It's also very low energy as, unlike electro-shocking, the radiation sources are "always on".
[1] https://www.cdc.gov/foodsafety/communication/food-irradiatio...
If you kill everything you take a competitive ecosystem of many species and leave raw materials for the next, most invasive colonizer which is often more likely to cause disease.
Sterilizing manure is likewise pointless unless kept in a sealed container, once you expose it to the world it's going to get something alive in it real quick.
To prevent disease you really need to do the opposite, let it ferment longer to allow the various waves of decomposers access to break down and metabolize as much as possible to get the resulting material further away from what it would be like inside a living body as the more different environments are, the less likely species will be able to thrive in both environments. (this is why fermenting pickles works, microbes that find salty veggies tasty won't find you tasty because you're not nearly so salty or acidic)
You do sterilize manure as mushroom culture to prevent competition with the fungi, but that is done in quite controlled environments.
Also, you don't put raw manure directly into soils. You need to compost them first for at least 6 months. That alone breaks down and kills harmful organisms.