This is probably a good subject for Soapbox as well. Firstly you should note that I ride racing bikes not MTBs as what I am about to say is more relevant to racers.
Most cattle grids aren't very cycle-friendly. For example I came across two different types on my ride on Thursday over from Littondale to Malham Tarn and back. Most are just lumps of iron like lengths of rusty rail at right angles to the direction of travel. Often the gaps are not equal across the whole grid and there can be some quite big gaps. Some are made up of steel tubes which roll when you ride onto them. If wet these are pretty hazardous.
The fear I would have attempting to cross these at speed is that if your front wheel turned from the straight ahead position the bike would stop dead and you would carry on without it!
To make it worse some of these grids were at the bottom of a steep hill with a severe climb up the other side, so no chance of a run at it.
Between Malham Tarn and Arncliffe there were loads and one had no warning sign.
For me 4-5 mph is a safe speed to cross them, so you need good effective brakes!
On a ride in October in Barbondale near Dent, I noticed that Lancashire have been putting in a new design of cattle grid where the cross pieces are like an inverted "V". They look like stainless steel and the ride over them is very smooth. I found that running over them at a reasonable speed, say 15 mph, was pretty safe.
They certainly get my vote.