Here is a thought experiment: who can pay more for those 17 kilos of grain? An American who wants his steak dinner or a family of Ugandans? What are the implications of the answer?
Unless there is some kind of apocalypse (rinderpest making it to the USA due to global warming) then the decline in meat production might be (relatively) gradual. The main use of antibiotics in agriculture (AFAIK) is simply to aid fattening animals as they are sick for fewer days of the year and so can put on more weight. Reduce antibiotic use and the market weight of animals will decline and either prices will have to rise or demand will fall as people move to other sources of protein - assuming there are reasonable/viable alternatives.
I'm not sure there will be many political effects - unless of course people are going hungry, however the economic effects might be interesting. If intensive meat production stopped then perhaps there would be more free-range or extensive meat available (certainly at a higher price). Environmentally this might not be a bad thing, for example, the integrated olive/chestnut, goat/pig agriculture we have here in Portugal is pretty good for wildlife. More animals wandering over a wider area would be good for vultures as well (which are in rapid decline globally). Admittedly this is probably a limited scenario but it illustrates that there are desirable alternatives to highly intensive agriculture.
Aside: The economics of meat production seem pretty crazy to me. I can go to my local supermarket and purchase several meals worth of fatty pork for less than the price of a couple of heads of broccoli and and few carrots. Given it takes a lot more resources to produce the pork that suggests that the system we have is seriously out of whack.
Caveat: Off course, we might not be able to carpet the globe in goats to make this all work. It's just not going to be possible (or at least very difficult) to do this with a population well in excess of 10 billion.
Regarding your aside: Cattle are usually not fed broccoli and carrots, and I am fairly certain you can get loads of their food for the price of some fatty pork. Even one of the higher-quality cereals, wheat, is sufficiently cheap that the price you pay for bread is almost entirely in manufacturing the bread and the various ingredients with less mass.
Animals do not represent the only viable food source for humans.
The global implications are less meat in everyone's diet, which is hardly a dire outcome. The majority of humanity is likely capable of subsisting on a completely plant-based diet. (Please note I am not a Vegan or Vegetarian, I'm just pointing out what should be obvious.)
I'm reminded of Asimov's Sci-Fi books where he included the idea of "Yeast vats and algae farms [which] produced basic nutrients, which were then processed with artificial flavors into palatable food."
Currently, we'd be able to live if we can't raise cattle, pigs and sheep at all - there would be implications in culinary habits, food markets, and agricultural equipment changes caused by switching from livestock to food crops; but nothing that should cause a total economic crash or destroy food security.
If we can no longer raise as many cattle, pigs, sheep etc. then what are the global implications.