Research Summaries: Canola and Peas in Livestock Diets

1.2 Energy Digestibility of Peas for Pigs

As feed formulation becomes more sophisticated, the more important becomes carbohydrate characterization. In peas, carbohydrate composition varies between the cotyledons and the hulls (Table 3) and between arvense and hortense varieties. During early development sucrose and glucose plus fructose dominate while the concentration of oligosaccharides (-galactosides) remains low. At maturity, the latter increase, while the former stabilize at a low level. The three main NSP residues, glucose, uronic acid and xylose, increase considerably during development (Reichert and MacKenzie 1982 {1467}; Daveby et al. 1993 {1325}). Deposition of these sugars results in cotyledon cell walls, which are rich in arabinose-containing pectic substances with xyloglucans and cellulose also being important components. The cell walls of pea hulls have a high content of insoluble residues, comprising cellulose, acidic xylans, pectic polysaccharides (uronic acids) and lignin (Canibe et al. 1997 {1261}).

Table 3 Carbohydrate composition of peas g kg-1 DM
Soluble non-cellulosic polysaccharides			Insoluble non-cellulosic polysaccharides
	Cotyledon	Hull		Cotyledon	Hull
S-Rhamnose	2	3	I-Rhamnose	0	3
S-Rhamnose	2	3	I-Rhamnose	0	3
S-Fucose	0	1	I-Fucose	1	3
S-Arabinose	22	22	I-Arabinose	18	17
S-Xylose	2	9	I-Xylose	2	108
S-Mannose	3	1	I-Mannose	0	3
S-Galactose	5	9	I-Galactose	3	5
S-Glucose	2	19	I-Glucose	6	0
S-Uronic Acids	7	73	I-Uronic Acids	10	43
Total	43	137	Total	40	182
Total NSP	95	859
Cellulose	12	540
Reference{1261} Table 1

Degradation of pea carbohydrates occurs throughout the gastrointestinal tract. The mainly pectic polysaccharides of the cotyledon are readily available to the microflora in the proximal segments of the large intestine while the acidic xylans and cellulose from pea hulls, have a more resistant nature to digestion requiring longer transit times (Canibe et al. 1997 {1261}). Pectic substances found in the soluble fraction were attributed a role in interfering with reabsorption of some endogenous secretions (Leterme et al. 1996 {1269}). Van der Meulen and Bakker (1991 {1376}) found pigs pea consuming hulls (200g kg^-1) had a significantly lower gastric pH than a low fibre control diet. Redox potential, conductivity, osmolarity, volatile fatty acid content and percentage of free water remained virtually the same for high fibre diets whether wheat bran, soybean hulls, beet pulp, sunflower meal or pea hulls were used as a fibre source.

Pea non-starch polysaccharides (NSP) are considerably more digestible than those of wheat NSP and Goodlad and Mathers (1991 {1370}) even suggested that the digestibility of galactose and uronic acids of wheat are enhanced by the inclusion of peas in the diet. The appropriate enzymes to hydrolyze -galactosides directly are lacking, hence digestion is ascribed to activity in the lower small and large intestine. Although this can possibly result in flatulence and diarrhea if levels are extremely high (Saini et al. 1989 {1568}; Leterme et al.1990 {1389}) -galactosides are an important energetic contribution to the pig (Leterme et al. 1990 {1389).

In terms of digestible energy, peas rank intermediate between corn and wheat. Le Guen et al. (1995 {1286}) indicated that pea starch being one third amylose, was not as well digested by monogastrics as amylopectin. In support of this Abrahamsson et al. (1993 {1337}) found starch digestibility declined significantly from 92.5% for a barley-SBM control diet to 90.7 % for a diet containing 33% light colored peas. Several explanations have been invoked to explain the relatively low digestibility of pea starch compared with cereal starches: the entrapment of starch in the fibrous, thick walled cells, the crystalline structure of C pattern starches within the granule which is more resistant to pancreatic amylase than the A pattern typical of cereal starch granules, the higher amylose to amylopectin ratio compared with cereal starches (Wursh et al. 1986; Ring et al. 1988; Gallant et al. 1992 as cited by Canibe and Bach Knudsen (1997 {1262}).

The determination of metabolizable energy (ME) or net energy (NE) is considered a more accurate evaluation of the energy contribution to pigs. In pigs, ME increases by 0.5% for every 1.0% less energy disappearing in the hindgut. Taverner and Curcic (1983 {1466}) found fecal energy digestibilities of peas and SBM to be equivalent, with peas 6% of the total digested energy occurred in the hindgut, whereas for SBM it was 25% indicating that a greater energy contribution to the pig was conferred by the peas. Ileal digestibility values are found in Table 4.

Table 4 Apparent energy digestibilities of peas
Variety	Titan d	Stegholdt d	Tipu d	Miranda d D	Princessd d d	Victoria d	Solarac cf	Solara
Methodology	T-C b	T-C	T-C	T-C	T-C	T-C	T-C	IRA a
Pig weight (kg)	35	35	35	35	35	35
Energy Digestibility
Ileal (%)	59.4	54.7	56.8	65.1	61.0	61.3	72.8	68.7
Fecal (%)	87.4	89.1	90.2	88.8	87.9	87.4	87.7
Reference	1292	{1292}	{1292}	{1292}	{1292}	{1292}	{1389}	{1389}
Table	5	5	5	5	5	5	2	2

^a Ileo-rectal Anastomosis; ^b Simple T-Cannula;

^c Spring variety most commonly grown in France; ^d Spring variety grown in Canada

Toasting (130^oC, 3-4 minutes) does not affect NSP composition or solubility, fermentability or digestibility (Canibe et al. 1997 {1261}). It does however affect the nature of starch digestion, increasing the fraction that is rapidly digested, at the expense of the slowly digested starch and resistant fractions. Toasting decreased the digestibility of a-galactosides as the heat destroyed the endogenous -galactosidase of the pea seeds and pigs lack this enzyme (Canibe and Bach Knudsen 1997 {1262}). Despite a loss of the crystalline structure of starch after toasting, the cell walls surrounding the starch granules remained intact. The benefit of extrusion of peas is readily seen in the starch fraction as ileal digestibility of starch significantly increased in both spring (97.1 to 98.9%) and winter peas (94.4 to 99.1%) (Bengala Freire et al. 1991 {1377}).