This article is a free sample from the Ingredient Issues library, which contains 150 articles by nutritionist Greg Aldrich, Ph.D, published in Petfood Industry since 2004.
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This column was originally published in Petfood Industry, January 2004.
The availability of fresh poultry and rendered poultry products coincided with the commercialization and industrialization of poultry production in the 1940s and 1950s; and feed values for poultry by-product meal (PBPM) were first established in the 1950s (Fuller, 1996). The volume of rendered poultry proteins in 2003 was estimated at 3,073.5 million pounds per year and the companion animal industry consumes about 23% (Pearl 2003).
By definition, poultry by-product meal (PBPM; Section 9.10) differs from poultry meal (PM; Section 9.71) only by the inclusion of "heads, feet and entrails" (Association of American Feed Control Officials, 2003). From a nutrient composition they can be virtually identical. A proposal in 1998 to drop the "by-product" designation from poultry products and replace it with the term "protein" was rejected by AAFCO. Is there a nutritional rationale for the different names?
Whether it is poultry meal or poultry by-product meal, rendering is required for stabilization and transformation of raw unused poultry parts into a form that can be easily stored and transported. Rendering, in its simplest description, is a sterilization, dehydration and resizing process. Extensive heating (ca. 280Â°F) drives water and fat from the bone and tissue. The fat is separated by pressing and the remaining "cake" is ground in a hammer-mill to a uniform particle size.
The rendering process can have a substantial impact on nutritional quality. Murray et al. (1997) reported that in dogs fed a diet containing rendered poultry by-product (meal), protein and total amino acid digestibility at the ileum were reduced by 10.75% and 10.53%, respectively, compared to a diet containing fresh poultry by-product; however, no differences in total tract protein digestibility were detected. Amino acid digestibility of PBPM can be further reduced by high rendering temperatures (Wang, 1997).
In a study comparing protein quality of various rendered chicken parts, Aldrich and Daristotle (1998) reported that the PER value of feet and bone/cartilage were 0.87 and 1.22, respectively; whereas, heads, viscera and gizzards/livers/hearts were 2.50, 3.04 and 3.08, respectively. The protein quality of these latter parts was comparable to backs/breastplate and whole birds without feathers (2.88 and 3.43, respectively).
Based on this data, one might conclude that the level of ash (bone residue) would have the greatest impact on nutritional quality of the meal. However, ash level in PBPM (16.3% vs. 7.2%) did not affect ileal digestibility of protein or amino acids, or total tract protein digestibility in dogs, amino acid digestibility in cecectomized roosters (Johnson et al., 1998) or protein quality (PER) in chicks (Johnson and Parsons, 1997). Low ash PM (9% ash) fed to dogs at 10.4% to 32.5% of the diet did not affect protein digestibility at the ileum (Yamka et al., 2003). In this study there was a linear increase in total tract protein digestibility; however, the protein concentration at the lowest levels in the diets did not meet AAFCO minimum requirements for maintenance. For the two treatments in which maintenance levels of protein were met (25% and 32.5% of the diet) there was no difference in total tract digestibility. If not ash, then it is likely that the lower protein quality of bone/cartilage and feet is associated with high levels of connective tissue and a reduction in the ratio of essential to non-essential amino acids. Bone/cartilage can be a component of either PM or PBPM, and feet a component of PBPM; regardless, adding these components to the raw material mix would likely reduce the quality of either meal.
Very few direct comparisons between "meal" and "by-product meal" are found in the literature. In a comparison of chicken meal (CM) and chicken by-product meal (CBPM) using a chick protein efficiency ratio (PER) model, no difference was observed in protein quality (Aldrich and Daristotle, 1998). Bednar et al. (2000) fed ileally cannulated dogs diets containing either PBPM or PM and reported that ileal digestibility of protein and amino acids was not different between treatments; however, total tract protein digestibility was 6.74% lower for dogs receiving PBPM than for dogs receiving PM.
Several different grades of rendered poultry products are available. Feed grade is seldom used in petfood because it contains a higher level of ash and lower protein content. Petfood grade is lower in ash (14%). A low ash poultry and (or) poultry by-product meal containing less than 11% ash and a higher protein content (67 to 70%) is also available for a premium price.
Very few studies have been reported in the literature for dogs and none were found for cats. With the information cited, differences in the names for poultry by-product meal and poultry meal are not supported from a nutritional perspective. Differences in quality of each product appear to be more associated with the rendering process and raw material mix.