I have been out of the field for sometime but I will venture an answer. The short answer is YES, pretreatment definitely affects the biomethane production. However, the question is so summarized and we will need more information in order to give better advise.

First of all what is the substrate. The is it cow manure, or rice stalks; is it goats manure or is it chicken manure. All these different substrates will mean different pretreatments.

The second factor is the process itself. what is the temperature range inside the digester? is it in the range of 30 to 40 or is it heated to more than 50 degrees centigrade?

So when you provide the above kind of information then it will be easy to see what kind of pretreatment can be appropriate. For solid materials definitely it is best to do size reduction to increase the surface area where the bacteria will be acting upon. For example in my experience, goat manure will need to be crushed before being put inside a digester. The amount of process water also needs to be looked at. Also one needs to be careful with C:N ratio and other such chemical factors. All of the above will affect the kind of pretreatment that you may need to do.