Introduction of Clearfield technology in wheat cultivars in Brazil

Wheat is an important cereal, cultivated in practically all Brazilian regions. One of the main obstacles to its cultivation is the occurrence of weeds, which compete with the crop for essential elements to achieve high productivity, such as water, light, nutrients, CO₂ and physical space. In the southern region, where wheat is widely cultivated, the weed species that stand out are ryegrass (Annual ryegrass), black oats (Avena strigosa), white oats (Avena sativa), turnip (Raphanus raphanistrum) and turnip (Raphanus sativus).

Chemical control of these weeds is carried out, in the vast majority of cases, with aryloxyphenoxypropanoate herbicides (FOPs), which inhibit the ACCase enzyme, in the case of poaceae species (narrow leaves). This group includes the herbicides diclofop-methyl and clodinafop-propargyl. Sulfonylureas (SU), which inhibit the ALS enzyme, are also widely used. In this case, these include metsulfuron-methyl, for the control of broad leaves, and iodosulfuron-methyl, which provides control over broad-leaved and narrow-leaved species (Rodrigues; Almeida, 2011).

However, in some regions, cases of resistance of some previously mentioned weed species to ACCase or ALS inhibitor herbicides have already been found. Examples include the resistance of ryegrass to ACCase inhibitors, and to SU (ALS), with cases of multiple resistance to glyphosate (EPSPS), in addition to the cross-resistance of turnip and turnip greens to ALS inhibitors (sulfunylureas and imidazolinones) (Heap, 2019 ; Cechin et al., 2017).

To contribute to the control of these weed species, from 2020 onwards, Clearfield (CL) technology will be available to producers in wheat cultivars, resulting from a partnership between Biotrigo and Basf. Clearfield technology, present in the TBIO Capricho CL cultivar, gives this wheat cultivar tolerance to imizadolinone herbicides (IMI), more precisely to the active ingredient imazamoxi. This is not a transgenic event, but rather an induced mutation in the gene ALS, which provides selectivity of the herbicide imazamoxi in these cultivars. This technology is already used in wheat-producing countries, such as Canada and Australia.

This technology will help producers manage some important weeds in wheat, such as ryegrass, oats and turnip greens. In the case of ryegrass, which can be considered the main weed in wheat crops, this technology could bring great benefits to the farmer, as it will be possible to control ryegrass biotypes resistant to glyphosate, ACCase inhibitors (graminicides ) and ALS inhibitors.

But if ryegrass is resistant to ALS inhibitors, how can the herbicide imazamoxi, an ALS inhibitor, contribute to the control of this species? The answer to this question lies in the resistance pattern of this species to ALS inhibitors. ALS inhibitors are divided into five chemical groups, and among them are sulfonylureas (SU) and imidazolinones (IMI). Ryegrass resistance occurs to the herbicide iodosulfuron-methyl, which belongs to the SU chemical group. However, the mutation in the gene ALS which results in resistance to iodosulfuron-methyl, does not always result in resistance to IMI, as is the case with imazamoxi.

In ryegrass and turnip greens, populations with a known resistance mechanism show that the mutation occurs in the Proline 197 position of the gene ALS. This amino acid is important for the binding of SU in the ALS enzyme, and less important for the binding of IMI herbicides (Powles; Yu, 2010; Heap, 2019). Therefore, in plants where resistance to SU is due to this mutation, IMI herbicides work efficiently and can be used to control and manage resistance. The same cannot be said for turnip, where the mutation identified in resistant biotypes (Tryptophan 574) confers resistance to both SU and IMI (Cechin et al., 2017). This same mutation has already been identified in turnip greens in other countries (Yu et al., 2012) and, therefore, knowledge of the resistance mechanism is essential to know whether or not imazamoxi will be efficient in controlling the biotypes present in each area or region.

Therefore, Clearfield technology in wheat cultivars in Brazil will be another ally in controlling weeds in wheat crops, including resistant weeds. However, it is worth noting that this technology must be adopted with caution, and be considered another tool, and not the solution to all problems. It is important that this cultivar is introduced into a cultivar rotation schedule, using it in 25 or 33% of the area cultivated with wheat each year, without repeating it in the same area in the following years. This will make it difficult to select weed biotypes resistant to the herbicide used, prolonging the useful life of the technology.