On your concern I will help you here to get the list of subject which come under Indian Forest Service Examination so that you can have idea.

Here are the subjects which come under the exam

Agriculture
Agricultural Engineering
Animal Husbandry & Veterinary Science
Botany
Chemistry
Chemical Engineering
Civil Engineering
Forestry
Geology
Mathematics
Mechanical Engineering
Physics
Statistics
Zoology

The candidates are not be allowed to take the following combination of subjects:
Agriculture and Agricultural Engineering
Agriculture and Animal Husbandry & Veterinary Science
Agriculture and Forestry
Chemistry and Chemical Engineering
Mathematics and Statistics
Not more than one of the Engineering subjects viz. Agricultural Engineering, Chemical Engineering, Civil Engineering and Mechanical Engineering

Here is the exam pattern
Paper I - General English 300 Marks

Paper II - General Knowledge 300 Marks

Paper III, IV, V & VI - Any two subjects to be selected from the list of optional subjects set out below. Each subject will have two papers. 200 Marks for each paper

Total marks for written examination 1400 Marks

Here is the eligibility criteria to apply for the exam

Should have a Bachelor’s degree with at least one of the subjects namely, Animal Husbandry & Veterinary Science, Botany, Chemistry, Geology, Mathematics, Physics, Statistics and Zoology or a Bachelor’s degree in Agriculture or Forestry or Engineering of a recognized university or equivalent.

Candidates appearing at their degree examination (with prescribed subjects) are also eligible to compete.

Candidates qualifying on the result of the written part of the examination will be required to submit the proof of passing the requisite examination along with their detailed application form to be filled up at that stage.

Age Criteria
21-30 Years on 1st July of the year of examination

Certain categories of persons as specified in the Notice are eligible for age relaxation.