As a life-long countryside dweller (but not a farmer) I've always wondered why the slurry from my local dairy farmer varies in smell so much when he spreads it on weekly basis on his fields. I mean, what governs the potency of the smell?

Is it down to how long it's been stored? Does fairly fresh slurry smell worse as it's being spread than old slurry, or vice versa?

Does the weather play a major factor? It would seem to make sense that in hot weather the slurry is fermenting faster and giving off more odour, but is that the case?

Sometimes there is barely any smell at all when he's spreading, sometimes it is extremely obnoxious to say the least.

I ask out of idle curiosity.