Vitamin Supplementation in Broiler Feeds and U.S. Survey on Fortification Rates

Abstract

This chapter covers a short review of the vitamin discovery period, followed by a discussion of the vitamins as nutritional supplements for poultry diets. These organic molecules perform within a complex metabolic system, and function in catalytic, developmental, and protective roles. Research in recent years suggests vitamins also play a pivotal role in the intestinal microbiome and “gut health” and may have direct effects on the establishment of a more desirable microbial population. Rapid changes in poultry genetics requires modifications in fortification rates, especially when less feed is required to attain these improvements. A survey on the vitamin fortification rates of broiler feeds in the U.S. is also included for discussion and comparison with a similar 1993 survey and the National Research Council. Some vitamins showed a wider disparity in fortification levels than others.

1. Introduction

Vitamins play a decisive role in the nutrition and health of all animals, including poultry. The requirements for vitamins and nutrients in general by poultry are affected variably by progressive genetics, disease, levels of production, and production systems. Production within an indoors environment places greater scrutiny on vitamin fortification levels in formulated feeds.

Many factors influence vitamin content of feed ingredients [1]. Whereas the level, stability, and bioavailability of vitamins in ingredients are highly variable and too low to meet requirements, commercial forms of vitamins offer greater stability, efficacy, and accuracy in dietary supplementation rates. Bioavailability and mixing characteristics can be maximized in synthetic forms that can be added to feeds in very small amounts (micrograms per tonne feed in some cases). Today, 12 individual vitamins are typically added to poultry feeds which provides nutritionists the opportunity to supplement each according to target.

As research reveals new information on the benefits of vitamin supplementation, this can impact vitamin fortification objectives. Studies on the effect of niacin on body phosphorus accumulation [2], or on the effect of the fat-soluble vitamins on carcass and meat quality [3], rooster fertility [4], and disease resistance [5], for example, can influence fortification rates in commercial feeds.

In 2017–2018, an unprecedented shortage in global vitamin supplies forced many nutritionists to scrutinize fortification guidelines as a means to control spiking costs and scarce supplies, a practice that most likely has affected today’s fortification rates.

2. Brief perspective of the golden age of vitamins

The history of vitamin research and discovery is certainly some of the more remarkable in science. The twentieth century highlights the period during which the vitamins were first officially recognized as being essential in nutrition [6, 7]. Leading up to the early 1900s, scientists were cognizant of the fact that a diet composed of carbohydrate, fat, protein, and salts was incapable of preventing certain disease-like maladies. In a speech known as the “vitamin theory”, Hopkins in 1906 noted that something in “astonishingly small amounts” was needed in animal diets beyond these basic dietary components [8].

Later, in 1912, Casimir Funk coined these yet-to-be-identified constituents as “vitamines” for “vital amines”, a term changed to “vitamins” in recognition that these were not all amines [8]. Names such as Eijkman, Funk, Stepp, and Hopkins—and many more—are popular references to the early investigations and observations that laid the foundation in “vitamin science” of the 1930s and 1940s—a period considered as the ‘Golden Age of Vitamins’ [7, 8]. The discovery of 13 vitamin groups covers only a few decades, starting with the first vitamin in 1913 (“factor A” that was later renamed vitamin A) and culminating in 1948 when vitamin B₁₂ was isolated and defined.

Yet, in his exhaustive review, McDowell [7] points out that some of the vitamin related maladies were recorded in Chinese literature as far back as 2600 B.C. That components of some foods and plants could cure many of these illnesses—broth of pine needles or juice of citrus fruits to cure scurvy, for example—was subtle acknowledgment that a basic diet lacked some nutritional but essential aspect. “Diseases” such as blindness, beriberi, scurvy, pellagra, and rickets went unrecognized as nutritional deficiencies by the earliest chemists, physiologists, and researchers. By 1900, only two or three of these were officially recognized to be associated with the diet.

In all, 17 vitamin-related Nobel Prizes were awarded from 1928 to 1967 on the discovery, isolation, synthesis, and structure of the vitamins [9]. In most cases, once the structure of the vitamin was chemically elucidated, the first synthesis was accomplished soon afterwards [10].

Today, economically important vitamin production occurs by chemical means, fermentation, or through the extraction from natural sources. Over the years, significant improvements in the stabilization and commercial product formulation were established in human and animal vitamin product forms. Advancements in the industrial production of vitamins makes possible the supplementation of vitamins of commercial animal diets in agriculture [1, 11]. The market size of the global vitamin supplement business was valued at USD 44.12 billion in 2020 and is expected to expand at a compound annual growth rate of 6.2% from 2021 to 2028 (https://www.grandviewresearch.com/industry-analysis/vitamin-supplements-market-report).

3. The vitamins

Vitamins comprise a group of organic compounds distinct from fats, carbohydrates, and proteins. They are considered to be (1) organic, (2) natural components of foods in minute amounts, (3) essential for normal physiological function, (4) associated with distinct deficiency symptoms when absent, and (5) insufficiently produced by the host [6]. Today, we recognize 13 different vitamins that meet these standards, of which 12 are routinely supplemented to commercial poultry feeds.

These molecules perform within a complex metabolic system, and function in catalytic, developmental, and protective roles (Table 1). As such, vitamins are essential for growth, development, maintenance, and reproduction, and mediate in synthetic and degradative processes and participate in catalytic functions. As opposed to macro ingredients in animal feeds, vitamins are required in comparatively small amounts to satisfy requirements, thus are considered micro ingredients in the realm of commercial feed additives. Depending on the animal species, some vitamins are produced in the body (for example, niacin from tryptophan; choline from methionine; vitamin D₃ from 7-dehydrochlesterol via ultraviolet light; ascorbic acid by most animals, including poultry), albeit at levels insufficient to meet demand for metabolic purposes. Survival depends on the presence of vitamins.

Vitamin	Solubility	Primary functions
Vitamin A	Fat	Vision, reproduction, membranes, bone development, hatchability, ataxia and weakness, ruffled feathers
Vitamin D3	Fat	Bone development (P, Ca absorption), immune function
Vitamin E	Fat	Antioxidant, cell membrane integrity, immune function, reduced platelet aggregation (blood clotting)
Vitamin K	Fat	Blood clotting, bone mineralization
Thiamin	Water	Energy production, and carbohydrate, fat and protein metabolism, nerve function
Riboflavin	Water	Energy production, and carbohydrate, fat and protein metabolism
Niacin	Water	Energy production, and carbohydrate, fat and protein metabolism
Pyridoxine	Water	Energy production, and carbohydrate, fat and protein metabolism
Pantothenate	Water	Energy metabolism
Biotin	Water	Carbohydrate, fat and protein metabolism, glucose metabolism
Folic acid	Water	Amino acid & energy metabolism, protein synthesis, immunity
Vitamin B12	Water	Related to methionine, choline and folacin metabolism, and fat and carbohydrate metabolism

Table 1.

Twelve vitamins commonly added to poultry feeds and their primary functions.

4. Vitamins are not a chemical class

Unlike other chemical classes such as the alcohols or aldehydes, the term ‘vitamin’ does not refer to a class of chemicals with similar structures or functions. Individual vitamins vary significantly in chemical structure (Figure 1) but are categorized in two groups based on solubility: fat-soluble and water-soluble. The four fat-soluble vitamins include vitamins A, D₃, E and K, while the nine water-soluble vitamins comprise thiamin, niacin, riboflavin, pyridoxine, pantothenic acid, vitamin B₁₂, folic acid, biotin, and ascorbic acid (vitamin C). Generally, the fat-soluble vitamins are more aligned with cell membrane components, whereas the water-soluble vitamins often function as carriers of several biochemical groups or act as coenzymes in metabolic reactions [12], albeit some exceptions do occur. Each vitamin is distinct in function and elicits different deficiency symptoms when absent or insufficient.

Figure 1.

The structure of vitamins varies considerably (www.compoundchem.com).

Fat-soluble Vitamins. These vitamins are primarily aliphatic and aromatic in nature and are absorbed in a manner similar to fats and oils. Being hydrophobic, they require emulsification in the upper intestinal tract of poultry through gizzard action and intestinal churning. Mixed micelles—a combination of free fatty acids, monoglycerides, and bile acids—deliver the fat-soluble vitamins to the microvilli surface for uptake into the portal circulation of poultry. For some animal species, regional intestinal differences generally exist for absorption: proximal for vitamin A, medial for vitamin D₃, and distal for vitamins E and K [13]. For poultry, the form of the vitamin can affect absorption site. Combs and McClung [6] note that vitamin D₃ absorption rate is fastest proximally, but owing to a longer feed transit time, the greatest amount probably occurs distally. Competition for absorption exists among the fat-soluble vitamins [13], which probably extends to the rate of accumulation for fat-soluble vitamins in eggs [14]. The magnitude to which interactions occur among the fat-soluble vitamins is impacted by supplementation levels in the feed [15].

Water-soluble Vitamins. Absorption is influenced by molecular weight, ionization status, and whether the vitamin is present as a weak acid or base [6, 12]. Intestinal uptake is favored by a small molecular structure and a weak ionic character. While niacin (niacinamide), pyridoxine, biotin and vitamin C are readily taken up, absorption of thiamin and B₁₂ face greater difficulty. Thiamin, B₁₂, folic, and vitamin C are absorbed by a carrier-mediated mechanism, but as levels in the feed increase, simple diffusion plays a greater role [12]. Less is known about potential competition among water-soluble vitamins. These vitamins are somewhat unique from their fat-soluble counterparts in that their many interactions with one another in metabolism can make it difficult to determine some of the individual vitamin requirements.

Once absorbed, the vitamins function in five basic roles in metabolism, as noted by Combs and McClung [6], and these include antioxidants, gene transcription, electron donors and acceptors, hormones, and coenzymes. Respective chemical reactivity and tissue distribution come into play for the completion of these roles.

Vitamins are generally considered nontoxic. Minimal body storage occurs among the water-soluble vitamins. Whereas fat-soluble vitamins are generally stored in the liver and other tissues, vitamin K appears to be rapidly metabolized and excreted. Factors such as age, duration of feeding, and form of the vitamin can affect tolerance to high levels. For example, hypervitaminosis is less likely with the vitamin D₂ as opposed to the more potent vitamin D₃ in poultry [16]. Dietary calcium and phosphorus can also influence the tolerance to vitamin D. Furthermore, laying hens may be more tolerant to high levels of vitamins as opposed to broilers. While 2800 IU vitamin D₃/kg fed for more than 60 days is listed as upper limit for chickens [16], a level of 102,200 IU D₃/kg triggered no adverse symptoms for laying hens over a 40-week period [17]. In part, the higher tolerance presumably is associated with the transfer of excess vitamin D₃ from the body to eggs, thus avoiding build-up in the body. Combs and McClung [6] categorize vitamins in four groups of toxic potential –

• Greatest potential—vitamin A, vitamin D₃

• Moderate potential—niacin/niacinamide

• Low potential—vitamin E, vitamin C, thiamin, riboflavin, pyridoxine

• Negligible potential—vitamin K (menadione), pantothenic acid, biotin, folic acid, vitamin B₁₂

5. Commercial vitamin forms

Moisture, heat, pelleting, oxidation, and reduction reactions can undermine vitamin survivability in feeds. These stresses are often associated with pelleted, expanded, or extruded feeds, as well as minerals in the feeds, or premixes with other additives that are hygroscopic in character. In addition, the level and bioavailability of vitamins that naturally occur in ingredients can vary considerably [1]. Agronomic factors influence vitamin content, as does plant maturity and harvest conditions, and the stability of naturally occurring vitamins seldom holds up to the rigors of feed processing. As such, many fail to meet standard requirements for poultry without supplementation from other sources.

Commercial product forms of vitamins are formulated to buffer these challenges that can adversely influence the amount the animal eventually consumes in the feed. Since vitamins vary widely in their chemical structure, the type of formulation and final product form for each can differ. In the end, the goal is to develop commercial vitamins to optimize handling, mixability, stability, and bioavailability. Some of the more common formulations include beadlet formation, spray drying, adsorption, crystalline powder, or coated powder in the final formulation.

Improvements usually encompass chemical or physical modifications (Table 2). Chemical modification of vitamin A, E and C, can stabilize the reactive hydroxyl groups through esterification. Antioxidants may also be included for added protection against reactions that are prone to occur in the presence of other factors such as moisture and some minerals. Physical protection is applied in various formulations to develop a barrier to protect against oxygen, moisture, or light. Differences exist across manufacturers and formulation technologies due to patents and proprietary techniques, hence, not all will equally protect against degradation.

Vitamin	Formulation	Purpose
Vitamin A	Ester in a cross-linked beadlet	Stability, solubility
Vitamin D3	Spray dry (SD), beadlet	Stability, uniformity
Vitamin E	Acetate ester, SD or granular	Flow, reduced dustiness
Vitamin K/menadione	Crystalline powder	Flow, handling
Thiamin	Coarse granular	Stability
Riboflavin	SD granular	Flow, handling
Pyridoxine	Fine granular crystals	Stability, mixing
Vitamin B12	Crystalline w/carrier	Distribution
Niacin	Crystalline	Flow, reduced dustiness
Niacinamide	Crystalline	Flow, reduced dustiness
Ca-pantothenate	SD	Flow, reduced dustiness
Biotin	SD	Distribution, handling
Folic acid	SD	Flow, stability, mixing
Vitamin C	P esterification, ethyl cellulose coat	Stability

Table 2.

Formulated changes in vitamins to improve function for commercial purposes.

The protection of the vitamin A molecule is a good example of chemical and physical technologies being utilized for one vitamin. Once chemically stabilized through esterification, further improvements are made by cross-linking with gelatin in a ‘beadlet’. Fructose and glycerine enhance the process to ensure protection against moisture and heat during feed manufacture. The protein-based coating can be hydrolyzed by low pH and intestinal proteases, thus releasing the vitamin A for absorption once consumed by the animal.

In addition to beadlet formation, improvements can also be made through encapsulation, adsorption and spray drying. Some vitamins are inherently stable, such as niacin or biotin, and require minimal formulation changes beyond grinding and sifting to improve particle size and particle distribution while reducing dustiness in the final form. Liquid or emulsified vitamins have some application but are not a common product form. For example, the liquid application to pelleted feeds may experience fewer losses via the pelleting process, which may allow nutritionists to finetune fortification rates. In addition, some costs-savings may occur by avoiding the costs to develop beadlets or product forms for improved stability.

6. Guidelines: Vitamin fortification in commercial feeds

Commercial vitamin supplementation rates of poultry feeds are provided by several sources, ranging from formal guidelines and university research to standards developed in field trials in commercial production. Guidelines or recommendations for vitamins are usually in addition to levels already present naturally in the ingredients being fed. Vitamins that occur in ingredients vary widely in bioavailability, as well as stability during pelleting of feeds.

The National Research Council (NRC) Nutrient Requirements for Poultry is considered one of several formal standards for the requirement of nutrients. Twelve vitamins are listed at levels summarized from research studies from published results of nutritional research (Table 3) [18]. Besides being quite dated, the most recent NRC [18]—and similar guidelines from other sources—excludes consideration for margins of safety to account for various factors that could influence vitamin requirements. Considerations for other factors—vitamin form, inadequate mixing, poor storage conditions, malabsorption issues, genetic change, lower feed intake, and stress status—are not considered, yet these factors can impact the supplemental levels necessary to attain optimal performance and production in commercial practice.

Units/MT feed*	Starter	Grower	Finisher
Vitamin A, MIU	1.5	1.5	1.5
Vitamin D3, MIU	0.2	0.2	0.2
Vitamin E, TIU	10	10	10
Vitamin K, G	0.5	0.5	0.5
Niacin, G	27	27	11
Thiamin, G	1.8	1.8	1.8
Riboflavin, G	3.6	3.6	3.6
Pyridoxine, G	3	3	2.5
Pantothenic, G	10	10	10
Vitamin B12, MG	9	9	9
Folic, MG	550	550	250
Biotin, MG	150	150	100

Table 3.

Vitamin recommendations [18] for broilers.

*

MIU = million international units; TIU = thousand international units; G = grams; MG = milligrams.

To this end, Applegate and Angel [19] note that “our perception and definition of a nutrient requirement has changed from first being a requirement, as a percent of a diet, to preventing a nutrient deficiency, to now being a requirement to optimize growth or egg production response per unit of nutrient intake.” Supplementation rates for nutrients as a feed additive can be adjusted to any particular goal: rapid growth (for example, the greatest body weight within a given time period) or optimal growth (where the conversion of feed to body weight gain becomes the primary goal), or to achieve the lowest cost per unit of feed or lowest cost per unit of meat produced. Economics play a dominant role in research objectives to define the requirement of vitamins in many cases.

The rapid change in broiler genetics is generally the most influential factor that affects long-term fortification levels. Broiler growth rates have improved 3–4% annually with less feed being consumed/unit live gain [20]. To this end, the feed conversion ratio (FCR) improved nearly 1.5% annually over the past 10 years [21]. Patricio et al. [22] presented calculations to show that 20% less feed was required to attain the same body weight in broilers over the span of nearly 20 years. Alternatively, the time required to attain a 2.31 kg slaughter weight has declined from 52 days 1995 to 40 days today [21]. Modern-day caged layers are considered “long life” layers, based on the production of 500 eggs in 100 weeks [23]. If dietary vitamin levels were held steady, for example, vitamin intake declines per unit feed intake while the requirement is increased for vitamins and other nutrients because of the higher production. So, founded simply on improvements in the bird’s ability to produce more with less feed, micro-nutrients such as vitamins require adjustments to offset a reduced feed intake.

Sources for vitamin fortification guidelines are offered by organizations other than the NRC. Genetic companies for poultry periodically conduct research to determine the most optimal nutrient levels for their genetic base, and this includes vitamins [24, 25]. Likewise, commercial vitamin suppliers present recommendations for vitamin supplementation rates [1, 26]. The Optimum Vitamin Nutrition^®, for example, proactively considers commercial stresses and conditions that influence vitamin supplementation of poultry feeds [1, 26]. This concept targets the health and productivity of poultry over a range of vitamin supplementation levels, as noted in Figure 2 (taken from [1, 26]). In this approach, average animal response refers to the animal response in terms of productivity: feed conversion, growth rate, etc., as a consequence of vitamin consumption. Total vitamin intake considers amount of vitamin provided in the diet from all sources and considers the bioavailability of that vitamin in feedstuffs. Deficient refers to a deficiency status relative to recommendations by the NRC and other similar sources. Sub-optimum vitamin intake results in subpar health and productivity and prevents clinical deficiencies, whereas OVN intended to compensate for factors that can negatively impact animal health, well-being, and productivity. While it is difficult to account for all factors that affect animal performance, this program considers many prominent factors that could influence vitamin supplementation.

Figure 2.

Optimum vitamin nutrition concept over the range of deficiency to special applications.

There is no shortage of evidence that targeted vitamin fortification can be beneficial. Luo et al. [27] highlight the benefits of higher vitamin supplementation for broilers exposed to coccidiosis under commercial conditions, in part, because of lower digestibility by diseased birds. In addition, folic acid at a level roughly 10-fold higher than average fortification rates in the U.S. increased gene expression for more muscle accretion in broilers [28]. On the other hand, more breast meat (P. major) accumulation occurred with 25-OH D₃in ovo injections [29], which is consistent with an improved mitotic satellite cell formation and breast muscle yield in broilers fed 25-OH D₃ [30]. Higher fortification is required to enhance egg levels of some vitamins for human consumption such as vitamin D₃ [17, 31] and folic acid [32]. So, in effect, vitamin fortification strategies or “requirements” are modified, depending on an improved return on investment, or to take advantage of effects in meat or eggs, or other endpoints [33].

7. Vitamins and intestinal microflora

Studies have illuminated an important relationship between vitamin fortification of feeds and intestinal microflora in mammalian species [34]. This also holds true for poultry, as noted by a number of recent studies that establish an important role for vitamins in the maintenance and promotion of a desirable microbiome.

In one of the first investigations [27], the diversity of the cecal bacteria was compared between vitamin-supplemented broilers at NRC [18] levels versus a non-supplemented group in which both groups were fed diets tethered to corn and soybean meal. With vitamin supplementation, the diversity of the microbiome was increased (P < 0.0001) with Clostridium being the most dominant genus of all species after a 28-day test period, whereas Faecalibacterium was second most abundant in those same birds. Escherichia/Shigella were found only in broilers without added vitamins in the feed, while Lactobacillus was present in those fed vitamins. This work indicated that vitamin supplementation encouraged the presence of advantageous bacteria, whereas the vitamin content supplied only by feed ingredients resulted in a microflora being notably less desirable. In similar work, aged layers benefited with an increased abundance of advantageous ileal and cecal bacteria, along with an improved laying rate and egg quality, when supplemented with a 2-fold higher vitamin level [35]. This work was tethered to the hypothesis that modern laying hen genetics require vitamin levels over and beyond today’s standards.

Although it is unclear in these studies which vitamins may have had the biggest impact, recent work with laying hens focused only on vitamin D₃. Laying hens fed a vitamin D₃-deficient diet were particularly susceptible to Salmonella enterica and gut mucosal damage [36]. Populations of less favorable intestinal bacteria such as Escherichia, Enterobacteriaceae, and Clostridia were elevated in vitamin D₃-deprived hens, whereas Lactobacillus and Bacilli and other desirable bacteria became more predominant when vitamin D₃ was supplemented (3000 IU/kg). Hens that were vitamin D₃-deficient experienced mucosal injury and extensive intestinal inflammatory response, leading the investigators to suggest that vitamin D₃ could be an important nutritional strategy to defend against Salmonella infection. More investigation is needed to understand the optimal vitamin D₃ level for this purpose.

In related work, dietary 25-OH vitamin D₃ (HyD^®; 69 μg/kg) in laying hens at high stocking density elevated bacterial diversity and improved intestinal function [37]. Villus height was significantly increased with 25-OH D₃ supplementation, as was oxidative capacity. In earlier work, high stocking density elevated blood corticosterone (an indicator of higher stress) and decreased the activities of several antioxidant enzymes [38]. Li et al. [39] reported that 25-OH D₃ improved some oxidant enzyme activities in the small intestine of hens maintained in higher density populations, which implies that the antioxidant effect of 25-OH D₃ might be indirectly improving morphologic features of the intestinal tract. In addition, the microbiota composition was improved with 25-OH vitamin D₃ which included typical vitamin D₃ supplementation levels, suggesting that 25-OH D₃ itself may be important to maintain morphologic and microbiome status.

Broilers challenged with Salmonella enteritis also benefited from vitamin C at 500 mg/kg with a reduction in damage to villus structure [40]. Vitamin C supplementation expanded cecal microbial diversity, such as the Firmicutes to Bacteroides ratio on days 21 and 35. Under heat stress conditions, cecal Lachnospiraceae and Ruminococcacaea populations were elevated when broilers were supplemented with vitamin E at 250 IU/kg (about 5-fold higher than commercial average), along with an organic Se complex [41]. Both cecal species are important butyrate producers from nonstarch polysaccharides and resistant starch.

As one of the essential nutritional groups, that vitamins can impact intestinal microflora and morphology is not particularly unexpected when considering bacteria also have requirements for certain vitamins and other nutrients. Certainly, these studies offer encouragement that vitamin fortification may include a consideration to account for a healthy and beneficial intestinal microbiome.

8. Egg deposition of vitamins

Partitioning of nutrients by the laying hen generally favors the egg over body tissue. For vitamin E, the yolk is considered the favored tissue for deposition, followed by liver, adipose tissue, dark meat, and white meat [42]. Similarly, the yolk preferentially accumulates folate, as compared to other tissues [43]. The lipid-rich yolk is an important reservoir for the fat-soluble vitamins, as well as for most of their water-soluble counterparts. The exception here appears to be biotin which is found in higher concentrations in the egg albumen [44].

Of the commonly accepted vitamins, all but vitamin C are present in the egg. Studies have established a direct link between vitamin levels in the feed consumed and the vitamin concentration of eggs of the hen [45]. Upon absorption, vitamins are delivered to the egg by one of several transport systems that go against a blood/egg concentration gradient. For example, biotin concentration in the egg is about 20-fold higher than in plasma, thus a receptor-mediated transport system works to go from low-to-high gradient [44]. The concentration gradient difference for riboflavin is nearly 7-fold [46], while folate in the yolk was 43-fold higher than in plasma [43].

Yet, owing to the chemical complexity of vitamins, and the absorption and body storage characteristics, as well as the biological variation inherent in hens, the feed-to-egg transfer occurs with a considerable range in efficiency across the vitamins. Whereas vitamin D₃ can be elevated to high levels in the egg through higher supplementation rates [17], other vitamins such as folate are deposited at a much lower rate (Table 4) [14]. Various factors such as dietary level, effect of other vitamins and nutrients, period of feeding high vitamin levels, and age of bird can influence the degree to which vitamins accumulate in egg yolk and/or albumen.

Vitamin	Potential increase	Comments
Vitamin A	2–3-fold	Includes retinol, retinyl esters, and retinal but compiled in yolk mainly as retinol
Vitamin D3	6–10-fold	Some work demonstrated a much higher accretion in eggs
25-OH D3	3–4-fold
Vitamin E	4–5-fold	Considerable variation because of influence of other fat-soluble vitamins
Vitamin K	2–5-fold	Research is limited
Niacin	2–3-fold	U.S. requires niacin fortification to some grain products for human consumption
Thiamin	≈2-fold	Limited data base
Riboflavin	2–3-fold	Refractory to accretion at high feed levels
Pyridoxine	2–3-fold	Limited data base
Pantothenic	2–3-fold	Plateaus quickly
Vitamin B12	3–4-fold	Good response but high feed level needed
Folic acid	2–3-fold	Corn-based feeds may favor higher accretion in yolk
Biotin	3–5-fold	Dietary excess largely goes into albumen

Table 4.

General guideline for egg accumulation of vitamins [14].

9. Commercial broiler vitamin survey

An assessment of vitamin levels used in commercial broiler feeds in the U.S. was recently completed [11], which was conducted in a manner similarly as an earlier study [47]. Supplementation rates were categorized according to feed phase and all vitamins are reported on a pure vitamin level to avoid any bias by differences in product form. Categories included the following: starter (≈day 1–14); grower 1/Grower 2 (≈day 15–28); finisher (≈day 28–36); withdrawal (WD; ≈day >36); Breeder. Commercial nutritionists provided their addition rates for vitamin premixes which allowed for an accurate calculation for each vitamin per metric ton (MT) of feed. Over 90% of the broiler production for the U.S. was accounted for in this manner.

Poultry feeds are commonly supplemented with vitamins through a vitamin premix that includes fat- and water-soluble synthetic vitamins. In some cases, the same vitamin premix was used across all broiler feeds, or multiple premixes may be fed over the production period. A vitamin premix is typically composed of four fundamental components: vitamins, calcium carbonate (densifier), rice hulls or wheat midds (carrier), and 1–2% oil (to reduce dustiness; adhere vitamins to the carrier). In this survey of commercial vitamin fortification levels, Figure 3 designates the actual vitamin premix addition rate in terms of kg/MT feed. From starter to WD, the addition rate declined from 0.53 to 0.34 kg/MT, or about 36%.

Figure 3.

Vitamin premix addition levels to broiler and breeder feeds [11].

Figures 4 and 5 illustrate the percent of respective vitamins found in the starter and WD vitamin premixes. These were calculated based on each vitamin in the pure form in the premix. The relative changes between a young bird (starter) and a mature bird (WD) fortification rate are noted in these figures, while those in the grower (not shown) premix ranked between. Niacin, vitamin E and vitamin A were some of the more notable changes as a percent of the total content.

Figure 4.

Broiler starter vitamin premix levels (% of the total active vitamin premix) ([48], unpublished).

Figure 5.

Broiler WD vitamin premix levels (% of the total active vitamin premix) ([48], unpublished).

Table 5 lists the average vitamin fortification levels across starter to WD feeds, as well as for broiler breeder feeds. The reduction in vitamin supplementation of feeds from starter to WD parallels the decline in requirements as the bird approaches market age. From starter to finisher, most vitamins declined 20–30%, while from starter to WD, the reduction was greater. The greatest decline occurred with WD folic acid being 52% of the starter level. Several vitamins ranged from 60% to 63% of the level in starter feed compared to WD, and these were vitamin A, vitamin D₃, niacin, pantothenic acid, and vitamin B₁₂.

	Type of feed
Units/MT feed*	Starter	Grower	Finisher	Withdrawal	Breeder
Vitamin A, MIU	9.29	8.25	7.00	5.73	10.58
Vitamin D3, MIU	3.73	3.42	2.89	2.35	4.30
Vitamin E, TIU	51.07	41.18	35.15	27.39	67.36
Vitamin K, G	2.52	2.35	2.03	1.50	3.42
Niacin, G	51.60	47.57	41.97	31.08	54.13
Thiamin, G	2.46	2.18	1.87	1.38	3.11
Riboflavin, G	8.49	7.62	6.69	5.15	11.14
Pyridoxine, G	3.57	2.69	2.76	2.12	4.74
Pantothenic, G	13.51	12.70	10.93	8.43	17.31
Vitamin B12, MG	20.56	15.84	15.77	12.57	27.23
Folic, MG	1329.7	1192.9	994.5	696.8	2046.3
Biotin, MG	180.9	131.1	142.2	100.3	267.9

Table 5.

Average U.S. vitamin fortification rates for broilers and breeders [11].

*

MIU = million international units; TIU = thousand international units; G = grams; MG = milligrams.

The 2022 commercial starter vitamin D level was 18.6-fold higher than NRC [18], while vitamin A, E, and K were 6.2-, 5.1-, and 5.0-fold higher in commercial supplementation rates. Differences between commercial water-soluble vitamins and NRC [18] were not nearly as high. Biotin, pyridoxine, pantothenic acid and thiamin difference the least, ranging from 1.2 to 1.4 higher than NRC [18]. The interrelationship among the water-soluble vitamins is recognized as a complicating factor when trying to distinguish individual requirements among that group [49, 50]. Whereas none of the differences between commercial and NRC for the water-soluble vitamin exceeds 3-fold, this is not the case for the fat-soluble vitamins.

The breeder vitamin supplementation premix seldom consists of more of more than one premix over the entire production period (Table 5). This survey found that Breeder fortification was higher than typically fed in the starter feeds across all vitamins, presumably because of the need for optimal fertility, egg levels, and chick viability.

Noted in Table 6 are the overall average values for the starter premix when compared with the average of the lowest 25% and the highest 25% of those values for each of the vitamins. When comparing the lowest 25% average with the highest 25%, vitamins E, K, and B₁₂, along with folic acid and biotin, declined to more than 50% of the highest 25%. All others declined to a lesser degree. Overall, biotin declined the most (from highest 25% to lowest 25%) while vitamin A declined the least. Such changes can be attributed to different factors, such as how much agreement there exists within research data for necessary fortification levels, to special effects a vitamin is perceived to have beyond meeting requirements for optimal bird performance.

Units/MT feed*	High 25%	Average	Low 25%
Vitamin A, MIU	10.66	9.29	8.31
Vitamin D3, MIU	4.74	3.73	2.93
Vitamin E, TIU	72.55	51.07	34.62
Vitamin K, G	3.76	2.52	1.57
Niacin, G	63.30	51.60	41.74
Thiamin, G	3.12	2.46	1.81
Riboflavin, G	9.88	8.49	7.34
Pyridoxine, G	4.55	3.57	2.58
Pantothenic, G	16.65	13.51	10.71
Vitamin B12, MG	31.63	20.56	13.02
Folic, MG	2017.64	1329.66	876.52
Biotin, MG	263.1	180.9	96.0

Table 6.

Comparison of high and low 25% with average U.S. vitamin fortification rates for broilers and breeders [11].

*

MIU = million international units; TIU = thousand international units; G = grams; MG = milligrams.

Figure 6 illustrates the percent coefficient of variation (%CV; standard deviation divided by the mean times 100) by feed phase and within each vitamin category. The lowest %CV across vitamin fortification levels existed within the starter feeds. Vitamin A and riboflavin exhibited the lowest %CV, suggesting the greatest agreement among nutritionists for these two vitamins and nutritional requirements. On the other hand, vitamin E and vitamin B₁₂ generally were the most variable across all feed phases, while the remaining vitamins fell someplace within these two groupings.

Figure 6.

Coefficient of variation of individual vitamin fortification rates for broilers across feed phases ([48], unpublished).

Listed in Table 5 is a comparison of vitamin levels from the current 2022 survey and the 1993 survey. In comparing the former versus present, it is worthwhile to mention that 2017–2018 was a period of global vitamin shortages. During this period, many nutritionists were forced to pare back their vitamin supplementation levels in order that all birds received some vitamins, as opposed to some not receiving any supplementation. Current vitamin fortification levels have not fully recuperated to this point and are reflected in the 2022 vitamin summary. Even so, the pressure for more skeleton and body weight in a shorter time requires higher vitamin fortification levels [51, 52].

In reference to the 1993 versus 2022 surveys, the 2022 vitamin levels were higher, especially for vitamin E and biotin, being 184% and 127% of the 1993 levels. Vitamin A, pantothenic acid, niacin, and riboflavin showed less increase, being 5%, 12%, 13%, and 20% higher in 2022 than in 1993. Considering the genetic improvements since 1993, overall increases in vitamin are not necessarily impressive, despite some nutritionists curtailing levels because of shortages.

10. Conclusion

Based on the scientific results presented in this chapter, the following conclusions can be drawn:

1. Vitamins consist of a class of micronutrients that are commonly supplemented to broiler feeds through a vitamin premix.

2. Vitamins represent an important group of nutrients with fortification guidelines that can be affected by stress, genetic change, and other factors.

3. In their natural form in feed ingredients, vitamins vary in levels, bioavailability, and stability in today’s feed manufacturing processes.

4. Through chemical and physical means, commercial forms of vitamins are variably stabilized and improved according to their inherent character to withstand environmental challenges.

5. Guidelines for vitamin fortification are developed by various organizations, some of which make allowances for a number of different factors that influence vitamin needs for proper growth and development and return on financial investment.

6. These micronutrients are vital for developing embryos, but not all vitamins are equally transferred from blood to egg, and this should be considered when supplementing breeder feeds.

7. Within recent years, the presence/absence and levels of vitamins have been shown to promote a more desirable microbiome diversity, while defending against Salmonella enteritis and improving overall intestinal morphology.

8. Today’s vitamin fortification levels exceed NRC [18] guidelines for the fat-soluble vitamins by 5-fold or higher, whereas differences with water-soluble vitamins are also higher but more subdued.

9. Relative to 1993, a similar survey nearly 30 years later showed significant increases for vitamin E and biotin, but less change occurred for other vitamins, while variability in fortification rates were the least variable in starter broiler feeds, and for vitamin A and riboflavin.