By John Patience, Iowa State University Department of Animal Science Professor
In my last column, I discussed various aspects of fiber. It is a topic of growing interest in the United States, because we are feeding diets with higher levels of fiber than was previously the case. We are also starting to learn that fiber is not an inert component of the diet, but it affects not only digestion but various aspects of gut function, physiology, structure and health.
The change in fiber use in swine diets happened quite abruptly in the mid-2000s when increasing quantities of corn were diverted to produce ethanol. This boosted the price of corn, so in order to save money, pork producers started using the byproduct of ethanol production — corn distillers dried grains with solubles — in their diets. Corn contains about 9% neutral detergent fiber while DDGS contains about 30% NDF; therefore, for every 1% of corn replaced with DDGS, the NDF content of the diet rises by about 0.2%. Whereas a traditional corn-soybean meal diet might contain 8 to 9% NDF, a diet containing 20% DDGS might contain 12 or 13% NDF — or even more, depending on the composition of the DDGS.
While high fiber diets were a novelty in the United States until recently, they were already relatively common in Europe, where byproduct feeding has been practiced for decades. European nutritionists were quite familiar with fiber, and researchers had been studying the subject for more than 50 years. They were thus more experienced than we were. However, American nutritionists adapted with incredible speed. When corn reached its peak price around 2008, practical, on-farm diets changed from 75% or more corn to, in some cases, less than 20% corn; byproducts made up the difference. This period illustrated very well how adaptable the pig is to diverse diet ingredients, and also how adaptable feed industry and pork industry nutritionists were in successfully formulating such diets.
Let’s step back and consider the human diet for a moment. This is important to us because when we read about fiber in newspapers or magazines or on the internet, or hear about fiber on the radio or TV, it is often in the human, rather than animal, context. In human nutrition, and especially in adults, fiber is often viewed as a good thing. It is well-known that the intestinal tract of the pig is quite similar to that of people, and the pig is therefore often used in studies of human nutrition. However, the conversation surrounding fiber in people is quite different from that in pigs.
Fiber in the diet helps to “move things along” in the intestine and prevent or relieve constipation. This is not as important in pigs — other than perhaps lactating sows — than it apparently is in people. Some types of fiber — known as soluble fiber, which we discussed in the previous column — are also known to lower cholesterol, which is associated with heart disease and to control blood glucose or blood sugar, something that is important for people who are at risk of developing diabetes.
Again, this is not so important to the pig. Fiber, and especially insoluble fiber, also gives a sense of satiety or fullness, and is commonly used to reduce calorie intake and prevent obesity. In contrast, pigs are eating as much feed as they can, so achieving satiety at less than ad libitum intake is not important, other than perhaps in the gestating sow. Therefore, in human nutrition, fiber is by and large viewed as a good thing.
In pig nutrition, we take a different view. In the young, which has limited feed intake capacity, the bulkiness of higher fiber diets often leads to slower growth rates and poorer feed efficiency. Therefore, fiber levels are often kept quite low in diets of the young pig; however, as the pigs grows, fiber levels can be increased because feed intake capacity increases in relative terms.
In older pigs, with greater capacity for feed intake, the results of feeding higher fiber diets are mixed. If diet energy concentration is maintained at the same level in both high- and low-fiber diets — often achieved by feeding more fat — and if the pigs are able to eat sufficient quantities of the bulkier diet, growth rate will be the same. But if the bulkiness of the diet makes it difficult for the pigs to maintain feed intake, then growth rate will decline as fiber levels increase. Often, higher fiber ingredients are less expensive than lower fiber ingredients, so there is a motivation to use them in pig diets — but to do so in a manner that results in a minimal impact on performance while increasing net income.
In hot weather, the risk of feeding higher fiber diets is magnified because the metabolic utilization of fiber generates body heat that in turn makes heat stress more of a problem. For this reason, in summer months, fiber levels may be reduced to offset this additional heat production; doing so will help maintain performance or at least reduce losses in growth rate that are typically observed in the summer months.
There are a few other considerations when feeding increased levels of fiber in the pig diet. Higher levels of fiber fed during the late finishing period will result in reduced carcass yield. A former graduate student at Iowa State University, Emily Weber, found that increasing DDGS from 30 to 60% of the diet lowered carcass yield by 1 percentage point, from 76.1% to 75.2%. Reducing fiber during late-finishing can reduce or eliminate this loss in yield; it really comes down to the feed cost of producing one pound of carcass rather than one pound of live body weight. That is why, to the greatest extent possible, such calculations should always be made on a carcass basis (carcass average daily gain, carcass feed efficiency, carcass feed cost, etc.) when higher fiber diets are being fed.
Finally, and we discussed this a bit last time, certain types of fiber might be beneficial to the pig in terms of gut health, and other types of fiber may be detrimental. This is still a field of science with more questions than answers, but it appears that in the case of intestinal disease, such as colibacillosis, soluble fiber may be beneficial to the pig, and insoluble fiber may be a bit harmful. Much of the research to date has been done in very intensive research systems, which allow us to understand mechanisms, but do not do as good a job of revealing how this might all work in the field. As I said, it is a growing topic of research and we will learn a great deal more in the coming years.