Resumen
The dietary inclusion of trace minerals (TMs), such as copper (Cu), manganese (Mn) and zinc (Zn), is of importance to cover the ever-evolving requirements for growth, production and reproduction in ruminants. Various sources of TMs are commercially available, such as inorganic (ITM), organic (OTM) or hydroxy (HTM) forms; however, their bioavailability and efficiency to improve ruminant zootechnical parameters may be highly influenced by ruminal solubility and effects on the rumen environment. The objective of this review was to compile the most up-to-date information on the ruminal solubility of ITMs, OTMs and HTMs and their effects on fermentation parameters and rumen microbiota, aiming to support specialists from the animal feed industry when choosing TM products for ruminant supplementation. Some commonly used ITM sources, like sulfates, have a high ruminal solubility, while oxides are less soluble. The ruminal solubility of OTMs is mostly found to be high; however, data on these TM forms are still lacking. Regarding HTMs, ruminal solubility is reported to be low; nevertheless, results are inconsistent. Considering rumen fermentation, ITMs show a negative effect, OTMs might improve, while HTMs do not affect parameters like dry matter degradability, volatile fatty acid production, pH or microbial protein synthesis. As for rumen microbiota, ITMs do not affect microbial populations; OTMs could decrease the abundance of some specific bacteria, like fibrolytic microorganisms, while studies with HTMs are missing or inconclusive. Further research is necessary to better understand the ruminal solubility kinetics of TM sources and the different interactions with fermentation parameters and rumen microbiota to successfully apply the precision TM supplementation of ruminants, tackling deficiency occurrences.