If your risk assent says that there is risk to your employees from the poultry muck( and I think that there is- when our staff go into poultry houses they wear RPE) they you must identify suitable protective for them. Your issue seems to be 1. Resistance to wearing RPE by the staff 2. Them having to wear it for long periods of time( a whole 7 hour shift). You will need to talk to your employees and find out what sort of equipment fits them best. It is quite likely that different employees will find that different masks suit them, so you might end up with a variety of masks in use. If you are intending to use a negative pressure face plate it must be face fit tested, to make sure it fits that individual. Ask you supplier to bring along a range of mask for them to try out. If you decide that your staff need use this RPE then you have a duty not only to supply it to them to make sure that they use it correctly. Positive pressure face mask like the ones you mentioned are more expensive to start with but they have a number of advantages, including the fact that they do not need face fit testing. Remember if you go for the disposable face mask option you will need to replace it at least every shift if not twice a shift ( depending on the conditions that they’re working under) so in the long term a powered respirator might make more sense. The newer ones are lighter that the older designs, so again see your supplier and ask them what they have. It think it is unrealistic to expect someone to work a whole shift wearing RPE and you will need to give the guys some more breaks which might reduce the level of reluctance to use the RPE.