**Author details**

selection pressure caused by the repeated use of herbicides with the same mechanism of action in conventional crop cultivars [14], of which a major determinant is the herbicide use rate [15]. The use of an herbicide (or herbicides from the same herbicide group) continuously for many years can drastically decrease the number of susceptible biotypes within the natural weed population and dramatically increase the number of resistant biotypes.

Resistance has increased rapidly since 1975, and today, there are currently 477 unique cases (species × site of action) of herbicide-resistant weeds globally, with 251 species (146 dicots and 105 monocots) in more than one million fields. Herbicide-resistant weeds have been reported in 90 crops in 66 countries [8]. The total area affected, although not estimated, may cover several thousand hectares of crops regularly treated with herbicides in countries such as Australia, Canada and the United States of America, as well as countries in the European Union and South America. Weeds have developed resistance to 23 of the 26 known herbicide sites of action and to 161 different herbicides [8], and no new mode of action has been mar-

Herbicide resistance in weeds is a global problem. Resistance to herbicides in arable weeds is increasing rapidly worldwide and threatening global food security. Resistance has now been reported to all major herbicide modes of action despite the development of resistance management strategies in the 1990s. Despite it being a known issue, farmers in many states reveal the problem of herbicide weakness when the resistance is present in the field; alike bad, occasionally, they are using other herbicide ingredients that have the same mechanism

Proactive, evolutionary-based weed management options that integrate both herbicides and non-chemical tools are of utmost importance in agriculture today [14]. As resistance is generally the consequence of using a single herbicide repeatedly, any proactive or reactive approach should take an opposite view: the use of a diverse method to avoid repetition as much as possible. Because of that, herbicide-resistant weed management practices most often recommended by weed scientists include (1) identification of resistant populations through diligent field monitoring; (2) biosanitary practices, such as cleaning equipment and removing and destroying resistant plants to prevent re-infestation of the field with resistant seeds or plant parts; (3) crop rotations and/or the use of competitive covers that allow the use of alternative mechanism of actions or that change the balance of weeds in a field or both; (4) cultivation and hoeing that provide weed control, which reduces reliance on herbicides; (5) using herbicide rotations and mixtures, which include compounds from classes of herbicides with different modes of action that control similar spectra of weeds; (6) using only labeled herbicide rates at labeled application timings; (7) introduction of new herbicides and herbicide modes of action to replace those herbicides failing due to resistance; and (8) controlling

Since 1996, herbicide-resistant crops (HRCs) have had a major effect on agriculture, particularly in the United States of America, Brazil, Argentina, and Canada [18]. The introduction of HRCs in the United States of America, for example, helped solve a major weed-management problem that was developing at that time—the evolution of weeds resistant to the acetolactate synthase (ALS)-inhibiting and protoporphyrinogen oxidase (PPO)-inhibiting

of action as the one already used, which deteriorates the problem.

keted since 1991 [16].

2 Herbicide Resistance in Weeds and Crops

weed escapes [17].

#### Zvonko Pacanoski

Address all correspondence to: zvonko\_lav@yahoo.com

Faculty of Agricultural Sciences and Food, Institute for Plant Protection, Skopje, The Republic of Macedonia
