IntroductionMethane released during ruminal fermentation plays an important role in global warming [1,2]. One of the aims in ruminant nutrition is to reduce the release of methane from the rumen, without adverse effects on digestibility, animal health, and productivity [3]. Moreover, improvements of rumen microbial biomass and the antioxidant status of animals are attractive targets in feeding management. This requires the best supply of nutrients and supplementary minerals in diets. Zinc is a vital trace mineral needed for animal productivity, immunity, rumen metabolism, and the antioxidant system [3][4][5]. Zn insufficiency can cause the weakness of the antioxidant system [6-8] and using high levels of dietary trace minerals, such as Zn, can improve animal health and performance [9][10][11][12].Diets are usually supplemented with Zn as inorganic (such as ZnO and ZnSO 4 ) or organic (such as Zn-amino acid complexes) sources. An organic mineral source offers further elements to animals due to its superior bioavailability [5,13]. At present, the usage of ZnO nanoparticles (nano-ZnO), with sizes of 1 to 100 nm, has increased in various fields such as mineral nutrition in livestock [4,7,14]. Nano sources of trace minerals have high bioavailability because of interesting properties such as the nano scale size, rapid and specific movement, higher area surface to volume proportion, surface activity, catalytic effectiveness, and absorption percentage [12,15,16]. Some researchers assessed the toxic impacts of Zn nanoparticles in animals [17][18][19]. However, there are studies supporting the beneficial effects of nanoparticles on animal performance, feed efficiency, and health as well as reduction of environmental pollution, due to the great bioavailability [15,16,20,21]. Wang et al. [12] mentioned that long-term oral Zn sulfate treatment was more toxic for animals than nano-ZnO. As reported by Singh et al.[22], feeding preruminant lambs with nano-ZnO instead of ZnO increased the Zn availability without causing toxicity. Nano-ZnO can improve villus height, crypt depth, and villus surface area in the gastrointestinal tract [12,14]. Sarker et al. [1] reported that supplementation with high levels of nano-ZnO (i.e. 500 and 1000 μg/g) decreased in vitro methane concentration compared with a control treatment (no Zn supplementation). Nanoparticles could be poisonous to certain microbes generating methane in anaerobic digestion [14,23]. Still, further studies are necessary to realize the possible helpful or harmful effects of nano-ZnO on animals [1,7,21].