Manganese is an essential trace element, which is necessary for cartilage development. Thus, it finds it extremely useful as a feed for various animals. As its properties contribute significantly to the ossification process, metabolism process, integrity of cellular membrane, most animal feeds use it in traces.

Manganous oxide is a safe source of manganese for all animal species/categories, provided that the current maximum total contents of manganese authorised in feed are respected. Dietary manganese does not appear to cause any adverse effects in consumers at usual intake levels; manganese intake resulting from the consumption of tissues and products of animal origin is low compared with the intake resulting from the consumption of food of plant origin. No essential differences in the manganese deposition in edible tissues and products of animal origin are expected from the use of manganous oxide compared with other inorganic manganese sources. The use of manganous oxide in animal nutrition is of no concern for the safety of consumers, provided that the current 

Also Used as a fertilizer supplement -

Manganese is readily assimilated by plants in this form, so that MnO is used as a fertilizer supplement in manganese-deficient regions.