I would guess that the approach to weed management is dependent on who is teaching the course and where the course is being taught. In part it may also be an issue in how to deal with the problem. A grower might want to quarantine all new shipments that come into his area to prevent some weed from invading his field. Yet one grower has neither the political power to make this happen nor the infrastructure to inforce the quarantine. That is the role of governments and police forces. At the point that a grower must do something it is already too late for regulatory weed management strategies to prevent the spread of the weed. Even if borders are secure enough to enable a quarantine approach, there will be smugglers to contend with.

    I would also suggest that by the time a problem has gotten large enough to mobilize the political will to enact a quarantine, that the effort is already too late. The quarantine may slow the spread but that is the best that can be hoped for. The problem is with detection. So I need to prevent a new weed from invading my island. Every car or truck entering my island is carefully washed. All produce is screened, all hay is sterilized.  Yet one seed lands 73 feet past the left side of my house. I have already lost, though I will not know this until several months later when the plant from that seed has produced 800 more and my scout happens upon one of these plants. I just cannot afford to have my entire island searched millimeter by millimeter twice per week to find that one initial plant. The other problem is with collective action. If even one farmer allows the weed to grow on his land, all the neighbors will suffer. Also who pays for taking care of the problem? Some farmers can't pay, others are perfectly happy forcing you to pay.

   The best the grower (as an individual) can do is deal with the problem after the weed has become established. Some places are luck to have a grower that is liked/respected/feared that has political connections and can energize collective action. However, internal conflicts and outside interests make quarantine a difficult proposition at best.

   Partly, the issue is frequency. most of the weeds that growers must deal with are already abundant in their fields. The threat of one new weed from some foreign land is mostly too far away given the 65 weed species that are already at your doorstep.

  Partly, it is economic. So I spent 10 billion to prevent this weed from entering my country. I did this every year for 20 years. This includes border patrol agents to prevent spread by land/air/sea, and shipments of goods that were lost because they could not pass quarantine in time. So what economic benefit have I gained that offsets this cost? Here is what we spent on quarantine _________ and if we had not implemented the quarantine here is the economic damage that would have resulted _______. Was it worth it? Mostly the problem is complex and some people will argue yes and others no. Both will be right depending on where the numbers come from.

This problem is further complicated by perception. So I can go into my field and spray with an herbicide. I have dead weeds and a clear knowledge that I improved my yield this day. In contrast I paid some government another 3% of my earnings to pay some official money to do something. A friend of my neighbor said it was to help stop a new weed that I have never seen from invading. If I don not have strong confidence in my government, this might as well be money to prevent Martians from landing on Earth. So far all the tax dollars spent to prevent a Mars invasion of Earth have worked.

One might also consider what defines a weed. A rancher imports a cactus to act as a fence for cattle. The cactus grows elsewhere and becomes a weed. If only it had stayed just where it was planted.

I think we should consider the dynamics of the phenomena.

Weeds are dynamics, people decisions etc. are also dynamics. Things change with time etc. The components of IWM depend on the priority and the role each element can play in a specific condition.

may be due to difficulties related this way

It is always said that prevention is better than cure or treatment or control or management...

Weed prevention is the initial step of weed management by checking the entry and establishment 8f weed seed and other weed propagates like rhizomes, tubers, stolons, runners, bulbs, fragmented plant parts etc from one place to another place so that it would not infest other area or become a weed by competing crop plants for so many resources need for plant growth and survival...its ok...

There are lot of strategies by which one can prevent weed like:

Clean irrigation and drainage channels

Avoid grazing of farm animals

Use of weed seed free crop seed

Clean farm implements

Etc etc...

Which is really very tough in real life situations when we are doing farming or cultivating any of the crop...as weed flora having certain characteristics whivh make it more resistant with best survival adaptations...as weed produces large amount of seeds than crop plants, weeds seed are too small , can disseminate by wind, water animals and other natural factors with some of the mechanisms...so prevention of entry and establishment of weed seed and other propagules are really very difficult by man made practices or management as there are fully natural factors are involved for their survival...

Prevention is the initial and first step while managing weed flora...but though, if doesn't work, one should go for eradication which is complete removal of weed plant from an area...it is also difficult as some of the weeds are bushy in nature , roots are barky which got well established in the soil...eradication is also done by using large amount of soil sterilents as well as drenching...

After that we came to control measures like physical, chemical, biological etc etc...

First, we should prevent weed entry and establishment by using above written measures...there are some other preventive measures like use of certified seeds, well rotten FYM and by adopting quarantine measures...we tried best to prevent weed,,after that one should go for control....

Weed prevention is free of cost measure...

Because of the multiple modes of spread