Trial 1: Level and source of vitamin D for modern genotypesBroilers of 2 modern fast growing genotypes were offered diets with either low (LD; 1000 IU/kg), medium (MD, 4000 IU/kg) or high levels of D3 (HD; 7000 IU/kg) and medium levels with the majority of D3 substituted by 25D3 (25MD; 1000 D3 + 3000 25D3 IU/kg). Performance, long bone mineralization and vitamin D status were measured at the end of the starter (d10) grower (d24) and nisher periods (d38), whilst bird walking capacity was assessed at d37 of age. Bird performance was unaffected by vitamin D supplementation but the degree of mineralization (bone ash %), walking capacity (Gait score; GS) and blood level of 25D3, which is an indicator of vitamin D status, were superior in birds offered the 25MD diets in comparison to the low levels of vitamin D (Figure 1). Our results suggested that 25D3 is the optimum source of supplementation, but also pointed out that vitamin D levels set by the EU may not need to be exceeded in diets with recommended dietary levels of Ca and P.
Trial 2: Level and source of vitamin D in parasitized broilers
Coccidiosis is a highly prevalent parasitic disease of the gastrointestinal tract, causing significant economic losses in broiler farms. Infections with coccidiosis have been shown to adversely affect mineral digestibility, fat soluble vitamin status and bone mineralization. In the second trial we explored the capacity of source (D3 vs 25D3) and level of vitamin D (1000 vs 4000 IU/kg) to promote bone mineralization in birds affected by coccidiosis. An established host-parasite model was utilized in which half of the growing broilers were infected with Eimeria maxima, one of the most commonly encountered species of coccidia. Infection reduced bone mineralization measured by tibia ash %, and body weight with the progression of infection. Overall, offering 25D3 or high levels of vitamin D increased performance, supported bone mineralization and vitamin D status irrespective of infection status. A higher vitamin D status was associated with higher levels of long bone mineralization, which is indicative of the bird’s skeletal health (Figure 2). Although there were no interactions between infection, level and/ or source of vitamin D on any of the aforementioned traits, infected birds which were offered low level of vitamin D3 had the worst feed utilization efficiency, the lowest vitamin D status and inferior long bone mechanical properties (Figure 3).
Trial 3: Level of Ca:P supply and source of vitamin D in parasitized broilersIn a follow-up final trial we assessed the efficacy of D3 and 25D3 in growing broilers, using the same host-parasite model, offering 25D3 or D3 (4000 IU/kg of feed) and diets with adequate total Ca to available P (High; 8.7:4.4 g/kg) or marginally deficient levels (Low; 6.1:3.1 g/ kg). Infection induced the expected effects, as the ones reported in trial 2. Results of this study indicated that birds on low Ca:P diets had reduced bone mineralization (Figure 4a), while birds offered 25D3 as the source of vitamin D and high Ca:P diets showed the highest degree of bone mineralization (Figure 4b). Finally, the efficacy of the two metabolites was similar, irrespective of infection status.
• Overall, our studies indicated that inclusion of 25D3 at commercial levels of vitamin D supplementation is the preferred source of vitamin D in both healthy and infected birds for vitamin D status and bone mineralization.
• In addition, birds at later stages of growth showed better walking capacities with the inclusion of 25D3.
• Finally, reduced levels of vitamin D resulted in reduced performance and skeletal integrity in birds challenged with a commonly encountered species of coccidia.
Future studies will explore whether benefits from supplementing with intermediate vitamin D metabolites such as 25D3 or 1,25D3 could be seen in commercial enterprises, where sanitary challenges are more multifactorial in nature (e.g. presence of pathogenic bacterial infections, and multiple species coccidian infections) and husbandry practices present additional challenges for the leg health of modern broilers.