Mr. mohammad yousefi there are two phases of wheat production, one is agriculture phase including agri activities like ploughing, sowing, irrigation, fertilizer and pesticides application and transportation of grain to the mill etc etc and second is industrial activites which comprise of electricity consumption, fossil fuel consumption, packing, transportation and distribution and final use. so first should design system boundary for your study that which type of activities and sources will be included in the study and then collect the resource input data for each activity e.g, diesel consumption in on site machines or CNG consumption in mill cars or trucks etc etc or quantity of fertilizer applied by growers means 2 tons or 2 mands etc etc. after getting all the relevant data then you will decided which one GHG protocol you will use for calculation of carbon footprint of wheat production, so you will get emission factor for each activity and put the values in the pre determined formulas or equations identified and developed by the said GHG protocol. so you will get CF for each phase and then sum up both the phases will provide you the total carbon footprint of wheat production.
i hope this will help you a little bit, i am trying to sent you a research paper on carbon footprint of wheat in ontario Canada, which will clear your concepts on the topic, by the way this topic is very interesting and i am working on this in Pakistan. Thanks
I suggest you consider following some of the published standards and protocols for carbon footprint assessment. Of particular value are the standards from the World Resources Institute and World Business Council for Sustainable Development, published under the GHG Protocol. The "Product Standard" will be of particular value to you. It is available at http://www.ghgprotocol.org/standards/product-standard . In addition, ISO now has a protocol for product-level carbon footprints and The British Standards Institute has a "specification" called PAS 2050 that may be useful.
for a proper LCA i recommend to follow the DIN EN ISO 14040 with the basics and 14044 with requirements and advise. There are plenty of commercial software, what you can do if you are running on a short budget is using openLCA, a freeware.
Than you will need data! Especially in the beginning you don't wanna get lost in the details. Try to get access to the Ecoinvent Database, currently V3.0 from the Ecoinvent Centre, Switzerland.
A very important issue that often is neglected is defining an equivalent process, e.g. if you measure the CO2 emissions per calorie, than you would like to compare it to e.g. the emissions be eating rice.
The second important issue with lots of controversy is whether to include indirect land use factors ore not.
conducting an LCA of agricultural systems is tricky. As Majid has mentioned the agricultural phase consists of soil mechanisation (tilage), sowing, fertilisation, PPA application, harvest, transport and storage (if applicable also drying).
There are a number of methodological choices you have to make and that also determines the databases you are able to use, e.g. inclusion/exclusion of capital goods. Others are system boundary,(growing season, whole year, crop rotation perid), how do deal with the straw (allocation), or whether to consider direct and indirect effects (LUC and ILUC). Then the question of representativeness of important datasets, e.g. of fertiliser production.
Moreover there are field emissions mainly from fertiliser application. You can use IPPC emission factors, reginalised emission factors (if available) or measured data.
Please be aware of the consequences of your modelling choices.