Pea seeds are a dual purpose ingredient. They provide half as much as crude protein as soybean meal with a lower rumen undegradable protein content, with the high energy value of a starch-rich feed (Schroeder, 2002). It represents one of the best quality and least expensive feeds, its low cost partly reflecting the low cost of transportation (Anderson et al., 2002).
Peas have a low starch degradation rate (from 4 to 6%/hour) that is much slower than that of cereals, such as barley (21 to 34%/h). This low rate decreases the risk of acidosis with high starch diets, as the pH lowest point is higher than that observed with barley when fed at a level of 70% with oaten hay (Valentine et al., 1987).
Pea seeds are highly palatable to ruminants (Hutson et al., 1981).
The in sacco nitrogen degradability of peas is very high. This is due to the large amount of highly soluble proteins but it is also an artifact of the in sacco method: grinding peas produces many very fine particles that pass through the pores of the nylon bag and are considered to be fermented in the rumen, which is not the case (Maaroufi, 2001). When peas are included in a diet with a limited fermentable organic matter, the fraction washed out from the bag is supposed to be lost and rejected as urinary nitrogen, which leads to an underestimation of the amount of protein available in the intestine (Cabon et al., 1997).
Nevertheless, the ammonia concentration in the rumen increases with the substitution of soybean meal and maize grain by peas (Vander Pol et al., 2009). Peas are an acceptable source of protein when properly balanced in the diet with by-pass protein sources. The nitrogen of peas is highly soluble (40%) (Aguilera et al., 1992). Grain processing methods substantially affect solubilisation and fermentation (Azarfar et al., 2007). The partition between the soluble and degradable fractions for both DM and N varies a lot between cultivars, for example from 58% to 85% for soluble N (Cabon et al., 1995).
Effects of processing
Increasing the fineness of grinding peas (from 6 mm to 0.8 mm sieve) increased their N degradability in the rumen by 12.3 percentage points (Michalet-Doreau et al., 1991). When grinding pea seeds, the chemical and physical characteristics of the differently sized fractions reflect the respective comminution laws of the hulls and the kernels, leading to a physical separation of the botanical constituents of the pea seed, with coarse fractions containing hulls and most of the parietal constituents, finer fractions containing kernels and cellular constituents, and the finest fraction being mainly composed of starch granules (Maaroufi et al., 2000a). These different fractions have quite different fermentation patterns (Maaroufi et al., 2009). Grinding pea seeds increases their fermentability, resulting in a decrease in the post-prandial in vivo pH (Giger-Reverdin et al., 2000) due to a higher concentration in soluble carbohydrates (Maaroufi et al., 2000b).
Autoclaving peas decreased both the soluble fraction and the fractional rate of protein degradation of the slowly degraded fractions, strongly reducing effective N degradability (Aguilera et al., 1992; Goelema et al., 1999). Toasting decreased both total protein digestibility and intestinal digestibility of rumen undegraded protein. These results can also be explained by the particle size reduction due to processing. However, toasting does not affect rumen undegraded starch or total starch digestibility (Goelema et al., 1999).
Extrusion increases the washable protein fraction of the pea (Goelema et al., 1999) and decreases rumen protein degradability (Aufrère et al., 2001; Walhain et al., 1992), but increases the rate of disappearance of starch in the rumen (Thewis et al., 1992; Walhain et al., 1992). However, a large compensation in N digestion occurred in the intestine for extruded blends of pea and full-fat rapeseed so that organic matter digestibility of concentrates was only slightly decreased by extrusion. Protein protection was not significantly improved when extrusion temperature rose above 140°C (Chapoutot et al., 1997). Extrusion did not improve the essential amino acid profile of extruded peas in the undegradable rumen fraction (Walhain et al., 1992).
Pelleting increased the washable protein fraction and the fractional degradation rate, thus decreasing the amount of rumen undegraded protein. This is due to the particle size reduction occurring during processing (Goelema et al., 1999).
Young dairy cattle
Peas can be included at up to 40 to 50% DM in concentrates fed to pre-weaned and weaned dairy calves. It can partly replace maize grain, barley and/or soybean meal (Schroeder, 2002). Peas may be the sole protein source for dairy heifers (Anderson et al., 2002).
Peas are palatable to dairy cows (Weiss, 1992). Replacing barley by peas increased DM intake and subsequently the yields of milk, fat and protein (Valentine et al., 1987). Peas can be included at up to 25% in concentrates for lactating cows (Anderson et al., 2002). Peas have been successfully substituted for soybean meal in cows producing up to 30 kg milk/d (Hoden et al., 1992), for soybean/canola meal (Corbett et al., 1995) and for soybean meal and barley in late lactation (Khorasani et al., 2001). At higher levels of production, they should be supplemented with better protein sources, especially some with higher concentrations of sulphur-containing amino acids. The substitution of soybean meal and maize grain with field peas in dairy cow diets at a 15% inclusion rate does not modify the organoleptic characteristics of milk (Vander Pol et al., 2008).
Peas have to be coarsely ground for dairy cow diets to avoid depression in total tract digestibility of nutrients (Vander Pol et al., 2009). Raw and extruded peas fed at 20% of the diet to cows in early lactation resulted in similar DM intake, milk yield and milk composition, though extrusion increased the rumen degradability of starch. Milk yield and composition were also similar to that of the soybean meal-based control diet (Petit et al., 1997).
As peas are very palatable, they are best used in diets where nutrient density and palatability are important, such as creep feeds where its optimum inclusion rate lies between 33 and 67% (Anderson et al., 2002). Peas can replace a mixture of barley and canola meal for young calves (Schroeder, 2002). They can also be used as an ingredient in creep feed to increase calf weight gain without impairing rumen fermentation and digestion (Gelvin et al., 2004).
For growing steers and heifers, peas should not constitute more than 25% of the total diet to prevent excessive protein intake (Anderson et al., 2002). It is a suitable supplement to maize silage for growing cattle (Gatel, 1995). At a level of 20% in a total mixed ration fed ad libitum, replacement of a maize grain and rapeseed meal diet by field peas increased DM intake without impairing fermentation characteristics: pH and NH3 concentration remained unchanged, and total volatile fatty acids and acetate concentration decreased. Nevertheless, average daily gain increased due to the increase in DMI (Gilbery et al., 2007). These results indicate that field peas can be included successfully into rations at levels up to 36% (diet DM) without negatively affecting growth and most carcass characteristics of finishing beef cattle (though effects on marbling score were variable). These data also indicate that the energy content of field peas is similar to that of cereal grains, such as maize and barley, when included in high-concentrate finishing diets (Lardy et al., 2009).
Feeding peas to steers results in growth and carcass characteristics similar to that obtained with dry-rolled maize, and improves objective and subjective tenderness, overall desirability and flavour of beef. Field peas could be fed to cattle and give positive attributes to the quality of the meat up to 30% inclusion in the diet (Jenkins et al., 2011).
For finishing lambs, peas have a similar energy value to maize (Loe et al., 2004). They were included at a level of 45% in a feedlot diet and replaced all soybean meal and part of the maize. Growth, carcass fatness, meat pH, colour parameters and cooking losses were comparable among groups (Lardy et al., 2002). The replacement of soybean meal with peas did not significantly affect growth and slaughter performances, and preserved meat quality (Lanza et al., 2003). The use of pea seeds increases the proportions of total n-3 fatty acids, and meat from lambs fed peas showed a more favourable n-6:n-3 ratio in the intramuscular fatty acid composition (Scerra et al., 2011).
Dairy goats in mid-lactation cope well with a total mixed ration including field peas at a level of 23% even though the values for protein digestible in the intestine seem to be underestimated (Maaroufi, 2001).