Diets for pigs may vary in composition and in their concentration of the energy containing nutrients (i.e., protein, fat, starch and sugars, and dietary fiber). To our knowledge, however, there is no information on how concentration of net energy (NE) is affected by dietary fiber concentrations in diets fed to group-housed pigs. Therefore, the objective of this experiment was to test the hypothesis that increasing soybean hulls in diets for group-housed growing pigs decreases NE and nitrogen balance.

Experimental design

Six diets contained corn and soybean meal (SBM) and increasing inclusion rates of soybean hulls (i.e., 0, 6, 12, 18, 24, and 30%). Ratios between digestible amino acids and metabolizable energy (ME) in all diets were equal among diets.

A total of 48 pigs (initial weight: 39.58 ± 3.39 kg) were used in two blocks of 24 pigs. The 24 pigs were assigned to 6 calorimeter chambers with 4 pigs per chamber. Chambers in each block were allotted to a 6 × 3 incomplete Latin square design with 6 diets and 3 periods. Therefore, each block provided 3 replicates per diet for a total of 6 replicates for each diet from the 2 blocks.

Pigs had ad libitum access to feed and water. Diets were fed for 13 days in each period. After a 7-day adaptation period, O₂ consumption and CO₂ and CH₄ productions were measured for 6 days to calculate total heat production. Fecal and urine samples were quantitatively collected for 6 days and analyzed for gross energy (GE) and nitrogen. Concentrations of digestible energy (DE), ME, and NE, the ATTD of dry matter (DM) and GE, and nitrogen balance in diets were calculated.

The statistical model included diet as fixed effect and group, period, and chamber as random effects. Contrast coefficients were used to determine the linear and quadratic effects of increasing soybean hulls in diets.

Results

Results indicated that daily feed intake was not affected by dietary treatments (Table 1). Weight of orts and feces increased (linear, P < 0.05) by increasing soybean hulls in diets, which decreased (linear, P < 0.05) the apparent total tract digestibility (ATTD) of dry matter and GE and digestible energy in diets. Urine GE excretion tended to decrease (linear, P = 0.051) and ME in diets decreased (linear, P < 0.001) as soybean hulls increased in diets. Total heat production and retained energy did not change, but respiratory quotient on fed status and NE decreased (linear, P < 0.05) as soybean hulls increased in diets. Ratios between ME and GE and NE and GE decreased (linear, P < 0.01) by increasing soybean hulls in diets. Nitrogen intake decreased (linear, P < 0.05), but fecal nitrogen excretion increased (P < 0.001) as soybean hulls increased in diets. The ATTD of nitrogen, absorbed nitrogen, and retained nitrogen decreased (linear, P < 0.01), but urine nitrogen excretion and retention were not affected by dietary fiber.

In conclusion, increasing soybean hulls decreased energy digestibility and concentrations of NE in diets, and decreased the ratio between produced carbon dioxide and consumed oxygen. Increasing dietary fiber decreased absorbed and daily retained nitrogen. Building on these results, future studies will be to generate prediction equations for NE using chemical composition, physicochemical characteristics, nitrogen balance, and nutrient digestibility.

Key points

• Concentrations of DE, ME, and NE are decreased by increasing soybean hulls in diets fed to group-housed pigs.
• Digestibility of nitrogen is reduced, but retention rate of nitrogen is not affected by increasing soybean hulls in diets fed to group-housed pigs.
• Daily absorbed and retained nitrogen are decreased by increasing soybean hulls in diets fed to group-housed pigs.

Table 1. Apparent total tract digestibility (ATTD) of dry matter (DM) and gross energy (GE), concentrations of digestible energy (DE), metabolizable energy (ME), and net energy (NE), and nitrogen (N) balance in experimental diets fed to group-housed pigs¹

¹Least mean squares represent 4 to 6 replicates per dietary treatment.

²P-value for linear (L) and quadratic (Q) effects of increasing soybean hulls in diets.

³TDF = total dietary fiber.