I cannot resist temptation to comment some points from our experience. I will use an example of a good friend to explain my position. Feeding broilers in many places worldwide, as it was 10 years ago in EU, sounds not "logical": imagine a room plenty of people, and one door is opened entering a waiter with nice food. Everybody will be fed, happy and even there will be reproduction between them. Then another door is opened and someone with a gun begins to shoot people inside room, later again more food is available, and later again more bullets are shot. Now change people in the example by bacterias (in general) in the intestine of a chicken, and change guns by antibiotics (growth promoter). Feeding and killing- feeding and killing...
When we started to think how to substitute GPA because they were banned, first we thought was to kill as an antibiotic.... is not possible.... but even with them, if you review your reports in veterinary field treatment because enteritis and similar, you will see they were not perfect! They failed too as alternatives (pro&prebiotics, acids,enzymes...) available can. Don't look for a silver bullet ... doesn´t exist, and didn´t exist.
There was a prediction done in 2005 about future of technical results without GPA ...and nothing was right! Broilers one year after being banned GPA were growing in very similar way as they were one year before. And nowadays, 10 years later, with different alternatives being proved, broilers are improving genetic standards, I would say that today we are more worried by carcass quality than for DWG, FCR, mortality...because everyone is getting good marks in.
Don´t look for silver bullet, use what you have as alternative depending of which is more reasonable for you and thinking bacterias are not bad, we need them to develop completely:" Full maturation of gut inmune system requires a complex flora. However, the precise composition of an optimal flora is still unknown”/ In mouse, is clear that nutrients and fermentation products also influence funcional status of the mucosal inmune system" (Kaspers 2015).
So maybe, modulation microbiota, balance host-guest is the key, try to choose wich bacterias you want because chickens (and us) need them... And for geting this target all production system must be involved: we have reviewed since 2006 all formulations of diets, raw material quality water, temperature-ventilation in housing, management of day old chicks. We have spent a lot of bullets looking for some factors that allow us complete activation of digestive physiology... but to produce with less antibiotics than today, can be done.
Many times we are creating the situation for imbalance microflora, f.ex. regular or bad thermostabiltiy enzyme used, too many antinutritional factors in raw materials...we are creating a situation for undesired bacterias that will consume nutrients and antibiotics.
A lot of things to do, but life exists after.