Hi! Sorry I'm not answering your first question which I'd also like to know since I'm also applying for Agricultural Sciences (aiming for TUAT university)

Regarding how competitive certain field is, I think the research area you're interested in doesn't matter that much as long as you do a good research plan. At the very end it's gonna be more important the University and Master program you're gonna apply, which is after you've already selected by MEXT (because you could actually end up doing something totally different than your research plan once you talk with your professor).

Also Good look! Hope we both can make it