Through analysis of the specific activity. The variability of activities and machines is so large in Agriculture that is very difficult, or even imposible, to define general design guidelies. Do not forget that the enviroment make the big difference. I remember a farmer complained that an ergonomist wanted to put a seatbelt on a tractor to work on hills with slopes of 15%. He was right when he said that would be the seat belt could be the cause of the death of the driver if the tractor overturns

In my opinion all mentioned factors are very important but I think we should always remember that in such system the man plays the first role, Such thinking help us design a better system (man-machine) with higher ergonomic quality. Of course general design guidelines will be only  little helpful  we need more detail  guidelines for specific class of Machinery.

In general design of any tools and equipment the most and important point is human comfort and safety. This can be achieved with proper man-machine design interaction with due consideration of human capabilities and limitation i.e. Anthropometric and Biomechanical. If above factors are considered and design suite to operator’s needs then hope drudgery and discomfort reduced and efficiency would improve substantially.

This review may be of interest!

In addition to the above, I would like to submit that  "Selection of proper material and its properities" should also play a vital role for effective ergonomic design.

I think, the design suitability is the matter of suitability of the characteristics of the human with the machine or tools or other things they use. It's depend on the goal, many aspects can be considered. So, the suitability of a machine design means suitability of: machine dimensions with anthropometry, work posture and tasking procedure with biomechanic, the tasking complexity with psycho-physiological workload, work condition & environment with the occupational health & safety, etc. 

I think, the design suitability is the matter of suitability of the characteristics of the human with the machine or tools or other things they use. It's depend on the goal, many aspects can be considered. So, the suitability of a machine design means suitability of: machine dimensions with anthropometry, work posture and tasking procedure with biomechanic, the tasking complexity with psycho-physiological workload, work condition & environment with the occupational health & safety, etc.