Sounds like a simple topic but boy is it complex. For each 100 grams of food a hen eats it produces 100 grams of manure. Doesn't sound like much but to feed the population of Australia with the eggs they want you need at least fifteen million hens and they will produce 1.5 Trillion grams of manure per day or in simpler terms 1,500 tonnes per day or over half a million tonnes of manure per annum. The good thing about all that manure is that when its produced by hens in cages it can be dealt with in an environmentally responsible manner. Its collected on manure belts and goes straight into the truck for either composting or spreading directly on land to produce crops.
Manure is an under rated valuable source of everything a plant needs. Unfortunately the key ingredients - phosphates, nitrates and potassium can be synthesized and added separately to boost plant growth so carting and processing manure into compost isn't competitive. With a carbon tax and later the Emission Trading Scheme manure which saves carbon and could be used to store it in soils should be more competitive.