The effects of dietary iodine content, milking system, and farming practices on milk iodine concentration and quality traits

Various management practices can influence milk quality traits in dairy cattle. As an example, an increasing investment in automatic milking system to substitute milking parlors has been observed in the last 2 decades in dairy farms which could have affected certain bulk milk quality traits. What is more, milking practices can also affect certain milk parameters; as an example, teat disinfectants containing iodine (IOD) are used in commercial farms where pre- and/or post-dipping is performed, leading to presence of some IOD in the bulk milk. However, this trace mineral is also supplied in cows' diet to fulfil their nutritional requirements, partly contributing to the milk IOD final concentration. Therefore, the aim of this study was to evaluate the sources of variation of milk IOD along with other traditional milk quality traits. A total of 91 dairy farms in north-eastern Italy were enrolled in the study. In each farm, diet and bulk milk samples were collected on the same day for chemical analysis. Concentration of IOD, in particular, was determined in both milk and feed with gold standard. Pearson correlations were calculated among the traits available for milk and diet, and a general linear model was used to test significance of fixed effects (feeding system, milking system, farming system, herd size, herd stage of lactation, and sampling month) on milk quality traits including the IOD concentration. In the case of milk IOD, diet IOD and presence of IOD-based pre-dipping and post-dipping teat disinfect application were also tested as fixed effects. Results showed a positive linear correlation between milk and diet IOD content (r = 0.78; P < 0.001). While milk IOD was also positively correlated with lactose content (r = 0.25; P < 0.05), dietary IOD was not correlated with other milk traits. Milk IOD content was significantly affected by dietary IOD (P < 0.001), IOD-based pre-dipping teat disinfectant application (P = 0.014), and herd composition (P = 0.044). Compared with conventional farms, organic farms showed lower protein content and greater somatic cell score (SCS; P < 0.05) but similar milk IOD. Milking system significantly affected only lactose content and SCS of milk. Sampling month was only significant for milk urea nitrogen (MUN; P = 0.041) and herd composition, feeding system, herd size, and herd average days in milk did not modify milk gross composition and SCS. In conclusion, dietary supply of IOD is the main factor affecting milk IOD concentration and findings suggest that IOD level in milk can be naturally improved in dairy cows by modulating the IOD content in the diet administered. However, further research is needed to evaluate the impact of IOD-based sanitizers on milk IOD.