5. Factors Affecting the Utilization of Peas by Poultry
Analysis of digesta from the proximal ileum of 3-week-old broilers indicated that semi-purified starch from peas (90.3%) was less well-digested than that from wheat (97.3%). However, pea starch digestion was comparable to wheat (94.4 vs. 97.6%, resp.) at the distal ileum. Other pea cell components had been removed from surrounding the starch during purification; therefore, the digestibilities are thought to reflect varying susceptibilities to enzyme hydrolysis in pea and wheat starches (Yutse et al. 1991{64}). The starch granules of the wrinkled pea cultivar, Scout, had deep fissures and grooves (Otto et al. 1997{5}) and it would be interesting to determine if the increased surface area would improve digestibility for poultry.
Storage proteins in some legume species are resistant to digestion by poultry. Vicilin is one such protein in peas; however, unheated vicilin is far more digestible than equivalent proteins in dry beans or soybean (Deshpande and Damodaran, 1989 {96}; Nielsen et al. 1988{101}). Although resistant proteins have been identified in peas, they represented only a small fraction of total protein when measured at the terminal ileum (Crévieu et al. 1997{4}).
Greater practical implications come from work examining
the effects of steam pelleting on field pea nutrient digestibility
in adult birds and 3-week-old broilers. Diets were based on ground
peas (2 mm sieve), or on ground peas that were subsequently steam
pelleted (4x30 mm die) and reground (2.5 mm sieve). Pelleting
produced a strong positive effect (P< 0.001) on AMEn and starch
digestibility across age groups, and a significant improvement
in protein digestibility (P<0.05) within the young birds (Table
6). Workers had previously noted a correlation (r=0.97, P<0.05)
between starch and protein digestibility (Conan and Carré
1989{86}). When this work was repeated, it was observed that
the majority of undigested starch was located in the large particles
(>0.5mm) in the excreta. It was hypothesized that this was
due to physical inaccessibility of protein and starch in the large
particles, a situation improved by steam pelleting. Digestibility
was not affected by regrinding of the steam pelleted peas, because
the mean diameter of particles ground once or twice was similar
(0.434 vs. 0.507 mm, resp.) (Carré et al. 1991 {57}).
| Table 6. Effect of pelleting on energy value and digestibility of protein and starch of spring smooth seeded peas
(cv. Finale), in young and adult cockerels (means (n=7) ± standard deviations) | |||
| AMEn (MJ/kg of DM) | young | ||
| adult | |||
| starch digestibility, % | young | ||
| adult | |||
| apparent protein digestibility, % | young | ||
| adult | |||
(Adapted from Carré et al., 1991 {57})
5b. Antinutritional Factors
Legumes are known to contain antinutritional factors (ANFs) which interfere with digestive processes, thereby reducing the nutrititional value of pulse crops for monogastric animals. Potential pea ANFs include amylase, trypsin and chymotrypsin inhibitors, tannins (proanthocyanidins), phytic acid, saponins (hypocholesterolemic factors), hemagglutinins (lectins) and oligosaccharides. Fortunately, the white-flowered, spring-seeded peas grown in Canada have probably the least requirement for ANF reduction when compared to other pea varieties (Valdebouze et al. 1980{140}) and pulse crops (Bond and Smith 1989{92}). Poultry are therefore able to enjoy a relatively large proportion of peas in their diets.
Analysis of European peas indicated that the trypsin inhibitor activity (TIA) of winter cultivars was twice that of those sown in spring, and that smooth peas had higher TIA than wrinkled varieties (Valdebouze et al. 1980{140}). Other studies found that the level of TIA was not linked to seed character (round vs. wrinkled) or CP level (Griffiths 1984{129}) but was significantly affected by cultivar and environment (Bacon et al. 1991{67}). European pea cultivars contained considerable variation in trypsin and chymotrypsin inhibitor levels, with average values similar to and double those of faba beans, respectively (Griffiths 1984 {129})
Trypsin and chymotrypsin inhibitors have been presumed
responsible for reduced protein digestibility in pea diets (Pisulewski
et al. 1983{132}) because they form stable complexes with
trypsin and chymotrypsin. A review article (Huisman and Tolman
1992{39}) suggested that production of cystine-rich trypsin and
chymotrypsin may then increase, placing further stress on birds
consuming pea-based diets deficient in sulfur amino acids. However,
studies indicate (chymo)trypsin inhibitors in peas have little
practical effect on poultry performance. Even when data was pooled
between spring peas and high-TIA winter peas, improved protein
digestibility was non-significantly correlated to a reduction
in TIA (r=0.71, Conan and Carré 1989{86}; r=0.79, Carré
and Conan, 1989{94}). Pancreatic enlargement (0.21 vs. 0.18%
live body weight) was noted in broiler chicks (0-28d of age) consuming
pea-supplemented diets (200 mg kg-1), but the birds had increased
weight gain, intake and FC (P<0.05) (Huisman et al. 1990
{77}).
Tannins, or proanthocyanidins, are polyphenolic compounds
that inhibit the activity of digestive enzymes including trypsin,
-amylase and lipase (Longstaff and McNab 1991{64}). They are
found in the hull (Griffiths 1981{138}) of colored-flowered peas,
but are difficult to extract and quantify using current methodologies
(Marquardt and Blackburn 1991{196}). Studies involving dehulling
confirmed that the nutritional value of a white-flowered, spring-seeded
cultivar was not affected by tannins (Brenes et al. 1993{37}).
Recent studies indicated that the yellow and green-seeded peas
commonly grown in Canada are devoid of tannins, but that the brown-seeded
cultivars may contain appreciable amounts of this ANF (<0.1,
<0.1, and 11.5 to 41.0g condensed tannins kg-1, resp; Igbasan
et al. 1997 {3}; Brenes et al. 1993{37}). A brown-seeded
and a green-seeded cultivar had reduced TMEn, but tannins were
not thought to be the only agent reducing ME since they were absent
from the green-seeded sample (Igbasan et al. 1997 {3}).
Antinutritive effects should not be seen with any practical inclusion
level of yellow or green peas because faba bean-based diets (5g
condensed tannins kg-1) supported excellent growth rates in young
broilers (5 - 26d of age) (Jansman et al. 1993{171}).
Oligosaccharides consist of a sucrose moiety -1,4-linked
to one or more galactose subunits. Dehulled, Canadian-grown peas
contained 44.2 to 56.1g kg-1 oligosaccharides (DM basis; Reichert
& MacKenzie 1982{133}). Monogastrics do not produce the -galactosidase
enzyme required to digest oligosaccharides, and these carbohydrates
may cause digestive disturbances when microbially fermented in
the hindgut of poultry (Saini 1989{89}). However, studies found
high digestibilities of oligosaccharides in cockerels (>90%)
and chicks (>70%), and indicated the birds were capable of
absorbing the organic acids which may have resulted (Carre et
al. 1995{13}). Addition of pea oligosaccharide extract (56
and 28g kg-1) to the diets of young broilers (7-28d of age) did
not affect performance or digestibility of dietary nutrients (Trevino
et al. 1990 {72}). Studies with pea protein concentrate
indicated significant levels of endogenous -galactosidase (Brown,
1991; Weins, 1992), which may explain the apparently negligible
antinutritive effects of oligosaccharides in pea-based rations.
5biv. Lectins, Saponins and Phytic Acid
Lectins, saponins and phytic acid are ANFs that occur
in peas, but either have mild effects or receive less attention.
The cotyledons of peas contain lectins (hemagglutinins), polysaccharide-binding
agents which cause hyperregenerative villus atrophy in the small
intestine. A general survey of legumes indicated that pea lectins
had low reactivity and were non-toxic (Grant et al. 1983{130}).
Laying hens had enhanced performance when fed heated versus raw
peas, which was attributed to the inactivation of lectins. However,
a high-trypsin-inhibitor variety of peas (Maro) was used in these
diets, and the improvement in performance was more likely due
to inactivation of TIA (Davidson 1980{144}). Saponins, are ANFs
composed of sugar and steroid or triterpenoid moieties. Studies
indicated that pea saponins were less hemolytic than those from
field or soya beans, but intermediate in toxicity to guppy fish
(Khalil and El-Adawy 1994{22}). It is not known what, if any,
effects pea saponins have on poultry. Peas have been reported
to contain 22 g kg-1 phytic acid, a cyclohexane compound with
six phosphate groups (Blatny et al. 1995 {14}). Antinutritional
effects associated with this compound include mineral-complexing
and inactivation of digestive enzymes, although it is not known
what effects pea phytate levels would have on poultry.
Results from various methods of fibre determination
are shown in Table 1. Initial reports indicated a high level
of pea hull fibre digestion by cockerels (Longstaff and McNab
1987{117}) but further study showed pea hull digestion was very
low (6.1%), and did not greatly exceed the available carbohydrate
content (30.95 g kg-1) (Longstaff and McNab
1989{84}). Indigestibility of the hulls may reduce the ME (Jorgensen
et al. 1996{12}) and digestible nutrient content of whole
peas; however, if available as a low-cost byproduct of the pea
processing industry, the hulls of low-tannin peas have potential
application as a diluent in broiler-breeder rations.