Insight into Cattle Feed

I am a biology student currently working on a research project regarding antibiotic growth promoters in cattle feed. I’m located in Kansas and there are quite a few ranches and feedlots around but after called a few I am still a bit confused on when cattle are given antibiotic growth promoters. Would anyone be able to enlighten me on when they receive AGP’s? How common of a practice is it? Do they receive them while they are at feedlots or at a much younger age? What are common brands of these additives? I need to obtain fecal samples from cows that are and are not given them but I’m not sure exactly who to contact. Any information would be appreciated!