6. Improving the Nutritional Value of Peas
Peas are a high-quality feedstuff for use in poultry
diets, supporting excellent performance and efficiency in all
types of birds. In the relentless effort to enhance production,
workers have attempted to improve the nutritive value of peas
for poultry through a variety of techniques.
Several forms of heat treatment have been documented to effectively eliminate many pea ANFs. Cooking at 100 C destroyed antitrypsin activity (Savage and Deo 1991{65}), and autoclaving at 130 C and 170 kpa for 3 minutes significantly reduced TIA levels (Conan and Carre 1989{86}) in peas. Tannins, lectins and trypsin inhibitors in round-seeded spring peas were virtually eliminated at minimum extrusion conditions of 105 C and 20.3% moisture (Poel et al. 1992{51}). However, dry heat ( 100 C, 24 h) was ineffective as a means of reducing (chymo)trypsin inhibitor activity in peas (Griffiths 1984{129}).
The practical poultry effects of heat treatment were
less encouraging. The TMEn of autoclaved peas (121 C, 15 psi,
30 min) was lower than that of heated peas (121 C, 30 min), and
cooking (simmered, 20 min) caused a non-significant reduction
in starch digestibility due to retrograde starch formation (Longstaff
and McNab 1987{84}). In addition, the vicilin storage protein
of peas became more resistant to in vitro digestion upon
heating, possibly due to changes in secondary and quaternary structure
(Deshpande and Damodaran 1989{96}). Neither AMEn or protein digestibility
were improved by autoclaving low-tannin Trapper peas, although
a significant improvement was seen when the treatment was duplicated
for high-tannin Maple peas (Brenes et al. 1993 {37}).
Micronization (110-115 C for 55 s) improved AMEn, APD and starch
digestibility relative to untreated peas fed to broilers (3-12
d of age; Igbasan and Guenter, 1996{10}); however, the peas were
tempered prior to micronization and then immediately roll flaked
, and this additional processing may have affected nutrient accessibility.
Grinding is the process that has the single greatest
effect on the nutrient value of peas. Relative to whole peas,
fine grinding (<1mm diam.) significantly increased (P<0.001)
the starch digestibility (75.6 vs. 88.1%) and TMEn (9.91 vs. 11.38
kJ g-1) in adult white leghorn cockerels; however, further improvements
were not seen when peas were heated, autoclaved or dehulled prior
to grinding (Longstaff and McNab,
1987{84}). The effect of ground faba bean particle size (0.5
vs 0.16mm mean diameter) was examined in 3-week-old broilers.
Reduced particle size enhanced starch digestibility (P<0.01)
and AMEn (P<0.05), but did not affect APD (Lacassagne et
al. 1991{213}). It is fortunate that pelleting further enhances
the nutritive value of ground peas because the time and expense
associated with fine grinding of feedstuffs could not be afforded
by industry.
Steam pelleting has been shown to improve the utilization of peas by poultry, an effect apparently due to increased nutrient accessibility rather than ANF destruction. The AMEn of peas was enhanced by steam-pelleting the complete ration, primarily through increases in pea protein and starch digestibility (1.8 - 4.6, 3.5, 5.4%, resp.). Pea starch digestibility in these diets was high (91-97%) and did not improve when the diet was repelleted (Carré et al. 1987{118}).
Gelatinization may improve the digestion of starch by poultry. Steam pelleting (80 C) was comparable to extrusion in terms of increasing starch gelatinization relative to that of untreated pea-based diets (38.6, 40.9 and 18.5%, resp.). Pelleting and extrusion produced similar improvements in weight gain and FC (P<0.05) and increased the AMEn of the diets relative to the control (3087, 3028 and 2877 kcal kg-1, resp.). However, the rations contained equal amounts of peas and whole canola seed (364.5g kg-1 each), and the treatments may have enhanced the nutritional value of both ingredients (Fasina et al. 1997{304}). Even at lower steam pelleting temperatures (65 C, 1/8" grind), significant starch gelatinization occurred relative to the unpelleted control (30.1 vs 15.9%; Fasina and Campbell 1997{305}).
Pelleting improved starch digestibility for both
young and mature birds and reduced AMEn variability so that the
average value was 3000 kcal kg-1 DM for both groups. Steam pelleting
improved the digestibility of spring pea protein without reducing
the level of TIA (3.47 vs. 2.98mg TIA/g peas) (Carré et
al. 1991{57}). Pelleted pea rations also have excellent pellet
quality (Leigh Campbell, personal communication).
Dehulling improves the nutrient content of peas because
it removes the indigestible, high-fibre hull and the ANFs it contains.
Studies with 10- to 17-day-old broilers indicated the dehulling
did not affect the AMEn or APD of low-tannin peas (Brenes et
al. 1993{37}) and this was supported by performance trials
(Igbasan and Guenter 1996{10}). Conversely, dehulled cotyledons
had improved starch digestibility and TMEn for adult cockerels
(Longstaff and McNab 1987{84}).
Peas generally contain low levels of ANFs, and therefore
require little genetic selection against these traits (Monti 1983{131}).
Two European varieties, Maro and Progreta, are high in trypsin
inhibitor activity; therefore, feed peas for monogastrics should
not contain this parentage (Bond and Smith 1989{92}). High tannin
levels are pleiotropic with colored petals in peas (Bond and Smith
1989{92}); however, most varieties grown in Canada are low-tannin,
white-flowered varieties.
Attempts to improve the nutritive value of peas through
enzyme supplementation have been largely unsuccessful. Broiler
and leghorn chicks tended to exhibit reduced weight gain and feed
intake when crude enzyme preparations were added to low-tannin
pea diets, but the same enzymes improved the FC of rations containing
high-tannin peas (Brenes et al. 1993{37}). Water extracts
from high-tannin peas have been shown to inhibit trypsin, -amylase
and fungal cellulase (Griffiths 1981{138}); therefore, selective
binding of ANFs to the supplemental enzyme preparations may have
contributed to improved performance. A commercial cellulase preparation
imcreased the digestibility of xylose from peas, but did not alter
significantly the TME value (Longstaff and McNab,1987{84}). Some
foreign studies (Jeroch et al. 1995{17}; Keller and Jeroch
1997{1}) and promotional materials (Charlton and Pugh 1995{16})
indicated an improvement in ME from enzyme supplementation of
pea-based diets. Peas showed a significantly greater AMEn value
(0.55 MJ kg-1) when mixed with corn versus wheat (Carré
et al. 1987{118}). Although adult cockerels were used
in this study, the difference in ME may indicate that the viscous
polysaccharides in wheat were reducing the nutrient availability
of the peas. In conclusion, peas themselves do not require enzyme
supplementation, but may benefit from the standard industry practice
of adding viscosity-reducing enzymes to wheat and barley diets.