varieties with the LibertyLink trait. Varieties with the WideStrike 3 traits, including Enlist traits (W3FE), express three copies of the pat gene and provide even greater tolerance to glufosinate. Data from Dr. Culpepper indicates glufosinate tolerance is slightly less that GlyTol LibertyLink varieties, but slightly better tolerance than the XtendFlex varieties tested.
When crop injury is observed on cotton plants, the response to glufosinate usually manifests itself as small necrotic spots on the leaves. In Figure 1 below, this response can be seen on several varieties with varying trait packages. Glufosinate is not translocated in the plant and thus the necrotic injury will only be observed on the leaves present at application and subsequent leaves will be healthy. With low levels of glufosinate injury (<15%), cotton plants easily grow through the injury and little (if any) yield loss would be expected to occur.
In summary, under the extreme environmental conditions of lush growing conditions and high humidity, some differences may be observed among trait packages in their tolerance to glufosinate. Additionally, as the number of tankmix partners with Liberty increase, the potential for crop injury increases for all varieties and traits. Occasionally, greater injury may be observed on WideStrike and XtendFlex varieties, but is unlikely to affect yield.