Supplementation of vitamin C would not normally be recommended for common livestock species (ruminants, poultry, swine and horses) under normal management and feeding regimes. As previously mentioned, stress conditions do affect vitamin C synthesis and supplementation considerations must take this into account. Kolb (1984) has summarized various types of stress that apparently have increased demands while reducing animals' capability to synthesize vitamin C: (1) dietary conditions: deficiencies of energy, protein, vitamin E, selenium, iron and starvation, etc.; (2) production or performance stress: high production or performance (e.g., rapid growth rates and high egg production); (3) transportation, animal handling and new environmental location stress: animals placed in new surroundings and stressful management practices (castration, debeaking, etc.); (4) temperature: high ambient temperature or cold trauma; (5) disease and parasite: fever and infection reduce blood ascorbic acid, while parasites, particularly in liver, disturb ascorbic acid synthesis and increase requirement for the vitamin.
Various studies have demonstrated beneficial effects of low supplemental levels of 50 to 100 mg per kg (23 to 45 mg per lb) of ascorbic acid in diets of broilers or laying hens exposed to heat stress (Kolb, 1984). Njoku (1986) reported that 200 mg of ascorbic acid per kg (90 mg per lb) of diet fed to broilers helped alleviate heat stress. Egg shell thickness increased for hens (El-Boushy et al., 1968), while livability, weight gain and immune response improved in broilers (Pardue and Thaxton, 1982) when heat-stressed birds received supplemental vitamin C. Peebles and Brake (1985) fed vitamin C to broiler breeders throughout a complete production cycle. They found fertility improved at dietary levels of 50 mg per kg (23 mg per lb) with improvement in hatch of fertile eggs due to a decrease in early embryonic mortality. There was no further benefit at 100 mg per kg (45 mg per lb) ascorbic acid. In turkeys, toms given 150 mg per kg (68 mg per lb) of dietary ascorbic acid increased semen volume by 28% and increased sperm concentration by 31% per ejaculate. It is thought that the ascorbic acid stimulates testicular activity by its involvement in the synthesis of steroid hormones (Dobrescu, 1987).
Higher amounts of supplemental vitamin C have likewise proved beneficial to poultry. Vitamin C supplementation of turkey breeder hen diets at a rate of 200 mg per kg (90.0 mg per lb) improved egg production 6.5% over a 24-week period (Noll et al., 1996b). Similarly, vitamin C supplemented at the rate of 330 mg per kg (136.4 mg per lb) diet reduced mortality and pericarditis in chicks infected with E. coli (Gross et al., 1988). The amount of vitamin C needed for this protective effect increased with higher environmental stress levels (Chew, 1994). Some commercial turkey operations have adopted the practice of feeding higher levels of vitamin C, riboflavin and vitamin E to prevent against a pale, soft and exudative syndrome (PSE) in meat.
Large doses of ascorbic acid of 2,000 to 3,000 mg per kg (909 to 1,364 mg per lb) diet fed in granular coated form to White Leghorn hens increased egg weight up to 5% and also improved egg specific gravity (Orban et al., 1993). Improvement in egg specific gravity might be associated with thicker eggshells because the high level of ascorbic acid caused more calcium to be deposited on the shell. Levels of 250 and 500 mg per kg (114 or 227 mg per lb) of granular ascorbic acid in the diet of White Leghorns that have been force rested resulted in increased egg production an average of 5% over that of hens on lower levels, and egg shell quality was improved (Zapata and Gernat, 1995).
In another study, Noll et al. (1996a) reported improved feed intake and water consumption by poults when their drinking water was supplemented with a stabilized ascorbic acid product. Similar results were reported in broilers by McKee and Harrison (1995).
In recent studies, ascorbic acid has been implicated in the fear response of poultry (Satterlee et al., 1994; Jones, 1996; Jones et al., 1996; Jones et al., 1999). This is an important consideration, since in a study involving 22 broiler farms, 28% of the variation in the feed:gain ratio could be attributed to fear of humans by the birds (Jones, 1996). Panic, hysteria and attempts to flee can have a negative effect on bird performance. In the 22-farm study, Jones (1996) determined that an improvement of 0.09 in F/G would be possible if fear levels of the farm with the most fearful birds were reduced to levels of the least fearful.
Supplementation with ascorbic acid at 1,000 ppm in drinking water for 24 hours prior to stimuli resulted in a significant reduction in the length and intensity of fear responses in broilers (Satterlee et al., 1994). In a study with Japanese quail (Jones et al., 1999), ascorbic acid was tested in two genetic lines, which had been identified for either low or high innate fearfulness. Ascorbic acid at 1,000 ppm for 24 hours prior to stimuli significantly decreased timidity in both genetic lines by 20-25%. In an earlier study (Jones, 1996), ascorbic acid supplemented Japanese quail (1,000 ppm for 24 hrs prior to stimuli) were less likely (P < 0.05) to avoid strange objects in the feed trough and responded significantly faster to regain tonic movement after the induction of tonic immobility.
These responses could be explained if the metabolic requirements for this vitamin exceeds the capacity for biosynthesis during periods of stress (Satterlee et al., 1994). An alternative suggestion (Jones et al., 1999) is that ascorbic acid may modulate corticosteroid levels, which are elevated during stress (Satterlee et al., 1993).
L-ascorbic acid is the most important of the several compounds that have vitamin C activity. Ascorbic acid is commercially available as 100% crystalline, 50% fat-coated, and 97.5% ethylcellulose-coated products and their dilutions. The more soluble sodium salt of ascorbic acid (sodium ascorbate) is also commercially available. Various derivatives of ascorbic acid, which are more stable than the parent compound, have been shown to provide antiscorbutic activity. These include L-ascorbate-2-sulfate, L-ascorbyl-2-monophosphate, magnesium-L-ascorbyl-2-phosphate, and L-ascorbyl-2-polyphosphate. When providing supplemental ascorbic acid it is advisable to use a stabilized form, and coating of ascorbic acid crystals with ethylcellulose is a suitable stabilization method. In storage experiments, ascorbic acid protected in this manner was found to be four times more stable than untreated ascorbic acid crystals (Kolb, 1984).
The coated (ethylcellulose) ascorbic acid showed a higher retention after processing than the crystalline form, 84% versus 48%. Retention of ascorbic acid in mash feed was fairly good, but with elevated storage time and temperature the stability was poor in crumbled feeds. Although retention of vitamin C activity in feed containing the ethylcellulose-coated product was low, it was 19% to 32% better than that of the crystalline form.
Since ascorbic acid can be synthesized at the tissue level by domestic fowl, it has been held by many nutritionists that exogenous supplementation of ascorbic acid to poultry would be senseless. Over the past several decades, the relationship between stress and ascorbic acid in poultry has been recognized; however, research data have been inconsistent and conflicting, making it difficult to establish requirement for this nutrient under all conditions.
The amount of ascorbic acid synthesized by the bird should be sufficient for normal growth and metabolism; however, there is evidence that ascorbic acid synthesis may not meet physiological needs under stressful conditions (Ferket, 1994). Pardue and Williams (1990) reported that plasma ascorbic acid levels in poults were depressed significantly by cold stress, beak trimming, and injection at one and 14 days of age. Stress causes a depletion of ascorbic acid at a faster rate than the bird's natural capability to synthesize this vitamin (Pardue and Thaxton, 1986). The ability of poultry to synthesize ascorbic acid and the amount needed or used changes with age, management, environment, disease and stress. Pardue and Williams (1990) reported that serum ascorbic acid concentration in poults increased 170% from hatch to 49 days of age.
Research data indicate that supplementation with ascorbic acid should be considered as a management alternative to prevent vitamin C deficiencies when poultry are stressed (Quarles and Adrian, 1989; Quarles et al., 1989; Cheng et al., 1990; Ferket, 1994; Jones, 1996). Vitamin C supplementation can modify the detrimental effects of environmental, nutritional or pathological stress (Jones, 1996; Jones et al., 1996).
