Fiber in feed ingredients used in diets for pigs is mainly used for energy, which is synthesized by microbes in the hindgut of the pig and absorbed in the form of short chained fatty acids. To estimate the energy that a pig can obtain in a particular fibrous ingredient, a digestibility experiment is usually conducted and the digestible energy (DE) and the metabolizable energy (ME) in the ingredient is determined. However, it is not known if the obtained DE and ME values are accurate for all inclusion rates of the feed ingredient in diets.

There are two reasons that differing inclusion rates of high fiber ingredients in diets fed to pigs might result in variable DE and ME values. Because energy from dietary fiber is obtained via hindgut fermentation, there may be a saturation point in the fermentation capacity in the hindgut of growing pigs. In addition, increasing dietary fiber increases the passage rate of feed through the digestive tract and thus reduces time for fermentation. However, it is not known if these potential issues affect the DE and ME in fibrous ingredients fed to pigs. An experiment was conducted to determine effects of inclusion rate of four commonly used high fiber dietary ingredients on the concentration of DE and ME by growing pigs.

Experimental design

A basal diet based on corn and soybean meal as well as a diet containing corn, soybean meal, and 30% corn starch (CS diet) were formulated. Six diets were formulated by including 15 or 30% corn germ meal, sugar beet pulp, or wheat middlings to the CS diet at the expense of corn starch. Two additional diets were formulated by including 15 or 30% canola meal to the CS diet at the expense of corn and soybean meal.

Twenty pigs with an average initial body weight of 30.6 kg were randomly assigned to one of the ten diets. After a 14-day adaptation period to the diets, feces and urine were collected for five days and analyzed to determine apparent total tract digestibility (ATTD) of gross energy (GE) and the concentration of DE and ME. Ileal digesta were collected on days 22 and 23 of the experiment via a T-cannula surgically fitted in the distal ileum of the pigs. Ileal digesta samples were analyzed to determine apparent ileal digestibility (AID) of GE, and apparent hindgut disappearance (AHD) was calculated as the difference between ATTD and AID. On day 24, indigo carmine (an indigestible blue marker) was added to the morning meal. The time the pigs began to eat was considered time zero. Ileal digesta and feces were monitored and the time of the first appearances of blue ileal digesta and blue feces were recorded.

Inclusion rate does not affect DE or ME concentration in high fiber ingredients

There was a linear reduction (P < 0.001) in AID of GE, ATTD of GE, and concentrations of DE and ME when 15 or 30% of canola meal, corn germ meal, sugar beet pulp, or wheat middlings were added to the diets (Table 1).

The AHD of GE increased (P < 0.05) linearly as inclusion of canola meal, corn germ meal, or sugar beet pulp increased. This reflects the fact that the proportion of energy derived from hindgut fermentation is greater when diets contain more fiber.

The time until the first appearance of the indigestible indigo carmine marker in the ileal digesta did not differ among pigs fed the ten diets. However, the time until first appearance of the marker in feces decreased (P < 0.01) from 2,670 min for the CS diet to 2,057, 2,329, 1,812, and 1,914 minutes for pigs fed diets containing 15% canola meal, corn germ meal, sugar beet pulp, or wheat middlings respectively. The transit times were further decreased  (P < 0.01)  to 1,755, 1,844, 1,210, and 1,686 minutes for pigs fed 30% canola meal, corn germ meal, sugar beet pulp, or wheat middlings.

When the contributions of each ingredient to DE and ME were calculated, inclusion rate did not affect AID, AHD, or ATTD of GE or concentration of DE and ME in any of the high-fiber experimental ingredients (Table 2). In other words, each ingredient was digested as efficiently at 30% inclusion as at 15% inclusion. This indicates that the ability of the microbial population in the hindgut to ferment fiber was not exceeded when pigs were fed diets containing up to 30% of high fiber ingredients, and that the reduction in transit time did not impair fermentation and synthesis and absorption of short chain fatty acids.

Key points

• The concentration of DE and ME decreases as fiber content of the diet increases.
• Passage through the hindgut was faster when pigs were fed high fiber ingredients, but the rate of passage in the small intestine was not affected.
• The ability of pigs to ferment fiber in the hindgut was not affected by inclusion of high fiber ingredients up to 30%.

Table 1. Apparent ileal digestibility (AID), apparent hindgut disappearance (AHD), and apparent total tract digestibility (ATTD) of GE, concentration of DE and ME, and transit time in experimental diets, as-fed basis

Table 2. Apparent ileal digestibility (AID) of GE, apparent hindgut disappearance (AHD) of GE, apparent total tract digestibility (ATTD) of GE, and concentration of DE and ME in canola meal, corn germ meal, sugar beet pulp, and wheat middlings at 15 or 30% inclusion rate

This report is based on unpublished research by Diego Navarro, Erik Bruininx, Lineke de Jong, and Hans H. Stein. The research was financially supported by Agrifirm Innovation Center, Apeldoorn, the Netherlands.