When evaluating the energy content of pig diets, producers and feed companies in the United States usually use the digestible energy (DE) and metabolizable energy (ME) systems. However, these systems do not take into account the heat produced by the animals during digestion, and thus the energy lost by pigs in the process of digesting and metabolising the feed.

Pigs fed diets high in fibre have greater feed intake, larger gastrointestinal tracts and increased hindgut fermentation relative to pigs fed diets containing less fibre. Therefore, they might be expected to have greater heat production as well. As a result, the DE and ME systems may overestimate the energy value of fibrous feed ingredients. Net energy (NE) takes heat production into account, and thus may be a more accurate estimate of the energy available to the pig.

An experiment was conducted to test the hypothesis that increasing dietary fibre in diets fed to growing pigs will increase heat production and decrease net energy values.

Experimental Design

The experiment was conducted in the Open-Circuit Respiration Laboratory at the Swine Nutrition Research Centre of the Ministry of Agriculture Feed Industry Centre (Chengde, Hebei Province, China).

Eighteen growing barrows with an average initial body weight of 54.4kg were used. Pigs were randomly allotted to one of three diets. The basal diet was based on corn and soybean meal. The other two diets consisted of the basal diet with 15 and 30 per cent wheat bran added, respectively.

Pigs were given eight days to adapt to the diets, then were placed in metabolism cages in open-circuit respiration chambers. The chambers measured oxygen consumption and methane and carbon dioxide production. Gas exchange and urinary losses of nitrogen were used to calculate heat production. Energy balance was measured over a five day period. Finally, the pigs underwent a two day pre-fasting period and a one-day fast.

Faeces and urine were collected during the experimental period, and analysed for energy content to calculate DE and ME in the diets. HP was subtracted from ME to determine retained energy (RE). The NE of the diets was calculated using the following equation:

• NE (kcal/kg DM) = [RE (kcal) + FHP (kcal)] / dry matter intake (kg)

Results

The apparent total tract digestibility (ATTD) of gross energy, crude protein, acid detergent fibre (ADF), and neutral detergent fibre (NDF) decreased linearly (P<0.01) as the level of dietary fibre in the diets increased (Table 1).

Table 1. Apparent total tract digestibility (ATTD) of nutrients and energy by growing pigs fed experimental diets
	Diet			P-value
ATTD, %	Basal	15% wheat bran	30% wheat bran	Linear	Quadratic
Dry matter	91.74	86.09	81.62	‹0.01	0.40
Crude protein	91.20	87.73	83.87	‹0.01	0.80
Gross energy	91.92	85.78	81.13	‹0.01	0.25
Crude fibre	69.54	54.93	39.32	‹0.01	0.93
Acid detergent fibre	79.60	61.30	52.22	‹0.01	0.05
Neutral detergent fibre	74.17	65.42	64.71	‹0.01	0.10

Table 2. Production of heat and methane and energy balance by growing pigs fed experimental diets
	Diet			P-value
Item	Basal	15% wheat bran	30% wheat bran	Linear	Quadratic
Daily HP, kcal/kg BW0.6	300.79	297.11	287.54	0.02	0.55
Daily FHP, kcal/kg BW0.6	192.75	177.4	198.08	0.74	0.22
Methane production , L/d	4.83	3.21	1.48	‹0.01	0.85
DE in diet kcal/kg	3,454	3,257	3,161	‹0.01	0.18
ME in diet kcal/kg	3,400	3,209	3,091	‹0.01	0.34
NE, kcal/kg	1,808	1,575	1,462	‹0.01	0.17

The heat production of pigs, measured in kcal per kg of metabolic bodyweight, decreased linearly (P<0.05) as dietary wheat bran inclusion increased (Table 2).

This was counter to the researchers' hypothesis. Fasting heat production was unaffected by the inclusion of wheat bran in the diets. This observation indicates that the pigs were not fed the diets for a long enough time to cause the size of their gastrointestinal tracts to increase. Another possibility is that physical activity decreased as dietary fibre increased, offsetting increases in heat production from metabolism; however, physical activity was not measured in this experiment.

The excretion of methane decreased as wheat bran inclusion increased, indicating that the fermentation of wheat bran is low. Wheat bran contains large concentrations of insoluble dietary fibre, which has low fermentability. The high fibre content also shortens transit time through the digestive tract, giving feed less time to ferment.

DE, ME, and NE all decreased linearly (P<0.01) as inclusion of wheat bran in the diets increased.

Key Points

The ATTD of gross energy, crude protein and fibre decreased with increasing inclusion of wheat bran in the diets.

Pigs' heat production decreased with increasing inclusion of wheat bran in the diets. Fasting heat production did not differ among pigs fed the different levels of wheat bran.

The concentrations of DE, ME and NE decreased as the inclusion rate of wheat bran in the diets increased.

This report is based on unpublished research by Neil Jaworski, Dewen Liu, Defa Li and Hans H. Stein.

October 2014