Research Summaries: Peas in Livestock Diets

Intoduction/Table of Contents

Section 2: POULTRY

II. The Use of Peas in Poultry Diets

M. D. Fleury M.Sc.

The nutrient profile of peas indicates that they are suitable for use in high-density diets such as those used in modern poultry production. However, incomplete poultry nutrient data for feed peas, combined with limited industry experience using this feedstuff for poultry, has prevented peas from replacing more traditional feed ingredients such as soybean meal. The following review is intended to outline what is currently known about the properties of western Canadian feed peas and to provide insight into ways in which peas can be most successfully incorporated into poultry diets.

1.0 Nutrient specifications of Canadian Peas for poultry

Peas contain moderate levels of crude protein (CP) and metabolizable energy (ME), making them suitable for use in the diets of all poultry classes. Peas are deficient in methionine but contain relatively high levels of lysine, which compliments the amino acid profile of locally-produced cereals and canola meal. Studies of Western Canadian pea proximate composition are summarized in Tables 1 and 2. The data agrees with that from analyses performed previously (Sosulski & Holt 1980).

1.a Moisture

Moisture analysis of 1100 pea samples by Norwest Labs for the preceding four years indicated that the average moisture content of feed peas is 12.2% with a standard deviation (SD) of 2.75 (Ken Mrazek, 2004 Norwest Labs).

1.b Crude Protein

Western Canadian studies have measured average feed pea crude protein contents between 23 and 24% (dry matter (DM)-basis, Table 1). Analysis of 1100 samples compounded over four years from throughout western Canada by Norwest Labs confirmed a feed pea crude protein level of 23.9% (DM-basis, SD= 3.17). Samples of a single pea variety ranged from 14.5 - 28.5% CP (dry, dehulled basis), indicating that environment has a large impact on pea protein variability (Reichert and MacKenzie 1982).

Several observations emerged from a study of yellow- (n=8), green- (n=2) and brown- (n=2) seeded pea cultivars grown in Manitoba (Igbasan et al. 1997). The peas had a broad range of protein levels (20.8 - 26.4% CP DM-basis), but this was not correlated to seed size or colour. Seed coat colour was also not related to amino acid concentrations. The concentration of most essential amino acids, including lysine, methionine and threonine, did not increase proportionately with CP level (Igbasan et al. 1997); supporting the concept that attempts to genetically select for increased protein content in peas would likely result in an erosion of protein quality (Gatel 1994).

Pea CP levels are determined on the basis of nitrogen content. Non-protein nitrogen was present in the seeds, which reduced the true nitrogen-to-protein conversion factor (5.25) from that normally used (6.25). However, it is recommended that all CP calculations continue to use the 6.25 factor, lest peas appear at a disadvantage to the similarly inaccurate protein calculations of other feed ingredients (Sosulski and Holt 1980; Mosse 1990).

1.b.i Protein digestibility

The constituent proteins of feed peas (ie: globulins, albumin, insolubles) and their sub-fractions are highly variable in terms of their digestiblities. Protein structure including hydrophobicity, glycosylation, beta-sheets, compact tertiary structure and disulphide bonds can have a negative impact on protein hydrolysis (Crevieu-Gabriel 1999). Although vicilin is one such protein in peas, unheated vicilin was far more digestible than equivalent proteins in dry beans or soybean (Deshpande and Damodaran 1989; Nielsen et al. 1988). However, the pea protein fractions that were less susceptible to digestion at the terminal ileum of 3-week-old broilers represented only a small proportion of total dietary protein (Crevieu et al. 1997).

Apparent protein digestibility (APD) differed significantly (P≤0.05) for a yellow-, a green- and a brown-seeded cultivar (76.7, 71.5 and 60.1%, resp.; Igbasan and Guenter 1996) when measured using 10-day-old male broilers. The APD of Trapper peas was 81% in 17-day-old broilers (Brenes et al. 1993). These APD values are reasonably close to the apparent amino acid digestibility values shown for 4-wk-old broilers (Table 2). Apparent protein digestibility for colostomized laying hens was 78.5% (Gruhn and Zander 1990). Chicken and porcine pepsinogen activities differ significantly, depending upon pH, incubation time and type of protein analysed; therefore, in vitro assessment of pea protein digestibility should approximate poultry gastric conditions (Crevieu-Gabriel et al. 1999).

1.c Fibre

High fibre levels in feedstuffs are not desirable because this component is not well digested or utilized by poultry. Fibre levels for western Canadian feed peas measured by Norwest Labs in 1100 samples spanning a four-year period were: crude fibre - 58 g kg^-1 DM; acid detergent fibre - 103 g kg^-1 DM; and neutral detergent fibre - 180 g kg^-1 DM. A European study (37 samples; Grosjean et al. 1999) determined that crude fibre was the only measured fibre component that had a significantly negative correlation (P<0.01) with AME in mash diets. Fibre components measured in pelleted diets did not have a significantly negative correlation with AME. Conversely, a high crude fibre level (99 g kg^-1) was implicated in the reduced AMEn content of peas for birds in-lay (2525 kcal kg^-1, as fed; Askbrant and Hakansson 1984).

1.d Starch

Starch is the main storage polysaccharide in feed peas. It exists in two primary forms: amylose and the more readily digestible amylopectin (Grosjean et al. 1999). The amylose content of smooth-seeded peas such as those grown in Canada is roughly 40% of total starch (Lloyd et al. 1996). Wrinkled peas, harvested green for the human food trade, have a mutation reducing the content of amylopectin and total starch, making them less suitable as an energy source for monogastric feeds (Grosjean et al.1999).

Starch values from twelve Canadian pea cultivars averaged 415 g kg^-1 ± 16.8 (mean ± SD; DM basis; Igbasan et al. 1997). This is lower than the average starch values (DM-basis) of 459 g kg^-1 (Conan and Carre 1989), 453 g kg^-1 (Cerioli et al. 1998) and 501 g kg^-1 (Grosjean et al. 1999) measured in extensive European surveys.

1.d.i Starch digestibility

Starch digestibility in peas is affected by factors that limit its accessibility. The structure of pea starch granules may be partially responsible, as indicated by the significant differences between the digestibilities of purified starch granules from high-amylose peas, regular peas and maize (75.2, 94.4 and 98.8%; P<0.05, respectively). In addition, the digestibility of starch granules was lowest in the largest particles of ground peas, suggesting that the matrix surrounding the starch granules restricted their accessibility. Total tract pea starch digestibility in 3-wk-old broilers was 95.7% in pea particles below 100 µm in diameter, and 84.4% for pea particles greater than 100 µm in diameter (Carre et al. 1998).

Human nutritionists have examined the relatively slow rate of starch breakdown and glucose absorption from peas because it moderated the peak in post-meal insulin production. This reduced glycemic effect was emphasized with the high amylose/amylopectin ratio pea genotypes (Skrabaja et al. 1999). The corollary was also examined in poultry nutrition. A broiler trial compared diets in which starch was degraded rapidly and absorbed from the jejunum (ie: corn, tapioca, rice) versus diets in which a proportion of the starch was also degraded in the ileum (ie: peas). Recent research indicated that broilers consuming diets which contained a minimum quantity of slowly degraded starch, specifically that available from peas (Weurding et al. 2001), had improved growth rates and feed conversion relative to isocaloric control diets containing rapidly-degraded starch (Weurding et al. 2003). These birds had an improved response to elevated lysine levels (Weurding et al. 2003b). It was suggested that this was due to the relatively continuous glucose supply: 1) sparing the catabolism of gluconeogenic amino acids, 2) providing a direct source of energy for posterior sections of the gut, 3) stimulating prolonged insulin production and therefore protein deposition, and 4) reducing the energetically inefficient conversions between glucose and its storage forms when glucose absorption spikes following a meal of rapidly-degraded starch. An in vitro method to predict starch digestion rate has been developed (Weurding et al. 2001b). This is an exciting area of research and may advance the goal of formulating diets to meet animal requirements at the metabolic level.

1.e Minerals

A comprehensive study of 12 pea cultivars was performed in Alberta (Jaikaran et al. 1995), the mineral values from which are shown in Table 1. A similar range in values was seen in four samples of a single cultivar of peas (cultivar Trapper) ranging from 14.5 - 28.5% CP (dehulled, DM-basis; Reichert and MacKenzie 1982 in Table 1), indicating that both cultivar and environment affect the mineral content of feed peas. Mineral analyses performed by Norwest Labs on 1100 samples over 4 years concurred with values from the previous authors (Table 1).

1.f Energy values

The energy value of peas for poultry changes drastically in response to processing, therefore the energy value used for feed formulation should be matched to the processing conditions intended for the diet. The average AME values for eleven feed peas ground (3000 rpm. 3-mm screen) and included in either mash or cold-pelleted diets of Isa Brown cockerels were 2851 and 3150 kcal kg^-1 (DM-basis, SD= 123 and 72, respectively; Grosjean et al. 1999). In addition, Carre et al. (1991) determined that steam-pelleting (4x30mm die, flow rate 2.85 kg/min, inlet 75°C / outlet 76°C) pea-based diets had a strong positive effect on apparent metabolizable energy (AMEn; P<0.001) in both adult (3069 vs. 2813 kcal kg^-1 DM) and 19-day-old (3016 vs. 2762 kcal kg^-1) broilers, when compared to mash (2-mm hammer mill sieve) diets. Brenes et al. (1993) measured significantly lower pea AMEn values in mash diets with 2-week-old broilers (2189 kcal kg^-1, assume as-fed basis).

The true metabolizable energy (TMEn) values of yellow-, green- and brown-seeded peas (n=12) administered as the sole ingredient to adult leghorn cockerels averaged 2916 kcal kg^-1 (Igbasan et al. 1997) which is in agreement with other values published for peas (Farhoomand and Pirmohammad 2002).

The AME of feed peas (500 g kg^-1) for non-laying adult hens was 3060 kcal kg^-1 DM (Perez-Maldonado et al. 1999). The AMEn from a yellow-, a green- and a brown-seeded pea cultivar (2420, 2460 and 1980 kcal kg^-1, respectivley) was determined at 50% dietary inclusion using 10-day-old male broiler chicks (Igbasan and Guenter 1996). The TMEn values of the same pea samples administered as the sole ingredient to adult leghorn cockerels were 2796, 3011 and 2629 kcal kg^-1, respectively (Igbasan et al. 1997). On the basis of this data it appears that high dietary levels of peas are better utilized by older birds; however details regarding sample preparation (ie: ground vs. whole) for older birds, and regarding the format in which energy levels were reported (ie: DM basis vs. as-fed) were not reported. Conversely, other researchers reported that bird age did not appear to affect the energy value of peas. Carre et al. (1991) reported that a single sample of peas had similar mean AMEn and standard deviation (SD) values for adult and 19-day-old birds in either mash (2762 vs. 2813 kcal kg^-1 DM) or steam-pelleted (3069 vs. 3016 kcal kg^-1 DM) diets.

The apparent metabolizable energy (AMEn) of pea flour "chips" from a yellow and a green-seeded pea cultivar (2748 and 2696 kcal kg^-1, respectively) was determined at 450 kcal kg^-1 dietary inclusion using 10-day-old male broiler chicks (Igbasan and Guenter 1996).

1.g Amino acids in feed peas

The amino acid content determined for feed peas by various researchers is shown in Table 2. Feed peas are an excellent source of lysine, but contain lower levels of the sulphur amino acids, methionine and cystine. The digestibilities of feed pea amino acids, as measured by several researchers, are also shown in Table 2. Igbasan et al. (1997) measured true amino acid availabilities for yellow- and green-seeded pea samples (n=10), and values ranged from 78.3% for total sulphur amino acids to 90.5% for glutamic acid. Brown-seeded cultivars had average lysine, threonine and total sulphur amino acid availability values of 71.9, 72.4 and 64.0%, respectively, indicating elevated tannin levels may have impaired digestibility. Amino acid content (Table 2) was determined for 12 cultivars of peas and amino acid prediction equations were developed (Table 3) (Jaikaran et al. 1995). Values from a European study of true and apparent ileal amino acid digestibilities (Table 2) are similar to the others listed, with the exception of valine and histidine. However, it was indicated that these ileal analyses were difficult to perform without overestimating true digestibility (Perez et al. 1993).

Mossé (1990 in Gatel 1990) indicated that a linear relationship existed between CP and amino acid levels, with methionine, cystine and tryptophan accumulating slowly in comparison to nitrogen-rich residues like asparagine and glutamine (Huet et al. 1987). Regression equations used for the prediction of some limiting essential amino acid values are shown below (Table 3). Alternatively, near-infrared reflectance spectroscopy (NIRS) calibrations developed by Degussa (Fontaine et al. 2001) were used to accurately measure the levels of most amino acids in peas

^a (AA, g kg-1 DM) = a * (CP, g kg-1 DM) + b
^b (AA, % of DM) = a * (CP, % of DM) + b