Weed management costs money and so some kind of economic threshold estimation is wise, but most managers opt for reducing risk, and the thresholds they use often lead to over-investment.  This is unsurprising because penalties for failure can be severe, and weed impacts vary from site to site in ways that are not easily predicted.  Typically a regional analytical approach is taken, but with weeds targeted in a more site specific way.  A competent manager will know that certain weeds are likely on specific site types, and that the consequences of lack of control from those weeds generally justify control.  However, just to be sure they are likely to over-invest.

The question is complicated by the longevity of forest crops.  Have a look at types of responses to site management treatments as outlined in the following papers:

Snowdon, P. and H. D. Waring (1984). Long-term nature of growth responses obtained to fertiliser and weed control applied at planting and their consequences for forest management. IUFRO symposium on site and site productivity of fast growing plantations, Pretoria and Petermaritzberg, South Africa

Mason, E. G. (1991). A forest establishment or regeneration decision framework. Proceedings of the IUFRO symposium on Efficiency of stand establishment operations, Rotorua, Zealand Forest Research Institute Bulletin

Mason, E. G. (1996). "Planning forest establishment operations with a computerised decision-support system: a case study analysis of decision-making over a full rotation." New Zealand Journal of Forestry Science 26.(1/2): 222-234

Mason, E. G. (2005). "Effects of soil cultivation, fertilisation, initial seedling diameter and plant handling on the development of maturing Pinus radiata D.Don on Kaingaroa gravelly sand in the Central North Island of New Zealand." Bosque 25(2): 43-55

Mason, E. G. and P. G. Milne (1999). "Effects of weed control, fertilization, and soil cultivation on the growth of Pinus radiata at midrotation in Canterbury, New Zealand." Canadian Journal of Forest Research 29: 985-992.

Mason, E. G., A. G. D. Whyte, et al. (1997). "A model of the growth of juvenile radiata pine in the Central North Island of New Zealand: links with older models and rotation-length analyses of the effects of site preparation." Forest Ecology and Management 97(2): 187-195.

I think I understand your question but it would be helpful if you included the full terms in addition to the acronyms ELT and CWFP.

What do you mean by those acronyms ELT and CWFP?

Att Euan Mason:

Thank you for your response... But again I think another problem with the "risk reduction approach" apart from overinvestment is the issue stewardship in the case of herbicides... Don't you think it is in direct conflict with global consensus to use herbicides/pesticides in a responsible manner..? I Think CWFP + ETL information can assist in reducing herbicide pesticide doses to crops... Can you comment on that...

Att  Bastiaan Brak and Prof Mashingaidze

ETL (not ELT) means Economic threshold Level

CWFP means Critical Weed Free Period

Thank you for asking for clarification.. I had overlooked that...

 Att Derek Law ·

Thank you for your response... However I believe that the world is moving towards Precision Agriculture (PA).... and I think quantification and objectivity are some of the basic principles that underpin PA in order to regulate intensity of any pest control measure that one can opt to implement... Moreover, whereas it is a matter of fact that a one size fits all approach is not the way to go for the diverse farming systems in existence, I believe it is necessary to develop weed management models that can be flexed to fit farm/farmer situations.....

May you comment on that.... 

I agree that responsible use of herbicides is important, and so is clear and accurate reporting of their impacts.  Herbicides vary hugely in their potential for harm, and harm also varies with dosage, timing and the nature of the environment where they are used.  I have no qualms about using herbicides in my experiments because I keep track of reports about what their impacts will be and I am careful to ensure safe application in the right conditions and with dosages that will harm neither people nor the environment. I am a certified herbicide applicator through the New Zealand Qualifications Authority.

In a weedy situation the effect of big and small weeds on the growth and yield of crop varies. So it is difficult to determine the population of weeds which can reduce  50 percent of the yield or growth of crop. So always we take the CWFP.

Dear sir,

As we know for annual crops 1/3 rd portion of ontogeny of the crop is susceptible for critical crop weed competition and 1/4th for sugarcane like crops. from this we conclude that CWFP and ETL are definitely considered and controlling the weeds at that time only rather than full period of the crop.

Thank you

The problem with economic thresholds for weeds is that the damage caused to crop growth and yield is not only dependent on the numbers of weeds but their size and species.  Economic thresholds are therefore difficult and cumbersome to define for different weed size and species scenarios and they will need to to be location  specific  to cover the wide variability in weed densities and species spectrum.  This is why economic thresholds have been applied sparingly as decision support tools.  However, at specific locations where weed emergence, species spectrum and impact of the weeds on crop growth and yield have been intensively studied, economic thresholds have been incorporated in decision support models that are used by farmers to determine whether or not to take action on weeds emerging in the crop.

I strongly endorse the views of Mr.Arnold

The density and biomass are more related parameters for weed management in any cropped fileds, therfore,whenever we plan weed management,its to be followed crop-weed interaction,like Critical Crop Weed Competition Period/Time of each crop in every ecosystems. So, in weed management, the ETL may not be a suitable parameter like insect/diseases management

Chinnuswamy sir,

Please clarify me that intially for some days even though some level of weeds are there in the plot i think so it won't effect on crop. there fore ETL=Critical crop weed competition.

Dear Dr.Sunil

Basically in any field crops,preferably annual crops at early 1/3rd of its duration will be critical for weed infestation. Even from the day of sowing /planting, the effect will be there,but with lesser magnitude. In weed management, ETL is not equal to CCWC period. Because, lesser number of bigger weeds cause more damage than more number of smaller weeds. Hence, it may not be appropriate to go with ETL in weed managment.

Thank you sir for your kind reply. there was some confusion thats all sir. i will agree with your answer.

ATT All

It is a general consensus among the discussants that use of ETLs or weed densities per se, cannot be a practical indicator of the critical period for weed control....

In particular, Dr.Chinnusamy puts in bold that "the smaller the weeds, the lower their competitive ability against crops" In which he was echoing Professor Mashingaidze's earlier statement....

It is common knowledge that weeds compete for vital resources with crops..... I would take an example of mineral nutrients...Taking Southern Africa into consideration, low soil fertility coupled with low fertilizerinput  is one of the major limiting factors to crop productivity..... With this in mind, consider a the impact of those many small weeds competing for the limited resources with the crop,at whatever growth stage of the crop !!! Would one find no correlation whatsoever between weed density and crop growth/biomass accumulation/yield???

I believe weed density has a story to tell... Ihowever stand to be corrected if my perception is erroneous......

Fungai, We have plenty of examples of correlations between weed density and crop growth rates. For instance see the first two attached publications.

Examples of individual weed counts and densities correlated with crop response are also available, but they aren't always so clear, perhaps because measurements of weed size and/or proximity are rough, and also impacts of weeds depend on relative sizes of crops and weeds and the extent of overlap in their niches in the ecosystem. Moreover, the nature of the environment, sometimes the micro-environment around a crop plant, can be critical; soil depth, physiography and even soil texture can vary on a micro scale.  The most limiting resource also matters.  You mention nutrients, but sometimes water or light are more important, and then other, less often considered influences such as shelter, allelopathy, and impacts of weeds on albedo/frost may also affect crop survival and growth. Nevertheless, we can model multiple influences of weeds on crops (see the third publication I've attached).

Article Effect of genotype and grass competition on growth and wood ...

Article Interactions between influences of genotype and grass compet...

Article Time vs. light: A potentially useable light sum hybrid model...

Weed management is along term goal. Even if we come out with ETL levels taking into account the economics of weed control that will only be for the current crop in the field. In some weed species like Chenopodium , Amaranthus, one plant can produce thousand of seeds. So a few plants in one acre area will not show any adverse effect on the yield of the current crop but surely they will enrich the weed seed bank and enhance the weed management costs in the succeeding cropping seasons. Hence, ETL does not work well in case of weeds..     

I do endorse Dr.Bhullar's view and further add that it may not be appropriate to go for Weed Management based on ETL as in the case of Insect or disease management. Instead we take Critical Crop Weed Competition Period (CCWCP) in any annual and biennial crops and in perennials, growth stages of weed for post emergence weed management and the soil moisture for pre-emergence weed management are taken as basis. Because, basically, more number of smaller weeds may have different effect on the productivity of crops and similarly, lesser number of bigger weeds may have high impact on the yield of crops. hence, it may not be appropriate to go for weed management on ETL basis and it is appropriate to go with CCWCP.