Although poultry synthesize vitamin C in their kidneys, a growing body of research has found health and performance benefits with dietary supplementation, especially during periods of stress. Studies to date have considered ascorbic acid supplementation in the presence of individual stressors ranging from beak trimming to marketing and have reported benefits from increased gains to improved carcass weights compared to unsupplemented birds. These benefits are related to losses of ascorbic acid in the tissues due to stress.
Recent work at the University of Illinois (McKee and Harrison, 1995) examined ascorbic acid supplementation in the presence of multiple concurrent stressors. The results of this study not only strengthen the case for supplementation, they shed further light on the interaction between multiple stressors, which are far more common in commercial production than the individual stressors typically studied by researchers.
In the study, McKee and Harrison used 10-day-old chicks to assess the effects of beak trimming, disease (coccidiosis) and heat stress alone and in combination when the chicks' water was supplemented with 0, 150 or 300 ppm of supplemental ascorbic acid. Thus, there were eight possible stressor combinations at each level of vitamin C supplementation. The researchers found a generally linear decline in performance as the number of stressors increased. This is in keeping with McFarlane et al. (1989), who studied as many as six concurrent stressors (ammonia, electric shock and noise along with the three stressors used by McKee and Harrison) but did not assess vitamin C supplementation.
McFarlane et al. originally hypothesized that the combined effect of multiple stressors would be less than the sum of the parts, theorizing that the general adaptive response to one stressor would take part of the burden of response to another. Instead, the researchers found that the cumulative effect could be closely estimated by adding the effects of the individual stressors. In fact, variations from this rule of thumb occurred so rarely (in only six percent of the total 166 possible stressor interactions) that the researchers suggested these exceptions might be spurious, due merely to random chance.
The results in McKee and Harrison generally agree with those in the earlier study. However, there are differences in the effects of individual stressors, and the 1995 study suggests that the effects of multiple concurrent stressors may be more than additive.
For instance, McFarlane et al. found that individually, each of the stressors except for noise depressed feed intake, feed efficiency and weight gains. When McKee and Harrison measured feed intake, they found a significant decline with heat stress alone, but not with beak trimming or coccidiosis as single stressors. Yet feed intake was lowest when all three stressors were present (Figure 1).
