**3. Benefits and risks associated with growing of herbicide-resistance crops**

Cultivation of HR crops is associated not only with numerous benefits but also with various risk factors. The most important benefit is simplification of weed control using herbicides (including nonselective herbicides in many HR crops), in which some crops are able to control weeds that other herbicide that cannot control without concern for crop injury. Also, HR crops are good solution for control of parasitic weed species, in which control is more complex due to their attachment for host (mainly crop) plants [23]. Thanks to flexibility to the time of herbicide application, possible combination with other herbicides and integration with nonchemical methods, this weed control approach made weed management more effective and efficient, which results in higher and more profitable yields. For example, the average increase of yield of glyphosate-resistant soybean in developed countries was 7%, while in developing countries, it was 21% [24]. The higher yield with better quality of seed is not a direct result of HR crop traits *per se*, but it is the result of improved weed control, which is mainly more effective than the conventional weed management systems [20]. These approach to weed control also became popular thanks to the absence of new herbicides with novel sites of action during the last few decades and no prediction for its appearance in the near future [5, 6]. Also, weed resistance to herbicides becomes widely spread and still growing problem, which is difficult to manage. In conventional weed management systems control of weeds which are closely related to the crop is difficult or impossible. ALS-inhibiting resistant sunflower would allow to use this group of herbicides to control *Ambrosia trifida*, *Ambrosia artemisiifolia*, *Cirsium arvense*, *Xanthium strumarium*, weedy *Helianthus annuus*, and other weeds belonging Asteraceae family, without injuring the crop [25]. Although there are controversial views about HR crops impacts on environment, it is evident that this weed control system is more beneficial to the environment than conventional systems. Namely, herbicides used in HR crops are usually more environmentally friendly than herbicides used in conventional crops. ALS-inhibiting herbicides, which are used in many nontransgenic HR crops, are very effective with relatively low use rates and low mammalian toxicity [26]. Also, it is clearly shown that glyphosate-resistant crops are beneficial to the environment by reducing fuel use and soil erosion and residues of herbicides in ground waters [20] with the help of reduced tillage. Also, glyphosate-resistant crop cultivation has decreased herbicide use by 17 million kg per yr in the USA [27]. Except described, there are additional benefits growing HR crops to which farmers also give great importance. For example, in case of glyphosate-resistant maize, soybean and cotton, growers highlighted as very important (**Figure 1**) consistency and protection from yield loss, application frequency and flexibility, a clean field, cost, crop safety, family and public health, water quality, etc. [28].

The cultivation of HR crops, whether they have been developed through genetic engineering or classical breeding methods, is fraught with risks, i.e., potential serious economic and ecological consequences. Unlike the HR crops, which have been obtained through conventional breeding methods, the cultivation of GM crops has been a cause of a number of debates, pertaining to the health safety of these products and the risks they present to the environment. The questions, which cause the greatest concern, are those which relate to: (1) direct and indirect toxic effects of products containing transgenes for nonspecific organisms; (2) the impact of modified genes and GM plants on biodiversity, ecosystems, and soil microorganisms; and (3) gene transfer from GM crops to their wild relatives and ecological consequences of this phenomenon [29]. Contrary to this, in the case of HR crops developed by conventional breeding methods, the greatest risk is transfer of genes responsible for resistance from those crops to related weed species, wild relatives, or conventional crops of the same species. Namely, described gene flow creating the hybrids between HR crops and weeds, the so-called "super weeds", resistant to herbicides. Their eradication subsequently becomes one of the major problems in agriculture. Also, gene flow can change the fitness of recipient biotype/species, whereby increase of fitness resulting in greater weediness, while its decreases lead to extinction [30]. In addition, genes responsible for resistance can flow from HR crops to conventional varieties, which could be the source for resistant genes flow to wild or weedy relatives [31]. Gene flow from transgenic to nontransgenic crops of the same species has been a major controversy, the cause of law suits, and a factor influencing commercialization of some transgenic crops. Some authors [32] highlighted that the risks associated with transgenic crops cultivation may be more pronounced in the centers of origin of crops than in the other territories because of the presence of wild progenitors and other wild relatives in centers of origin.

The occurrences of volunteer populations of HR crops can also be leading to high risk. Namely, seed dissipation during harvest lead to the appearance of volunteer plants the next season

**Figure 1.** The important benefits growing of HR crops from the farmer point of view, made based on survey between 1176 glyphosate-resistant maize, soybean, and cotton growers [28].

generally in crop production of some crops. Negative consequences of volunteer plants are yield and quality reduction of the crop which they have invaded, contamination of harvested seeds, and maintenance of harmful insects and diseases. In the case of HR crops that volunteer plants basically represent resistant weed populations, which can be a source of pollen, which can contaminate the nonresistant crops or pass the resistance traits onto the related weed species. The control of volunteer plants, which has originated from HR crops, is impossible in the following cultures in which these herbicides are applied as a weed control measure. Therefore, volunteer plants of glyphosate-resistant cotton could be a problem in glyphosateresistant soybean as subsequent crop [33] or volunteer glyphosate-resistant canola and wheat could be problem in weed control in conservation tillage system [34]. Also, seeds from volunteer plants of GM crop can contaminate harvest of conventional subsequent crop [35].

very important (**Figure 1**) consistency and protection from yield loss, application frequency and flexibility, a clean field, cost, crop safety, family and public health, water quality, etc. [28]. The cultivation of HR crops, whether they have been developed through genetic engineering or classical breeding methods, is fraught with risks, i.e., potential serious economic and ecological consequences. Unlike the HR crops, which have been obtained through conventional breeding methods, the cultivation of GM crops has been a cause of a number of debates, pertaining to the health safety of these products and the risks they present to the environment. The questions, which cause the greatest concern, are those which relate to: (1) direct and indirect toxic effects of products containing transgenes for nonspecific organisms; (2) the impact of modified genes and GM plants on biodiversity, ecosystems, and soil microorganisms; and (3) gene transfer from GM crops to their wild relatives and ecological consequences of this phenomenon [29]. Contrary to this, in the case of HR crops developed by conventional breeding methods, the greatest risk is transfer of genes responsible for resistance from those crops to related weed species, wild relatives, or conventional crops of the same species. Namely, described gene flow creating the hybrids between HR crops and weeds, the so-called "super weeds", resistant to herbicides. Their eradication subsequently becomes one of the major problems in agriculture. Also, gene flow can change the fitness of recipient biotype/species, whereby increase of fitness resulting in greater weediness, while its decreases lead to extinction [30]. In addition, genes responsible for resistance can flow from HR crops to conventional varieties, which could be the source for resistant genes flow to wild or weedy relatives [31]. Gene flow from transgenic to nontransgenic crops of the same species has been a major controversy, the cause of law suits, and a factor influencing commercialization of some transgenic crops. Some authors [32] highlighted that the risks associated with transgenic crops cultivation may be more pronounced in the centers of origin of crops than in the other territories because of the presence of wild progenitors and

The occurrences of volunteer populations of HR crops can also be leading to high risk. Namely, seed dissipation during harvest lead to the appearance of volunteer plants the next season

**Figure 1.** The important benefits growing of HR crops from the farmer point of view, made based on survey between

other wild relatives in centers of origin.

42 Herbicide Resistance in Weeds and Crops

1176 glyphosate-resistant maize, soybean, and cotton growers [28].

Intensive and repeated use of the same herbicides with the same mode of action in HR crops mainly increase selection pressure on weeds, which would most likely lead to an increase in the selection of HR weed populations. Today, at least 36 weed species have evolved resistance to glyphosate, EPSPS inhibitor (the main herbicide in transgenic HR crops), and at least 159 to ALS-inhibiting herbicides (the main group of herbicides in nontransgenic HR crops) [36]. In addition to these concerns, other negative effects are also possible: herbicide drift can damage conventional crops of the same species, the genes responsible for resistance can be transferred onto conventional crops, characteristics of nontarget plants can be modified, biodiversity may be damaged, and the environment and soil properties can be changed due to the changes in the crop production technologies.

Due to the dangers of the mentioned potential risks, the research into these issues, with the aim of developing suitable prevention strategies, as well as solutions to these problems, should they arise, has been intensified. Consequently, plenty have dealt with the issue of the gene transfer from HR crops to their relatives (wild/weedy forms or conventional crops) [37–42], the study of gene stability in recipients [43, 44], the study of crop-weed hybrid's fitness [41, 45–47] and the competition between crop-weed hybrids and sensitive weed plants of the same species [45, 46, 48].
