As promised last Tuesday, I'm going to continue with the pig theme for a few long posts. I hope you'll stick it out as I could use your help at the end of my next piece, and you will better understand the issue after reading this one and the upcoming installment.
I wanted to get a picture of the whole digestive tract spread nicely in a line from stomach to end, but it was darn cold the day I slaughtered the pig and all I managed was a pile of guts. I did not keep the stomach attached... At least the photo shows the size difference between the small intestine and the large intestine. In the upper right-hand corner you can see the twine I used to tie the bung shut to prevent contamination of the carcass.
Optimizing carnivory or herbivory calls for radically different digestive tracts and strategies. Of course, there is the middle path straddled by rodents, bears, raccons, pigs, and humans. But even with the internal organs "optimized" for herbivory there is still room for the continuum to connect to carnivore. As an example, deer eat birds.* I find this particularly illuminating because deer are ruminants. The question for me is, how far can I push pigs toward herbivory? And not just a vegetarian diet since grains and beans are "vegetarian". I want pigs that perform with forages and fodders as a large part of their daily intake.
It is thinking in this vein that rubs me the wrong way when people, particularly farmers, say something to the effect of, "pigs can't eat grass because they're monogastric". I suppose it is useful as a shorthand explanation since most people then make the mental leap to the human gut, and think, "yeah that makes sense". The metabolic advantages of rumination, which allow for more complete extraction of nutrients from fibrous feeds, is one reason cattle are more widely raised for meat than horses are. All other things being equal, a steer will gain more weight on a given amount of hay than a similar size gelding. But horses are monogastric, so how is it they can thrive on coarse hay that even a young cow won't grow well on? The answer is they have huge colons. Rather than fermenting plant fiber in the stomach they wait until it is in the colon/caecum to set the cellulose breaking bacteria to work. Herbivores that use this digestion strategy are known as hindgut fermenters because the bacteria they harbor for breaking down cellulose and other plant fibers reside in their "hind" guts. Ruminants and their close cousins (camels) are also called "foregut fermenters". In their case the majority of plant roughage digestion takes place early in the digestive tract "at the fore of the gut" so to speak. When it comes to hindguts, larger and longer allow for more complete extraction of nutrients. In bigger tracts, the digesta's residence time is long enough to let colon bacteria work away on the cellulose fibers and liberate nutrients. Smaller hindgut herbivores like groundhogs tend to be more selective about what they eat, choosing the plant parts that are higher in protein and easier to breakdown. Some, like rabbits, even engage in coprophagy and send stuff through twice to extract goodies they missed on the first pass.
Pigs have bigger and longer guts than humans of the same weight, and thus they are capable of surviving on forage rich diets. Farm pigs reach mature weights far beyond humans - like 400-800 pounds for sows and 500 to well past 1,000 for boars. At that size pigs are able to derive a lot, or even 100%, of their daily nutrition from forages. I've read many anecdotes about people raising pigs largely, or even exclusively, on forages. I put stock in these accounts because even the conventional texts (I, II) claim that gestating sows can thrive on diets composed almost exclusively of alfalfa.
Pigs can and do eat grass, but they get better mileage on more digestible fodders. The colon and caecum in a pig is smaller than in horse or pony of comparable weight, and therefore they don't do well on coarse grasses. If they could, you'd see a lot more "grass-fed" pork at the farmer's market. More digestible means less fiber and therefore more of the plant can breakdown and be absorbed enzymatically in the small intestine. Rumen breakdown of roughage has the advantage of liberating energy and protein molecules prior to the intestines. Most protein absorption occurs in the small intestine. With hind-gut fermentation of fibrous feeds a lot of potential nutrients pass through the ileum unabsorbed because they're locked away. Fodder crops like those in the brassica family and some legumes can have quite low fiber levels, which means they will provide a reasonable dose of protein to the ingesting animal, no matter its fermentation method. In fact, they're so low in fiber they can cause problems for ruminants if they compose a large portion of daily intake. Rumanints actually need to ruminate, which they can't do without fiber.
While foregut fermentation is an advantage when it comes to liberating nutrients from fibers, it hinders feed conversion efficiency on digestible feeds. All the trillions of bacteria residing therein need to be fed too, and they take their cut whether the feed is easy or hard to break down. Some pigs can convert 3 lbs of corn/soy feed into 1 pound of liveweight gain. They have little in the way of a bacterial colony to support in the upper part of the digestive tract. Cattle need twice (or more) the feed for each pound of gain on a similarly concentrated ration. In my next installment I'll go into much greater depth about why I don't want to feed a bunch of corn and soy to my pigs. For now, suffice it to say, I don't want to feed them a lot of a "standard hog ration".
Young pigs are not terribly proficient at digesting roughage. In my (limited) experience they need to get well past 100 pounds before their digestive tracts are substantial enough to extract much feed value from grasses and their ilk. In addition, young growing animals need a higher plane of nutrition than mature animals do. Young stock anabolize (Is that a word? If not it should be) constantly and this requires a relatively greater amount of protein. I'm sure there are genetic pieces to this puzzle too - because they grow faster on grains, and 99.99% of US raised pigs get substantial grain rations, there is no selective pressure toward better fiber digestion. In fact, the selective pressure exerted by industry is toward less ability to digest cellulose. The digestive tract has a maintenance cost of its own. On a concentrated, easy to absorb ration, a smaller gut allows for more energy to go to growth of the animal. A bigger digestive tract will act as a "drag" of a few percentage points on a CAFO pig. In pig factories every single % is pushed toward maximum production, no matter the cost - be it animal (non)welfare, environmental externalities, unpleasant working space (for humans), etc. On my farm under my management, the more nutrition a hog can get from forage the better, even if it costs a little bit of conversion efficiency in a feeding trial against a CAFO pig on a hot ration.
PS - In learning about this topic I've begun to wonder how much hind-gut fermentation humans are capable of. I suspect adult humans are capable of deriving a significant number of calories from greens and vegetable fibers if we get the right bacteria in our colons and supply them with enough appropriately fermentable substrate. Assuming I'm correct about that, it would be yet another reason to be skeptical of low-fat diets. The absorbable products of hind-gut fermentation are mostly short chain fatty acids, so it seems to me we ought to be able to thrive with them in our diets whether they come from endogenous bacteria or exogenous food sources. I realize I'm on logically thin ice here - but it seems reasonable... :) But heck, even the august governmental and non-profit institutions that pedaled low-fat, low cholesterol diets for decades have backed away from those guidelines recently.