Sentence:  There is also a strong preference to avoid the cave effect associated with the front of most horse <WSD>trailers</WSD> and a strong desire to face the large opening between the top of the rear doors and the roof of the <WSD>trailer</WSD>. 
Meaning of the word marked with <WSD> tags:  An unpowered wheeled vehicle, not a caravan or camper, that is towed behind another, and used to carry equipment, etc, that cannot be carried in the leading vehicle.