Not every day will be bright and sunny – and that is a good thing! I have taken some wonderful photographs on overcast days.
One advantage to shooting with overcast is that you do not have to be concerned about your human subject squinting and the light is more even. Also, you can shoot longer into the day instead of having to worry about just early in the morning or just last in the afternoon to get good light – there is no issue of the light becoming harsh.
What should you pay attention to with your settings on an overcast day? Well, ISO is the main issue. ISO in the days of film (I know many of you may still be using film so this applies to you!) measured how sensitive the film you were using was to light – the higher the number, the more sensitive. Now using digital ISO is a measure of the sensitivity of the sensor in your camera – the higher the number, the more sensitive – but also the higher the number, the more grain or noise.
If you are shooting on an overcast day you will have to raise your ISO to get the correct setting for your images. Usually on a cloudy day, my ISO is at at least 640 and if I am shooting action I need even more sensitivity in order to freeze the action at a higher shutter speed, so I may go to 1000 or even higher.
Most of the newer digital cameras have very little noise with higher ISOs and newer films have much less grain. Try to experiment shooting under different light conditions, and you will expand your opportunities to get good photographs of horses.