1. HILLS
Fact – to maintain the same speed going up a hill that has only a 20 degree incline (which is not very steep) requires SIX times as much oxygen as on the flat. This is a direct indication of the increased energy required, so remember this for your 'mental fuel tank' as to how much juice your horse has left to complete the event.
Individual preference – some horses like to go at hills like semi-trailers i.e. they canter until it is too much of an effort, then trot then walk the rest of the way and when they get to the top, they will happily trot or canter down the other side. If they do not have an easy roll down the hill action, they may suffer jarring of the joints, chest and shoulder muscles, but if they have an easy gait downhill and you are a balanced rider, it can be distance gained for little energy used.
2. HEAT
High environmental temperatures obviously add to the heat load for your horse, who is already generating plenty of his own through exercise. Horses can only lose heat by the evaporative effect of sweating and by direct loss of radiant heat; both of these mechanisms are enhanced by a breeze, so beware sometimes of slow speeds on closed-in forestry tracks. Under those circumstances, it can be better to move along a bit faster, creating your own breeze to enhance evaporative cooling.
Horse sweat has more salt in it than blood i.e. it is hypertonic, so as well as fluid your horse is losing essential salts or electrolytes and it is vital that both the fluid and the electrolytes are replaced adequately during the event; don't wait until the end and don't start with a horse that is not fully fluid and electrolyte loaded or has poor gut activity. Plenty of roughage is essential for the hind-gut to function efficiently and act as a fluid and electrolyte reservoir.
Beware also of really dry heat with a bit of a breeze, as the sweat will dry so quickly that you may not notice how much your horse is losing, so check for the dried white salts on the coat. When your horse drinks, count the number of swallows as it is about 4 to the litre.
3. HUMIDITY
Possibly the most dangerous of all, except for (H)ego! High water content in the air means that sweat does not evaporate effectively and this is a major impediment to your horse's heat-loss mechanism. Check the weather forecast for the area the ride is in and talk to the locals to get an idea of the humidity, because high humidity is one thing that can consistently cause high vet-out rates.
So watch your speeds, especially up hills and during the heat of the day. Make distance in the early morning then slow down; use water on then scraped off, repeatedly (with your hand is OK) at strapping points to assist cooling; back at base use water on/water off repeatedly plus fanning with a towel if there is no breeze and add lots of ice to the water if it is available. Check your horse's temperature before you start and 10 minutes later to assess how much cooling you need.
4. (H)EGO
Definitely the most dangerous of all; it is an outright horse killer. Always ride to the conditions on the day; sure do as well as you can, but don't set out to beat someone else and therefore stop listening to your horse.