By one measure (using phytohemagglutinin-P as the mitogen), lymphocyte proliferation was more than 90 percent greater in the birds receiving the highest level of vitamin E supplementation (100,000 IU per ton) than in those receiving no supplementation. Compared to the chicks receiving the control level of 10,000 IU per ton, birds receiving this highest level of dietary vitamin E had 15 percent greater lymphocyte proliferation.
Second, increasing dietary vitamin E supplementation appeared to influence the development of T cells as shown by the greater proportions of helper to killer T cells in the thymus, spleen and blood. The researchers noted that helper T cells produce cytokines that are required for the activation of both B cells and killer T cells. In addition, helper T cytokines also influence the class of antibody produced by the B cells, such as immunoglobulin (Ig) G or A, and can activate components of nonspecific immunity (especially macrophages) to increase the efficiency of that line of defense.
Third, there was a significant improvement in B cell activation with increased vitamin E supplementation when the broilers were injected with S. pullorum, a B cell antigen, at six weeks of age. Ten days later, the antibody response in chicks receiving the 100,000 IU/ton level was 29 percent greater than in those receiving no dietary vitamin E supplementation and 13 percent greater than in the chicks receiving 10,000 IU per ton.
Based on these results, the researchers said that increased vitamin E supplementation promises to better support immunocompetence, especially in young birds and those with a compromised immune system. That point also comes through clearly in field studies that measured improvements in health, performance and profitability with increased vitamin E supplementation in commercial operations.
Boren and Bond (1996) reported field work in which 1.52 million broiler chicks received either of two levels of dietary vitamin E supplementation in their starter feed: the standard 30,000 IU or 218,000 IU per ton of starter feed (33 or 240 ppm). Whole bird condemnations, septicemia-toxemia and inflammatory process were 34, 25 and 61 percent lower, respectively, in the broilers receiving the higher vitamin E level. Also, the feed-to-gain ratio was 2.3 percent (an average of five points) lower for birds receiving the increased supplementation.
These results were then calculated in economic terms for a complex with 1 million birds and typical feed costs, conversion rates and marketing weights. The reduced condemnation rate would produce savings of $7,668 per week, assuming a value of $0.3545 per pound of dressed broiler meat. These savings in themselves would pay for the $7,312 weekly cost of the increased vitamin E supplementation for 1 million birds. Then, the five point difference in feed-to-gain ratio would be worth an additional $25,750 per week for 1 million birds, assuming an average overall feed cost of $200 per ton. Even if feed cost only $150 per ton, the five-point improvement in feed efficiency would still save $19,315 per week for 1 million birds.
In a field study in Northern Ireland, McIlroy et al. (1993) measured the profitability of increased vitamin E supplementation when broilers were under far less disease pressure. In this work, 43 flocks (categories C and D in Table 1) exhibited no clinical signs of any disease, but bursal samples did show subclinical IBD. The birds received either 161,800 IU of vitamin E per ton of feed (178 IU per kg) or the operation's standard supplementation level of 43,600 IU per ton (48 IU per kg), with each treatment fed throughout the life of the birds. Performance was measured only by percentages to maintain confidentiality.