I'm from Picterra and we're building a web platform that allow drone profesionnals to uploads orthophotos and then build detectors for any kind of objects. And we have a few users using it successfully on cattle.
Check out this blog post: https://picterra.ch/2019/03/25/a-crash-intro-into-ai-powered-object...
You can then sign up for free on the Platform. You can also PM me if you have more questions.
Why do you need to count cattle or trees ?
To count trees is easy job with overhead lidar and p;roblem has been solved 20+ years ago.
To count cattle you need to seperate cattle first since shading fools the count.
My friends from Texas just attach radio sensors or transmitters (RFID) to get exact count at pocket money
In theory you can count heads only but need to use smart AI based pattern recognition algorithms.
Theory and practice is very old and successful
To count trees you need to fly overhead with lidar in winter time (for no trees).
Alike problems have been solved many years ago.