## 4 Discussion
### Influence of Vitamin Form and Vitamin Level on the Growth Performance of Piglets  
To our knowledge, this study is the first to compare vitamin forms (non-microencapsulated vs. microencapsulated) and vitamin levels (NRC, 75% CRV, and 100% CRV) under commercial feeding condition. Such a study is critical to determine the optimum combination of vitamin forms and levels to optimize pig growth, feed efficiency, and ultimately profitability in commercial pig production. The results here could be utilized by commercial pig producers to inform investment decisions. Our results suggest that high level of vitamins in the diet is the preferred feeding strategy. However, the form of supplemental vitamins (non-microencapsulated vs. microencapsulated) did not influence the growth performance.  
The results of our study agree with those of previous studies related to the effects of vitamins on the growth performance of pigs. [PERSON] et al. fed weaned piglets a control diet (basal diet without vitamins) or diets containing 0.5, 1, 2.5, or 5 times the NRC requirements for vitamin A, vitamin D, vitamin E, vitamin K, niacin, pantothenic acid, riboflavin, and vitamin B\({}_{12}\)[31]. Pigs fed the control diet or the basal diet with 0.5 times the NRC requirements of vitamins had lower growth performance than the other groups, whereas the pigs fed five times the requirements of vitamins had the highest ADG and ADFI. [PERSON] et al. fed weaned piglets a control diet (a basal diet without vitamins) or diets containing 0.05%, 0.10%, 0.25%, or 0.50% commercial vitamin premix with 11 vitamins at levels of 73-640%, 147-1280%, 367-3200%, and 733-6400% of NRC requirement estimates [32], respectively. In the overall experimental period, increasing the supplementation of vitamins resulted in linear increases of ADFI and ADG. In our study, increasing the vitamin level did not affect the growth performance of weaned piglets in phase 1 (d 0 to 14). These results suggest that the indigenous vitamins in natural feed ingredients or in tissue reserves can provide adequate amounts of vitamins for only a short period of time after weaning. Moreover, regarding the missed effects of vitamin on growth performance during phase 1 (d 0 to 14), [PERSON] et al. speculated that indigenous vitamins in the feedstuff, tissue reserves in the pigs during weaning, or the lower growth rate of the pigs immediately postweaning could provide an adequate supply of vitamins for a short period postweaning, resulting in no performance response due to the supplementation of vitamins [33]. However, during phase 2 (14 to 28 d), and for the entire 28 d period, ADG increased with an increase in dietary vitamin concentration. Commercial vitamin recommendations for piglets improve feed efficiency and ADG, reducing this amount to 25% of the commercially recommended vitamin level is enough to optimize performance. Our results suggest that the commercial vitamin recommendation level can maximize pig growth performance, but this level far exceeds the threshold for best performance. This may result from the addition of vitamins in commercial vitamin premixes to prevent vitamin loss during storage, thereby ensuring optimal animal performance, and maximizing marginal utility for profit. Due to the limitations in the present study, we could not determine the specific quantity of vitamins for optimal performance and whether the responses were due to a single vitamin or a combination.