Consider following steps:

1. At least find out LD50.

2. Based on proposed bioactivity, select best suited animal model

3. Find out if dose Dependant curve is reported for that extract and respective biological activity

4. If reported then improve upon 

5. If not reported then go ahead 

It will depend on initial screening which type of experiment you want to design. If you want to perform antimicrobial bioassay than you should go for serial microdilution single or double dilution. You can make  a dilution like this 40mg /mL to 10 times diluted 0.675 or lower than this. You have test all the conetrations one by one than you can find a range than select the dose in marginal increasing order. 0.1, 0.2, 0.4, 0.8.1.6, 3.2 microgram/mL for bacterial cultures, it will be increased 2.5 times in case of fungal bio assays. You can also select a random dose to have an idea about MIC < MBC and ID50, LC50 and LD50 in regular toxicity bio-assays.  

Dear researcher

If you want to do animal experiment then you should follow the OECD guidelines to check the LD50 of particular extarct from which you can decide the effective dose of your extract. 

Dr. Mishra's responce is quite certain if the intended experiment involve animal studies. 

I agree with Dr. R.K. Upadhyay explanation. It will depend on which type of experiment you want to design.

First of all, LD50 should be calculated and then assess the effective dose of plant  extract.

Hi Srinivas,

I agree with Ravi. Defining starting crude extract concentration depends on what type of screening you want to perform.

For example, if you want to test a 2mg/ml concentration, you should just weigh 2mg of your extract and dilute it in an appropriate solvent (DMSO is frequently used) such as to have 1ml of solution. From this starting point, you can prepare subsequent serially diluted concentrations for further screens...

Good luck

Fabrice

Thanks & Happy New Year

Dear Srinivas

your question have two parts, first, you want to know how to do for extraction the compounds from the plants, and the second part is how to prepare the extract to do biological test, is it right?

for the first part, you have to use solvent less polar and increase the polarity, for example, is it usual to use dichloromethane and then methanol, or you can start using hexane, dichloromethane  and then methanol. The extraction can be in soxhlet or may be percolation. Then you should evaporate the solvents and finally you have your dried extracts and continue following Ravi or Fabrice advices.

Good & thanks.

Consider following steps:

1. At least find out LD50.

2. Based on proposed bioactivity, select best suited animal model

3. Find out if dose Dependant curve is reported for that extract and respective biological activity

4. If reported then improve upon 

5. If not reported then go ahead 

Thanks.

thank you @ Prof. Ravi K. Sharma sir

Thanks for consultancy.

Dear Yadav

The attached information may provide the required information you are looking for.

Good luck.

Thank you sir @ B.R. Rajeswara Rao

how do i decide dose of plant extract for antimicrobial activity