The graphics and controls were nicely done. I do enjoy the implementing of an upgrade system in these type of "move goods" games. I would like to see more though. I don't know about other people, but simply "going for the high score" isn't enough to keep me interested in playing a game. I need upgrades to work for. Specifically things that not only change the handling of my vehicle, but the looks also. I like to see a bigger engine when i upgrade to one.
Another critique I have is while a lot of these games focus on the whole "physics of the things in the trailer moving around when you fly through the air." It is rare when it FEELS like I am actually pulling something with weight. You do it a little bit, but I want to have the feel that If i try to brake with a bunch of livestock in the back it will be harder then a lot of carrots. Maybe implementing more diverse upgrades will help with that, such as better traction wheels that grip better (whereas before if you didn't have the upgrade pulling heavy loads up hills would be very difficult or impossible, and breaking would just cause you to skid).
One more minor problem I had was with the reminder to push forward AND up at the same time on every big hill. Doing it for the first big hill is fine, but if the person playing can't figure that out after being told once, then they have bigger problems besides just not being able to get over the hill :P. It was just a minor thing though, but kinda brought me out of the feel of trying to do the level myself (even though I already knew to).
Overall though, it was a fun game. It is good to see that this genre of games are starting to get better, instead of the type that you normally see it as being something easy for a programmer to do when they are working on trying to practice making games. Or something that websites crank out quickly. I really want to see more. The whole "trailer breaking off" is a good concept (and i liked attatching it at the beginning. Made it seem more like you were doing the work). But the only real time the trailer would break off is when you were already dying to begin with. It never really served as a warning that you took a jump too hard, because you would already be crashed or lost all your animals in the air already.
Please continue with the genre. I feel like your level designs were fun, and the last ones started getting quite interesting, but not overly difficult to completely discourage playing the game through.