How valid are the National Research Council nutrient requirement estimates for poultry?

The National Research Council (NRC) is the last surviving independent organization to publish nutrient requirement data for farm, pet and lab animal species.

The published values are often accepted as ‘standards’ especially in developing countries, and by government organizations charged with establishing guidelines for legislation. The question often asked is “are NRC values of commercial relevance today?” and if not, “what is the basis for such discrepancies?”

The National Research Council operates under the mandate of the National Academy of Sciences based in Washington, D.C. Over the last 50 years, the NRC has itself formed subcommittees in order to define the nutrient requirements of various animal species including most of the important farmed animals together with needs for laboratory animals, cats and dogs. Over the last 20 years, the Poultry Subcommittee has published recommendations on nutrient requirements each 7-10 years (1977, 1984, 1994). In Europe, the now defunct Agricultural Research Council likewise published nutrient recommendations for farm animals, again every 10 years or so.

The mandate of the NRC species subcommittees is to provide unbiased reviews and recommendations regarding the nutrient requirements of the various animal species. The last NRC Poultry Subcommittee was established in 1990, the outcome of which was the 9th Revised Edition of Nutrient Requirements of Poultry published in 1994. Within this publication are nutrient requirements for egg-layers, breeders, broilers, turkeys, pheasant and waterfowl. In establishing any nutrient requirement value, the committee members are given one simple, albeit very restrictive directive, and that is to base such values only on data published in referred journals. The idea behind this mandate is to prevent the use of potentially biased commercial data. This directive is particularly restrictive to estimating certain nutrient needs, since there has been a lack of scientific research and publication on many topics over the last 40 years.

This situation dictates the reliance on very dated literature estimates of certain nutrient needs. On the other hand, everyone recognizes the increase in growth rate of broilers and turkeys that has occurred over the last 40 years, and the increased egg output of modern layer strains. For this reason, the NRC estimates are often criticized as not representing the needs of modern strains of commercial poultry.

Much of the older data are based on research studies involving purified or semi-purified diets. In fact, the NRC (1994) publication has a section solely related to description of reference diets used in classical requirement studies. These diets often contain isolated soybean protein or casein as a source of protein and amino acids, and dextrose, starch and sucrose as a source of energy. Cellulose was often used as non-nutritive filler in these purified diets. Such diets are highly digestible, and are not encumbered with facets of variable nutrient availability, yet can be criticized as not being of relevance to commercial feeding. It is very difficult to crumble/pellet diets with these purified ingredients, and today mash diets are obviously of little relevance to the broiler and turkey industries.

Although the foregoing discussion highlights inadequacies in NRC published values, NRC still provides the best unbiased assessment. The reader has to be aware of the relevant potential confounding factors, and adjust actual feeding levels accordingly. Just as no two commercial broiler starter diets will ever be identical, formulating all nutrients within a diet solely to NRC (1984) levels shows a lack of understanding of nutrition and feed management.

Another factor of relevance today in establishing diet nutrient specifications is the trend towards specialization in nutrition. Certainly, many broiler nutritionists will never formulate diets for laying hens, while turkey nutritionists have little interest in specifications for egg laying stock. Our degree of specialization leads us to source more specific material, while NRC provides a general overview of all species. In research, specialization tends to be even more extreme, where researchers in amino acid metabolism, for example, at best pay cursory attention to trace mineral levels in their diets. In fact the complexity of research today dictates that we can answer questions on at best a very limited number of nutrients at any one time.

While the requirement tables were eventually published in 1994, the information base for the latest Nutrient Requirements of Poultry were collated in 1991, making the ‘current’ information now some 14 years old. Fourteen years is a long time in terms of productive performance of layers, broilers and turkeys. Obviously growth rate and egg mass output have increased in this period, and for some traits this is in the order of +20%. All research becomes ‘dated’ quite quickly, relative to the ongoing changes in genetic gain, and this situation must be a consideration in reviewing historical dated information.

Another major factor to consider in reviewing NRC (1994) data is the assessment criteria used to establish various nutrient requirements. For broiler chickens, virtually all nutrients are assessed in terms of optimizing growth rate, while for layers, the measurement criteria is simply egg production and egg weight. Over the last 20 years, commercial goals have evolved, and these impinge on nutrient needs and feeding programs. For the broiler chicken, the needs for lysine now relate to not merely growth and feed utilization, but also breast meat yield and carcass quality per se. Broiler chickens today are marketed over a vast range of weights/ages and in some instances these may be as mixed-sex or separate sex flocks. Most broiler genetic companies also have stock with different growth and carcass characteristics. In the future, we may also produce broilers with enhanced meat nutrient profile relative to human nutrition. Yet another major change has been the move to controlled environment housing of broilers, which itself impinges on the birds’ nutrient needs and growth potential. Of late, there has been the impetus to consider manure loading of nutrients during formulation of most poultry diets.

An interesting scenario has occurred with broilers since 1994, and that highlights the importance of continual need for reappraisal of feeding systems. In the mid-1990s, metabolic disorders such as ascites, sudden death syndrome, and leg disorders together accounted for 3-5% mortality in male broilers. In order to counteract such problems, it was common to feed lower energy and/or lower nutrient dense diets, at least for part of the grow-out period. Today such disorders are much less problematic, due to genetic selection, and consequently there is little need for any period of under nutrition. Consequently over a 15 year period we have gone from a situation of selecting nutrients for maximum growth followed by a 5-6 year period of consideration for tempering growth, back to today’s goal of maximum growth rate.

For egg production we no longer have the luxury of formulating solely for egg numbers per se, which is the basis for much of the NRC published nutrient values. There is now interest in egg composition, both in terms of nutrient profile as it impacts human nutrition, as well as component/solid yield for egg processing. There has always been concern about optimizing egg shell quality, and this becomes more critical today with white egg strains capable of producing 330 eggs in 365 days within reasonably large commercial flocks. The current trend of maintaining layers at 26- 28oC in modern housing systems imposes a fairly predictable limit to feed intake, and so allows for greater precision in selection of diet nutrient levels.

These evolving on-farm conditions, together with advances in feed processing, mean that nutritionists cannot expect that single nutrient values, whatever the source, will be applicable to feeding birds under all farm conditions.

To this point, this paper has focused on the validity of NRC (1994) values for the major nutrients. We invariably question these nutrient values most frequently, since they are the most expensive nutrients, and usually have greatest impact on performance and there is continuous release of ‘new’ information. Over the last few years, we have been re-evaluating trace mineral needs of poultry, and in this instance NRC (1994) is essentially the only reference. There has been a dearth of information on requirements for trace minerals, essentially due to the fact that they usually contribute less than 0.5% to the cost of a diet. There has been little trace mineral research conducted in the last 10 years, and even within the NRC (1994) publication, many trace mineral requirement values are based on quite dated research. Most commercial diets provide 2-5× the level of trace minerals relative to NRC (1994) values. While NRC (1994) values are often referred to as minimal values, for other nutrients such as amino acids, we do not usually provide such 2-5× levels of ‘insurance’ as occurs with the trace minerals (and vitamins). One reason for concern with trace minerals is consistency of quality and availability of the mineral within inorganic salts.

We have previously reported on the success in using drastically lower levels of trace minerals (80% less in the diet) provided by Bioplex^{^TM} proteinates of consistently high and predictable bioavailability. The impetus for this work was reduction in manure loading of minerals, especially zinc and copper (Leeson, 2003).

Broilers grew at comparable rates, while excreting 38% less zinc and 20% less copper in the manure. We have confirmed these results in a subsequent study, in which broilers were fed inorganic trace minerals or just 7% of the same level of minerals as a Bioplex^TM source, and again observed comparable performance. We have subsequently conducted a trial with laying hens, again using Bioplex^TM vs inorganic sources of trace minerals (Table 1). In this 32 week study, layers were fed conventional inorganic trace minerals, just 20% of these levels as Bioplex^TM minerals, or diets totally devoid of trace minerals. Birds fed inorganics or Bioplex^TM minerals performed the same, while those fed diets devoid of minerals produced slightly fewer eggs with reduced egg size. Layers fed Bioplex^TM or no trace minerals produced manure with identical levels of Zn, Mn and Cu. In both cases, the levels of zinc in manure were reduced by 67% while for manganese and copper, manure output was reduced by 80% and 10%, respectively.

^{Table 1. Effect of trace mineral source on egg production (28-60 weeks age).}

_{To enlarge the image, click here}
_{NS, no significant difference.
1- 28 day period.
2- 20% of inorganic mineral level.}

These recent findings with trace minerals suggest that the ‘low’ values published by NRC (1994) are probably more appropriate today than they were at time of publication, when environmental issues were not being considered.

The NRC (1994) values for nutrient requirement are a sound starting point for formulation of commercial poultry diets. A single source of requirement values is unlikely to be considered by any nutritionist, and this situation applies to NRC values. Likewise no one set of standards can be applicable to the array of commercial situations that arise during feeding, and it is evident that all requirement values must be periodically scrutinized so as to accommodate improved genetic potential of birds. Nutrient needs also must be reassessed as our end-point goals change, and this is exemplified with impending legislation in metal accumulation in manure.

References

Leeson, S. 2003. A new look at trace mineral nutrition of poultry. In: Nutritional Biotechnology in the Feed and Food Industries. (T.P. Lyons and K.A. Jacques, eds) Nottingham Univ. Press, Notts, UK.

National Research Council. 1994. Nutrient Requirements of Poultry. 9th Rev. Ed. NAS-NRC, Washington, D.C.

Author: STEVEN LEESON
Animal & Poultry Science, University of Guelph, Ontario, Canada