As is well documented, pulse crops are known to contain trypsin inhibitors, tannins, lectins (haemagglutinins) and saponins. The effects of these antinutritional factors (ANF) vary depending on the ANF in question. Pigs in general do not suffer serious consequences to their presence in the diet, however, age and size of pig are important considerations (Arentoft et al. 1991 {1383}).
Antinutritional factors vary among cultivar. In
peas, tannins are only found in arvense (dark colored flower)
varieties (Grosjean and Gatel 1989 {1411}) where they are concentrated
in the testa (Griffiths 1981 {1474}). Trypsin inhibitor activity
(TIA) is greater in winter than spring and smooth than wrinkled
pea varieties (Valdebouze et al. 1980 {1584}). Canadian
peas, the majority of which are spring seeded, white flowered
varieties are characterized by very low levels of ANF of little
nutritional significance. In a Canadian pea variety trial only
8 out of 56 samples exhibited trypsin inhibitor units (TIA x 1.9)
greater than six. The two highest cultivars contained 10.3 and
9.6 trypsin inhibitor units (100% DM). In comparison, half of
the 16 Austrian winter pea (Maple pea) varieties tested were above
6.0, with the highest being 10.7 trypsin inhibitor units (100%
DM) (Slinkard and Tyler 1993 {2057}).
Despite the low levels, trypsin inhibitors remain the most troublesome ANF in peas. Both trypsin inhibitor activity (TIA) and chymotrypsin inhibitor activity are independent of protein content. In peas 10% of TIA is found in the hulls and 90% in the cotyledons, which is proportionate to their respective weight distributions in the whole seed. Chymotrypsin inhibitor activity is higher in peas than in beans whereas the reverse is true for TIA (Griffiths 1984 {1461}). In peas TIA and chymotrypsin inhibitor activity are highly correlated (r=0.986) suggesting a double headed trypsin/chymotrypsin inhibitor analogous to the Bowman Birk inhibitor found in soybeans (Grosjean and Gatel 1989 {1411}). Interestingly, Arentoft et al. (1991 {1383}) concluded that pea inhibitors are different from Bowman-Birk and Kunitz soybean trypsin inhibitors. They reported that young piglets (several days to 8 weeks of age) showed 2-10 times higher inhibition than full grown animals. In addition they found the degree of inhibition to vary among species and stressed the importance of using animal species of interest at the correct age and determining 50% inhibition as the benchmark measurement. Boisen (1989 {1407}) proposed that an extraction procedure at pH 2 with added pepsin followed by an incubation of the extract with porcine trypsin gave the best evaluation of trypsin inhibitors in all types of pig feeds. Using this method they found that porcine trypsin was inhibited less than bovine trypsin by either legume or cereal inhibitors.
Both pea and bean proteolytic enzyme inhibitors remain stable at temperatures at or below 80C and their inhibitory properties are unaffected by pelleting. Both are slightly reduced at 100C and completely denatured by extrusion (Grosjean and Gatel 1989 {1411}) or autoclaving (Griffiths 1984 {1461}; Fan and Sauer 1994 {1292}). TIA does not decline with time as levels remained the same after storage for one year (Grosjean and Gatel 1989 {1411}).
An early explanation of the mode of action of protease
inhibitors was that as a consequence of inhibition of proteolytic
enzymes in the digestive tract, the pancreas was stimulated to
synthesize and secrete more enzyme protein. Ultimately this resulted
in not only a hyperactive pancreas but also an increased demand
for amino acids by this organ. In addition the higher loss of
inactivated proteolytic enzymes depresses the apparent absorption
of sulfur amino acids, since both methionine and cysteine are
relatively consistent amino acids of the proteolytic enzymes themselves
(Griffiths 1984 {1461}).
Lectins (haemagglutinins) are proteins with the capability of binding sugars present as glycoproteins. Lectins are typically measured according to their ability to agglutinate erythrocytes (red blood cells) through cross linking surface glycoproteins (Huisman et al. 1990 {1559}). Although this binding is not a physiological concern lectins similarly bind to the mucosa of the intestinal wall damaging the epithelial cells, depressing nutrient absorption, reducing activity of brush border enzymes, and causing hypersecretion of endogenous protein with the shedding of damaged cells. Increased production of mucins and a loss of plasma proteins to the intestinal lumen, also contribute to decreased nitrogen digestibility and retention. Occasionally scouring may further reduce weight gain and feed conversion (Huisman and Van der poel 1994 {1293}). Despite this dire description, in a comparison of 8 pea cultivars, lectin in peas were only one tenth of that found in soybeans (Valdebouze et al. 1980 {1476}).
Saponins are complex material based on a sugar linked
to a steroid or triterpenoid moiety. Saponins have detergent
like effects as illustrated by their haemolytic capabilities when
incubated with erythrocytes attributed to their interaction with
cholesterol in the erythrocyte membrane. Legume extracts vary
in their ability to lyse red blood cells. For peas, beans and
soybean extracts, sheep red blood cells were more sensitive to
saponin extracts than rabbit red blood cells. Survival of guppy
fish was lowest for those exposed to crude saponin extracts from
beans (3.8 minutes) as compared to peas (6.6 minutes) or soybean
extracts (11.4 minutes). The different sensitivity according
to saponin source is indicative of structural differences among
saponins. Saponin extracts from peas were separated into seven
fractions whereas the saponin extracts from beans or soybeans
were separated into only six fractions (Khalil and El-Adawy 1994
{1302}).
c. Effects of ANF on pig performance
Overall, the effect of pea ANF on pigs appears relatively slight. Using two varieties of Canadian peas with varying TIA (1.12 and 4.60) in semi synthetic diets, Gabert et al. (1996 {1271}) found only minor effects on the quality of exocrine protein secretions relative to fababean (which have a higher content of tannins than peas) or SBM diets. In a second study using the same diets no differences in concentration, flow or composition of total, protein bound or free AA in the pancreatic juice of 18kg pigs were noted (Gabert et al. (1996 {1270}). Following a series of experiments examining the effects of pea ANF in young (10-15kg) piglets, Le Guen et al. (1995 {1278}; 1995 {1286}; 1991{1340}; (1991 {1369}; 1991 {1375}) suggested a possible threshold for ANF. In their studies isolated ANF alone did not account for the entire decrease in digestibility observed when raw peas were fed in comparison to an extracted pea protein isolate devoid of ANF. Both diets were balanced for NE and amino acids. Alternatively, they suggested that ANF activity may depend on the protein source associated with them.
For older pigs (33-74 kg) Buraczewska et al. (1991 {1379}) observed no increase in pancreatic secretion with a diet containing 26% peas (white flowered Opal variety) compared to a 95% barley diet. Tryptic, chymotryptic and amylolytic activities were not appreciably altered for the barley or pea barley diets when measured as trypsin inhibitor units ml-1 pancreatic juice. These results indicate that the trypsin inhibitor and tannin levels occurring in peas do not negatively affect pigs even at a body weight of <10 kg.
In conclusion, is unlikely that ANF occurring in Canadian peas are of sufficient level and type to affect piglet performance. Additionally in practical diets their low concentration deems their impact negligible.
d. Antigencity of pea proteins
Antigenicity results from circulatory antibodies
being formed against pea proteins (LeGuen et al.1991 {1369}).
Antigenic proteins induce gut motility disorders in calves and
if lectins induce epithelial damage such disorders could be amplified
and the uptake of antigenic proteins from the intestinal lumen
favored (Le Guen et al. 1993 {1340}). However, in one
study (winter pea Frijaune) with piglets (5.7 -7.5 kg) serum antibody
responses to pea legumin or vicilin were detected in piglets whether
or not the sow's diet contained peas. However absorbance values
for piglets in prior contact with pea proteins either indirectly
from maternal immunity or direct consumption of the mother's diet
were higher than for those piglets with no contact (+77% for legumin
and +172% for vicilin). The observed immune response may have
indicated an immune response rather than a food allergy. With
a true food allergy, gastrointestinal distress would have included
diarrhea, weight loss and poor growth none of which was apparent
(Le Guen et al. 1991 {1369}).
The pigment level of peas varies with protein content.
Low protein peas appeared very bleached (faded yellow) whereas
high protein content peas were a bright yellow. Xanthophylls
were the predominant carotenoids in these peas (Reichert and MacKenzie
1982 {1467}).