An animal feed supplement in a solid, weather resistant block form and method of making is disclosed. The molasses based feed supplement block, intended for consumption primarily by ruminant animals, although not limited thereto, contains magnesium oxide as a blocking agent. The feed supplement also comprises a water absorbent clay, a phosphorous source and an edible fat or oil in sufficient quantities to set into a solid block in a relatively short period of time at ambient temperatures. A preferred block composition is prepared by a method which comprises admixing the molasses, water, phosphorous source and water absorbing clay under high speed shearing action for an effective amount of time to disperse the clay, admixing edible fat and dispersing it in the mixture and then admixing magnesium oxide to obtain a viscous nutrient mixture that thickens quickly. The mixture is then poured into molds or packages and solidifies to form a hard depackagable block.