Tl;Dr: There is only one FDA approved drug for farm animal pain in the U.S. (and that drug is not approved for any of the painful body modifications that farm animals are subjected to), FDA approval might meaningfully increase the frequency with which these drugs actually used, and addressing this might be a tractable and effective way to improve farm animal welfare.
The current state of farm animal analgesics
I was at the pain in animals workshop at NIH (I believe there will be a video of the whole conference available at some point), and was struck by the discussion of farm animal pain medication. As you probably know, farm animals in the U.S. almost never get pain medication for acutely painful procedures such as castration, tail docking, beak trimming, fin cutting, abdominal surgery, and de-horning. What I was not aware of until this morning is that there is only one FDA approved medication for ANY farm animal analgesic, and that medication is specifically approved only for foot rot in cattle. This means that if a farmer wanted to voluntarily use an analgesic on their chickens when they cut off part of their beaks or on their pigs/cattle when they castrate them, they must use drugs off label. There are plenty of such drugs available, but by using drugs off label, the farmer would risk being held responsible for any residue of pain medication in the animal's meat (One presenter mentioned that they surveyed 1187 cattle farmers and 64% reported perceiving the FDA as a barrier to the use of analgesics on cattle, but I don't have the study and people say a lot of things on surveys anyway, so take that for what its worth). In contrast, the EU, UK, and Canada have much higher standards for food residues in other domains (hormones, antibiotics, etc.) but have nevertheless approved several pain medication for several procedures in species of farm animals. As a result, these drugs are much more commonly used there.
When I hear people discuss the lack of pain medicine in the U.S. farms, it is usually in the context of farmers being irresponsible or animal welfare laws being to lenient (neither of which I deny). However, getting drugs approved is a pre-requisite for an improvement on either front.
Why aren't there more options?
I collected opinions from scientists, anesthesiologists, veterinarians, FDA representatives, USDA representatives, and representatives from Elanco (unfortunately, not from representatives of relevant pharmaceutical companies) why there are not more options like there are in other countries. Among the people I spoke to, there was near universal agreement that safe and effective drugs already existed. Some pointed out that there are logistical obstacles for implementing these drugs on the farm level. For example, factory farms spend very little time on each procedure and conduct them in an assembly line sort of fashion. Topical anesthetics, on the other hand, must be left on the skin for a couple minutes in order to work. This makes drug companies feel farmers will not adopt this practice, so they think it is not worth the trouble to get these drugs approved. A panelist from an animal welfare standards and labeling organization mentioned in their panel discussion that when pain management is mentioned on consumer labels, consumers don't buy an animal product at all (unlike when, say cage-free or other welfare metrics are labeled--which is good to know for its own reasons). Others mentioned that they don't think drug companies have had the economic case fully made to them--in other words, that it has gotten easier to pass drugs through FDA in recent years and there is a market, but the drug companies are somehow unaware of this opportunity or mistaken about the cost-benefit calculation because they don't believe there is a public or farmer level interest in these drugs when in fact there is (I am highly skeptical of this kind of reasoning but I don't think it should be ignored). A recurring theme was that there are different standards of specificity for approval in the EU than in the US. I don't have a great grasp on the nuances of these differences, but many people seemed to think this is major factor. For example, in the US, a drug must be approved as effective for a specific procedure, while in the EU, it is approved by species and category of procedure (e.g. soft tissue surgery). This might make drug companies feel hesitant. FDA representatives noted that FDA accepts foreign studies as evidence for approval in terms of residue analyses, safety, and effectiveness.
What can be done about this?
On the one hand, it is hard to understand how the economic incentives for drug companies are not there--the drugs do not seem to require serious innovation given the existing drugs in other countries and off-label uses here, and even if the drugs were used by only a very small fraction of farms, the number of farm animals is so large that seems to me it would be profitable. On the other hand, drug companies are highly rational and sophisticated, so something about this reasoning is probably mistaken or else they would have done so already.
One possibility is that the FDA needs more public pressure to speed up approvals or even recognize reciprocal relationships between farm animal medications approved in the EU/UK/Canada/etc. Obviously there is the risk of approving an ineffective drug, but in my opinion, there are sufficient drugs that seem very likely effective with no risk of human health harm from residues that this is not a major risk. I'm sure they already get pressure from drug companies, but pressure from animal welfare organizations is a different sort of pressure that might elicit more public sympathy and strike FDA officials as having less of a direct conflict of interest. Another possibility is that drug companies can be directly engaged to understand why they haven't developed these yet. If organizations that have a proven record of getting corporations to adopt cage-free eggs, for example, perhaps the missing piece is an assurance that if these drugs are approved, these same organizations will exert pressure for corporations to commit to using these new drugs. I also think more public attention to this (easier said than done) specific issue of lack of available drugs in the US is worth pursuing. Obviously, people have limited appetite for hearing about things, but it might motivate drug companies to go through the trouble of getting new drugs approved.
These strategies are not silver bullets, and I'm sure that there are people already working on this. However, I think this is worth paying more attention to from the perspective of welfare improvements.