Do Excessive Vitamin D Concentrations Improve or Impair Broiler Growth Performance and Bone Quality?

There has been much research recently to refine broiler requirements for calcium (Ca) and phosphorus (P), especially since the routine inclusion of phytase in broiler diets (Li et al., 2016). Phosphorus and Ca play significant roles in bone metabolism and vitamin D is also pivotal to maintaining bone integrity through the modulation of Ca metabolism (Li et al., 2017). The broiler requirement for vitamin D according to NRC 1994 is 200 IU/kg while, commercially, dietary concentrations as high as 5000 IU/kg are applied (Sakkas et al., 2018). Thus, given the genetic improvement of the current broiler and the refinement of Ca and P requirements, reassessment of the use of excessive dietary vitamin D concentration is timely.

The experiment was conducted to determine the response in growth performance, including live body weight (LBW), average daily weight gain (ADWG), average daily feed intake (ADFI), feed conversion ratio (FCR) and bone ash content including tibia ash and toe ash, of three concentrations of vitamin D3 (1600, 3200 and 6400 IU/kg diet) at different levels of ileal digestible phosphorus (IDP) and Ca. In the factorial experiment, six hundred day-old, male Ross broilers fed diets based on wheat, sorghum and soybean meal, were raised for 21 days in floor pens. Diets contained two ileal digestible P (IDP) concentrations (2.0 and 2.5 g/kg) and two Ca concentrations (3.5 and 4.5 g/kg) and each diet was supplemented with phytase. Starter and grower diets were fed from Day 1 to Day 14 and Day 15 to Day 21 respectively. Each treatment had 5 replicates and each replicate had 10 birds.

Growth performance and FCR were not affected by the IDP concentration but increasing the IDP concentration significantly (P < 0.05) increased toe ash content. Calcium, however, had a significant (P < 0.01) positive impact on LBW, ADWG, FCR and ash content but not on FCR (P > 0.05). In contrast, vitamin D₃ supplementation showed no effect on growth performance or ash content. However, there was a positive interaction of vitamin D₃ with IDP content (P< 0.05) on LBW and with Ca (P< 0.05) in regard to bone ash content which is generally used as a means of quantifying bone mineralization.

It appears, from this study, that the high inclusion rates of vitamin D₃ in broiler diets, used by industry, do not impair bird performance or bone health. There is an indication, depending on P and Ca status, that higher concentrations of the vitamin may be beneficial for growth and bone quality. These results again emphasise the complex interrelationships among these three nutrients and the need for more research to unravel the nuances of these interactions.