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 fat concentrations in diets fed to group-housed pigs. Therefore, the objective of this experiment is to test the hypothesis that increased soybean oil in diets for group-housed growing pigs will increase NE and nitrogen balance.

Experimental design

Six diets contained corn and soybean meal (SBM) and increasing inclusion rates of soybean oil (i.e., 0, 1.5, 3.0, 4.5, 6.0, and 7.5%). Ratios between digestible amino acids and metabolizable energy (ME) in all diets were equal among diets.

A total of 48 pigs (initial weight: 41.98 ± 3.32 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 oil in diets.

Results

Results indicated that the ATTD of GE and DM decreased and then increased (quadratic; P < 0.05) by increasing soybean oil in diets although the absolute differences were very small (Table 1). Concentrations of DE (quadratic; P < 0.013), ME (linear; P < 0.001), and NE (linear; P < 0.001) in diets increased by increasing dietary soybean oil. The respiratory quotient tended to be reduced (linear; P = 0.059) with increasing dietary fat. The ATTD of nitrogen decreased and then increased (quadratic; P = 0.013), but retention of nitrogen (% of intake and % of absorbed) increased (linear; P < 0.05) as dietary soybean oil increased.

In conclusion, increasing dietary soybean oil increased energy digestibility and concentrations of NE in diets, and decreased the ratio between produced carbon dioxide and consumed oxygen. The linear increase in nitrogen retention (% of intake and % of absorbed) indicated that more body protein was synthesized as increased dietary soybean oil made more energy available for the pigs.

Key points

• Concentrations of DE, ME, and NE are increased by increasing soybean oil in diets fed to group-housed pigs.
• Digestibility and retention of nitrogen are increased by increasing soybean oil 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.

²AEE = acid hydrolyzed ether extract.

³P-value for linear (L) and quadratic (Q) effects of increasing dietary fat.