Depends on the feed, if you look at someone growing chickens commercially, they probably mix their own feed, so they choose what goes into it... Antibiotics are not cheap, so they are unlikly to use them unless they are needed.
Aaron Z
This is true, no milk is allowed to be saved from a cow that has been treated with anti-biotics and chickens must be anti-biotic free before going to market. I don't recall the time delay required for the "free state" to be accomplished.
Commercial chicken growers are supplied everything (chicks, feed, pickup and slaughter) by big commercial companies like Tyson. We have tons of chicken houses here in Arkansas (4 within 2 miles) and daily trucks go by with feed to the houses and about every 2 months, trucks go by loaded with chickens. We also have egg houses but I don't know how they handle feed. I did work with a man who raised chickens for Tyson and he said they have to do exactly what Tyson says including the type of feeders that they use. All chicks are fed anti-biotic feed when small to ward off bacterial infections. They are also vaccinated for one disease (How do they do that TV show) while still in the egg. I feed chick starter which is a medicated feed to my home raised chicks. They grow off better and have fewer deaths that way. This is discontinued when they get large enough to handle pellets which are not medicated.
Also, on antibiotics, there is generally a "holdoff" period where you cannot sell the milk or butcher an animal after they get antibiotics to make sure they get out of the animal's system.
AS for GMO products- if a plant is still around it has genetically mutated over millennia or else they would become extinct. The man made GMO is nothing more than a sped up process to help nature out. Man has produced plants that are disease resistant, bug resistant, better rooted, bigger etc all because the existing plants were dying out or not producing enough food to feed us. Many of these are nothing more than hybrids. GMO danger is nothing more than internet and political hype.
Plants and animals have been modified genetically naturally for as long as they have been on earth OR they have died out because of climate change, disease etc. Every living organism must genetically modify itself if it wants to continue to exist. Humans have changed drastically from the caveman days in both form and eating habits, so are we genetically modified?
As for chemicals used, GMO plants make for less chemical use in most instances. No one wants to eat chemicals in any form. Pesticides are banned for days or weeks (depending on the chemicals breakdown time) from harvest so that we aren't eating pesticides. I still wash or peel my fruit and vegetables before eating just in case some third World provider doesn't conform to our guidelines.
As for lips going numb? I doubt that had anything to do with pesticides. I have been around pesticide use on the farm for the first 25 years of my life, spilling it on me, blowing out stopped up spray nozzles etc. and never have I experienced any numbing of the lips from them and I am talking commercial strength applications that you cant buy without being a licensed pesticide applier.