The beads further comprise a sustained-release overcoat comprising an effective amount of a hydrophobic material selected from the group consisting of (i) an acrylic polymer such as copolymers of acrylic and methacrylic acid; (ii) an alkylcellulose such as ethylcellulose; (iii) other commonly used retardant coatings such as shellac, zein, and hydrophobic waxy-type products, such as hydrogenated ca