I don't think there are many viable options that meet your criteria. Similar to human medicine, "Natural alternatives" are prone to lack of efficacy due to varying/inconcistent formulations, unproven pharmicokinetics, or even lack of bioavailablity of the putative active ingredient. Additionally for food production animals, I would suspect many of the "alternative" products would not have been thoroughly tests for meat/milk withdrawl periods. I would suggest that if you want to go "natural" the best approach is to manage the animals so as to prevent outbreaks/disease. These management approachs could aim to mitigate stress (e.g. shipping, minit new animals entering herd), optimize vaccination programs, provide adequate nutrition, appropriate parasite controls etc. Prevention could be perhaps the best natural approach.   _{Your Comment}