A multirotor will have limited range and flight time of about 15 minutes. If your batteries expire and you have to set it down in the brush, then you'll have to go get it, assuming that it can be found.
A fixed wing Remote Control airplane would be a better option to locate missing heifers at a distance. Fixed wings have longer flight times and greater range. However you would not be able to hover and study thick brush.
There are hundreds of flying wings. Here's 2:
http://team-blacksheep.com/products/prod:tbscaipirinha - 35 minute flight time.
http://www.crashtesthobby.com/albatross-46.html
A video that will give you a better idea:
Be aware that you'll still need about $500 of long range radio gear and $150 for a camera and transmitter. Monit0rs for you to see what your plane sees are varied $125 and up, or goggles are $250 or so.
A local RC flight club could be a source for hiring a pilot and plane who is just itching for a real world job to buy more gear with whatever you'd pay them. Substantially less than $400/hr.
