If you don't want to look at all of the references, the last one is the best.
This statement regarding benefit of rearing Broilers on DL in comparison to cage rearing:
http://agritech.tnau.ac.in/expert_system/poultry/Broiler Management.html
- Birds are not having access to the unidentified growth factors in deep-litter system.
This pertains to the bacon, that sits beside your eggs. Still associated with DL. I found it interesting:
Benefits. Many studies have been done in order to research potential
advantages and disadvantages of the
deep litter system. ... “Pigs in the
deep litter systemhad greater color score and rate of cooking meat, while they had lower drip loss and cooking loss than loins from concrete-floor
system housed pigs.”
Deep litter - Wikipedia
https://en.wikipedia.org/wiki/Deep_litter
Then, this info:
http://agritech.tnau.ac.in/animal_husbandry/ani_chik_poultry rearing.html
Deep litter system
Advantages:
1. It is an economical
2. Hygienic, comfortable and safe to birds
3. Built up litter supplies vitamin B12 and Riboflavin to the birds
4. Controls diseases and vices
5. It increases the efficiency of production
6. Materials such as paddy husks saw dust, dried leaf, chopped straw and groundnut kernels depending upon the availability can be used as litter materials.
Points to be considered while adopting deep litter system
1. The deep litter system should always kept dry.
2. Only right numbers of birds should be housed
3. The house should be well ventilated
4. The litter should be stirred at least once in a week-wet litter if any should be replaced immediately with new dry litter and birds must be fed a balanced ratio.
5. The time starting deep litter system should be in the dry period of the year as it allows sufficient time (At least two months) for bacterial action.
6. Placing of water should be given due attention to keep litter dry.
(Source: Dr.Paul Pricely Rajkumar, AC&RI, Madurai )
Here's a nice article about the benefit and ease of DL:
https://www.backyardchickencoops.com.au/the-key-to-a-deep-chicken-litter-system
And finally, these paragraphs from the following article:
https://www.grit.com/animals/chicken-health-deep-litter-zm0z13jazgou
Alchemy magic
Over many years showing countless visitors through my poultry house, I’ve found that — if my visitor has ever been in a chicken house — at some point she will sniff the air with a puzzled look and ask, “Why doesn’t it stink in here?” When that happens, I know I’m on the right track with manure management.
But the transformation of “nasty” to “pleasant” is just part of the magic. Remember the comparison of the deep litter to an active compost heap — the process in deep litter is driven by the same busy, happy gang of microbes. And among the metabolites of the microbes — byproducts of their life processes — are vitamins K and B12, in addition to other immune-enhancing compounds. The chickens actually ingest these beneficial substances as they find interesting things to eat in the litter. Don’t ask me what they’re eating, but chickens on a mature deep litter do little other than scratch and peck. This is alchemy indeed: What started as repugnant and a potential vector for disease has been transformed into a substrate for health.
Should you think I’m spinning fairy tales, know that scientific experiments have borne out the benefits of a bioactive deep litter. In 1949, a couple of researchers at the Ohio Experiment Station published research on deep litter. I urge you to read the full report, but to summarize: One experiment compared two groups of growing pullets, both on old built-up deep litter, one group receiving a complete ration, the other fed a severely deficient diet. Mortality and weight gain in the two groups were virtually identical. In another experiment comparing pullets fed a severely deficient diet, groups on old, thoroughly bioactive litter suffered far lower mortality (7 as opposed to 23 percent) and achieved much higher weight gain (at 12 weeks, 2.34 compared with 1.64 pounds) than those on fresh litter. Both these and further experiments demonstrated: “Obviously, the old built-up litter adequately supplemented the incomplete ration.”
The Food and Agriculture Organization of the United Nations confirms these observations: “Microorganisms thrive on the manure in the litter and break it down. This microflora produces growth factors, notably vitamin B12, and antibiotic substances, which help control the level of pathogenic bacteria. Consequently, the growth rate and health are often superior in poultry raised on deep litter.”